Internal storage conditions in the meat cold store are characterized by three factors: temperature, humidity and air circulation rate. These are governed by functioning refrigeration equipment and can be modified by changing the operating conditions of that equipment. Changes in operating parameters can be made to achieve different storage conditions or to counteract naturally occurring variations in external conditions.
The refrigeration plant has to rely on sensing elements to control internal storage conditions that reflect any deviation and on command mechanisms that can be activated to offset the alerted change. Sensing elements and command mechanisms are linked either manually, with operators periodically taking readings of the sensing elements and accordingly activating the command mechanisms, or automatically, usually when an electric signal from the sensing element activates the command mechanism. Between these two extremes of regulation there are some intermediate operating methods.
Within the sensing, command and functioning circuit of a refrigeration plant there are many elements, mainly electric (relays, disjunctors, contactors), which are not specific to refrigeration equipment but they must be familiar to the maintenance personnel for effective troubleshooting.
Automatic refrigeration is somewhat more complicated than manual as it has to incorporate certain devices for the easier handling of the refrigerating fluid (refrigerant).
Generally speaking the physical conditions (temperature, humidity, and to a certain degree air speed) established in any determined area of a cold chamber are not strictly constant over a period but oscillate between a superior and an inferior value, these changes recurring periodically in a somewhat regular pattern. This cycle can be divided into two periods: during the first the refrigerant distribution equipment functions, and eventually the refrigerating installation; in the second period this installation is off-cycle. This “on-off” operating mode is usual in conventional refrigerating plants.
Each physical condition to be controlled has its own peculiarities so it would be useful to look at each one in detail.
Temperature distribution inside a cold chamber is one of the most difficult features to control. As the temperature differs in the various parts of the chamber its distribution depends on correct design, on the stacking patterns chosen and on air circulation rate.
The space (and product) temperature is directly controlled by locating the thermostat in the interior space of the cold room (or on the product itself, but this is not the case for meat products). However it may be controlled indirectly by clamping the sensor of the thermostat to the evaporator so that the thermostat controls the evaporator temperature. The former method is indicated where close control of the space and product temperatures is necessary, as in chilled meat storage. The second method is better when it is necessary to ensure total defrosting of the evaporator, even if fluctuations in temperature are minor. It is employed for operations above freezing.
When the temperature is controlled by a thermostat or similar device the sensor element must be placed in such a way that it controls a temperature close to the average temperature of the chamber. To determine this position ordinary thermometers are distributed inside the chamber and, after a few readings, the point with the most representative temperature is easily located. When the temperature in the cold room has reached an equilibrium with the stored produce (which may take from one day to a week), the temperature of the air is practically equal to that of the produce.
Heat dispersal through the floor and walls influences the temperature distribution inside the store; an appropriate insulation thickness and correct installation will favour even distribution. If in spite of this insulation differences in temperature in the store become quite extreme, faulty stacking should be considered responsible.
The thermometer or the sensor of the thermostat that measures the temperature of the micro-environment of the store should be placed at mid-height on a wall far from doors and openings, a few centimetres away from and not directly touching the surface and, whenever possible, in the middle of one of the longer sides of the room.
When several coolers are used to refrigerate the chamber the sensor should be placed at an equal distance from them, usually on the opposite wall.
If the temperature to be maintained is close to 0°C, with a risk of freezing the stored produce, the coldest place should be chosen for the sensor, sometimes near the cooler, but in any case close to the floor.
For frozen products the same principles apply, though small temperature variations are not a major problem as some large frozen storage chambers where the refrigeration plant is run continuously during the night and stopped during the day allow for a certain oscillation in temperature.
The position of the sensor and the reading and command instruments has to be such as to allow easy access even when the chamber is completely full of produce.
A direct-reading thermometer (mercury or alcohol, graduated to 1/5°C) should be placed beside the sensor of the thermostat to be read in the morning and evening.
Thermometers used for temperature monitoring and refrigeration plant operation should be placed where temperature control is considered most necessary. However it is advisable to monitor temperatures at more than one location, particularly in large chambers. The measurement of product temperature is also recommended.
Storage conditions should be verified at least once a day.
The thermostat, which is the device to control room temperature, consists of a sensor (working in the same way as the thermometer) and an emissary (electric contactor) that transfers the information given by the sensor. It works on an on-off basis; modulated control is not used in refrigerating plants. The instant that the refrigeration plant must work or stop is determined by the sensor, but it gives an indication only, which is transmitted by the emissary to the electrically acting element, the compressor, for automatic operation. Thermostats control the temperature level of a refrigerated space by starting and stopping the compressor driving motor.
Once the maximum temperature level is reached inside the cold chamber the thermostat closes the electric circuit that starts the driving motor, and when the minimum temperature level is reached it opens the circuit stopping the motor. These cut-out points are fixed so that the thermostat can maintain temperature conditions inside a refrigerated space. The span between the two limit values is called the “temperature differential” and it represents the maximum difference that occurs. As long as the temperature of the refrigerated space is kept within the fluctuation limits, the functioning of the refrigeration machinery does not change.
For space temperature control, when the sensing element is located on a wall far from the cooler, the differential is ordinarily 2–3.5°C, but sometimes for chilled produce small differentials are established (about 0.5°C). When the sensor is clamped to the evaporator, controlling indirectly the space and product temperature through the evaporator temperature control, the differential used must be larger, from 8° to 10°C or even more, to avoid short-cycling of the equipment.
When the thermostat controls the space temperature directly its average temperature is approximately halfway between the cut-in and cut-out temperatures.
Correct adjustment of the temperature differential is essential if the refrigeration plant is to operate efficiently. When it is too small there will be a tendency to short-cycle, starting and stopping frequently, which affects the working life of the equipment. When the differential is too large the on and off cycles will be too long, resulting in excessively large fluctuations in space temperature.
As well as the equipment differential the “effective differential”, which indicates the actual fluctuation in storage temperature, must be considered, as it is always larger due to the inertia of the machinery.
Although the “range” of the system is defined as the difference between the cut-in and cut-out temperatures, it must not be confused with the differential. The values of the temperature difference in both cases coincide but the range affects the temperature level at which the control is operating, while the temperature differential does not affect this level. The range and the differential are adjusted and controlled with only one thermostat placed in the refrigerated space, but the adjustment of one implies the modification of the other. For temperature ranges some tolerance must be allowed, taking into account that the accuracy of automatic equipment is generally in the range of 1°–1.5°C.
The more sophisticated control systems use electrical sensing elements (electric resistance, thermistors) and electronic controllers, and the output of the controller is fed to a servo system to operate pneumatic or electric valves.
The temperatures of the cold rooms can be recorded intermittently or continually; the reading may be either directly transmitted to the control room or checked by the supervisor who notes down the reading in each room once or several times daily.
Thermometers should be calibrated annually, but sophisticated measuring systems need more frequent calibration; thus, automatic devices should be verified at least once a week.
Reading points must be easily accessible and thermometers should be protected against shock.
In summary, thermostats are used for automatic control of the temperature level in the cold room, their function being to start and stop the refrigeration plant by controlling the electric motors driving the compressor and the condenser fan, and also activating the solenoid valve.
Maintenance will prevent the malfunctioning of a thermostat. With the pressure bellows type, if the sensing element has lost its charge it has to be recharged whenever possible otherwise the thermostat must be changed. If electrical contacts are poor owing to a worn or corroded contact point then they should be replaced, though in an emergency they can be cleaned to continue operation. Poor electrical connections must be cleaned and tightened.
The relative humidity in a cold room is an indication of the equilibrium between the water evaporated from the stored produce and its removal from the air by the evaporator.
Relative humidity influences loss in meat weight during storage. This mass loss can be of considerable importance economically (less weight, spoiled appearance) and nutritionally.
The recommended levels of relative humidity for refrigerated storage provide adequate protection against micro-organism development, but they generate a certain loss of mass due to water evaporation that is accepted as unavoidable. Total losses are at a minimum within a relatively narrow range of relative humidity, say 80–90 percent.
Relative humidity inside a cold store is governed by many factors: quantity of product in store, type and method of packaging, stacking patterns, air motion, system running time, type of refrigeration system control, temperature difference, amount of exposed product surface, heat and water vapour infiltration, outside air conditions, and length of the working cycle of the refrigeration installation.
Of these, temperature difference (TD) is the most important. It should not be confused with the evaporator differential; TD is the difference between the temperature of the air entering the evaporator and the temperature of saturation of the refrigerant corresponding to the pressure at the evaporator outlet. The TD of an evaporator can be chosen in function of the produce, the evaporator geometry, the operating time of the refrigeration machinery, evaporator frost deposit and the type of refrigerant feeding into the evaporator. The smaller the difference in temperature between the evaporator surface and the space, the higher the relative humidity in the cold chamber (the contrary also being true). Notwithstanding other factors the following figures give an approximate guide for TD in evaporator design to achieve the desired relative humidity when the cooler works under forced convection conditions.
|Design TD (°C)||4.0–5.5||5.5–6.5||6.5–8.0||8–9||9–10|
|Relative humidity (%)||95–91||90–86||85–81||80–76||75–70|
The IIR publication Packing station for fruits and vegetables (1973) includes a table for the estimation of the relative humidity in a cold store from the air cooler surface temperature and storage space temperature. For chambers with storage capacities between 500 and 1 000 tonnes relative humidity should remain sufficiently high (about 88 percent or more) when TD is about 10°C or lower. Where the capacity is below 500 tonnes there is a rapid fall in relative humidity.
Obviously the difference in temperature between the evaporator and the space is directly related to the size of the evaporator compared to the amount of heat that must be removed. To increase the cooling load of a given evaporator surface the TD value must be increased, and, whenever possible, the air speed over the evaporator.
Evaporators with large surface areas are more expensive and occupy more space in the store, so it is more costly to construct a cold store operating at a high relative humidity. This, no doubt, is why most cold stores present humidity problems.
However, a large evaporator surface is not enough to achieve a high relative humidity. The operating efficiency of the evaporator is as important, and is influenced by design, air distribution, refrigerant feeding system, distribution and control; e.g. a constant pressure valve will avoid too low an evaporating temperature.
Weight loss is not only influenced by the space relative humidity but also by air circulation and length of the operating cycles of the refrigeration machinery, which in turn influence the relative humidity. These factors interact and are finally influenced by the design and operation of the cold store. Here is some practical advice concerning design and operation to reduce weight loss:
Different procedures to control and maintain relative humidity in the store are only effective within certain limits. For instance, hermetic wrapping of frozen products may induce progressive ice growth in the interior of the package, the rate of growth depending on the storage temperature and on the range and frequency of temperature fluctuation. Ice formation can be reduced by lowering the storage temperature and temperature differential and by limiting storage temperature variations.
There are ways to raise the relative humidity inside a cold room, but they are only temporary: water sprinkling or spraying; ice spraying or dusting; water vapour emission; passing air flow along a packed tower. These must be considered only as emergency or supplementary operations, as most of the incorporated water goes straight to the evaporator, uselessly increasing energy consumption and requiring more frequent defrosting of the evaporator.
When using water sprinkling or packed towers to humidify the atmosphere, care should be taken that water droplets evaporate before reaching the product stacked in front of the cooler. The humidifying equipment and its control system should be incorporated in the overall air circulation system.
A system capable of supplying 0.25 g of water per kg of refrigeration capacity (1 g/kcal refrigeration capacity) should maintain a relative humidity of about 95 percent.
Sometimes in winter or during cold weather too high a humidity can cause problems. As the cooling equipment has to function at much less than its actual capacity working periods become very short and cycling-off periods very long; this allows the humidity level to rise and reach values that can be dangerous for the proper storage of meat. The atmosphere of the chilled storage chamber may have a relative humidity of 92–95 percent or even higher during most of the off-cycle which favours bacterial growth, making the meat sticky and smelly. Excess humidity can be avoided by: dividing the evaporator into several elements; reducing the superficial temperature of the evaporator; prolonging the working time of the refrigeration equipment; or, finally, by direct dehumidification of the atmosphere by passing it through a water absorbent.
The extension of the refrigeration working period implies that some heat has to be introduced into the cold store to increase the naturally occurring heat load. This can be done by using electric resistances or dehumidifiers in small mobile units designed for cold chambers with high humidity problems. The evaporator of the equipment condenses the excess atmospheric humidity and consequently the air passing over the condenser is heated, leading to the on-cycling of the main refrigerating plant. The electric resistances or the dehumidifiers are governed by a hygrostat, though a couple of hours' daily operation should be enough to keep humidity at the desired level.
The heat furnished by electric resistances represents additional energy consumption, as the heating power should be about 20 percent of refrigeration power. The small dehumidifiers on the contrary consume as little as one-fifth of the total heating power, and besides removing humidity from the air they reduce the amount of water that condenses (and eventually frosts) on the surface of the cold room evaporator.
The cooling surface of the evaporator can be reduced by dividing and isolating some of the elements. This lengthens the operating time and also reduces refrigerant evaporation temperature. Both factors favour the reduction of relative humidity.
The humidity level in the cold room can be measured and controlled by using a hygrostat or hygrometer; all instruments should have a fast response. There are different types of hygrometer currently in use.
Hygrometers are often unstable and can easily become out of order; they are very sensitive to dust, their accuracy decreasing when dirt accumulates on the wet parts. An intense maintenance programme must be established and they must be tested periodically, using the wet and dry bulb psychrometer, keeping the wick clean and fitting the thermometer bulb tightly; the air velocity over the wick must be at least 30 cm/s. If a recording type hygrometer is used, the recording can be centralized in the control room. Automatic humidity devices that indicate and/or record must be regularly calibrated.
Careful records should be kept to ensure conditions are correct and also to gain experience in storage operation. Relative humidity at temperatures below 0°C necessitates special techniques and precautions.
Air is the secondary refrigerant that removes heat from the produce and its surroundings and carries it to the evaporator where it is cooled and discharged again into the refrigerated space. Heat, both sensible and latent due to water vapour condensation, is absorbed by the evaporator surface.
Air movement inside the cold chamber serves two main functions: first, the atmospheric temperature and relative humidity are homogenized to keep them reasonably uniform; second, evaporator efficiency is improved, in that the heat transfer coefficient is increased as air speed rises above it.
Air movement may be enhanced by differences in air density, known as natural convection, or may be promoted by mechanical action which is called forced convection. Fans or blowers are used to control the air speed in the chamber and also over the evaporator. An air circulation pattern is established inside the cold room, the evaporator being the focal point, while the rest is governed by smaller forces that direct the air as it moves through the stacking and recirculation spaces.
The uniform quality of air circulation is determined by measuring the differences in temperature, for instance, in distinct zones inside the cold chamber itself. The smaller the difference the better the air distribution.
When using forced convection systems, uniform atmospheric conditions depend on the air being dispersed with reasonable consistency, and if the circulation pattern involves much horizontal travel the product stacking design and the air flow pattern should be compatible. Stacked rows of produce sometimes serve to regulate the air flow by creating a uniform resistance all across the room without interfering with circulation.
The air circulation pattern is dependent on several factors: fan air flow; devices for distribution and suction; quantity of stored produce and stacking patterns; insulation efficiency; dimensions and shape of the chamber; external ambient temperature; pressure differences due to air flow; operating condition of the evaporator and frost accumulation. From this it can be seen that the circulation pattern changes according to the moment of the operating cycle and the state of the evaporator surface.
Air distribution in the cold store can be achieved via ducting or by using long-throw fans. The throw of the air delivery must be sufficiently farreaching to ventilate adequately all points of the cold room; when the space is too large for one evaporator unit to maintain adequate distribution several units should be strategically allocated inside the room.
There are two ways to install ducting. For each air cooler an individual duct is fitted beneath the ceiling of the cold room, occupying part of the store space, or ducts are located behind a false ceiling and/or floor. The other solution is to build a false ceiling over the entire ceiling area into which all the air coolers discharge; the air is thus distributed over the whole cold store area. Installing the air distribution system behind the ceiling has the advantage of recessed lights, thus avoiding projections, and a trim appearance which facilities cleaning, but above all the assurance that air distribution follows the desired pattern. The main disadvantage is the high cost of installation and operation.
The proper operation of air distribution through ducts requires a good ducting design, correct installation and regular maintenance.
Free air jets with high air velocities and horizontal throw are used nowadays as they have proved to be efficient without being costly if they are properly installed with enough space between stacks. The main problem is that the system relies on a restricted air passage and sometimes on low velocity of air leaving the evaporator.
It is clear that the most important factor in air distribution, no matter which system is used, is the stacking pattern. An air channel width of about 10 cm must be respected in the direction of main air flow. The crosswise distance between stacks is not so important. There must be no obstacle between the ceiling and a level tangential to the lower air outlet, allowing the formation of the jet flow pattern.
If the stacks are intended to regulate air distribution, the pallet rows must be perpendicular to the sense of air movement, particularly when the chamber is partially filled, and the first stacks introduced should be placed close to the evaporator (see Figure 4). The air circulation in the cold room must be correct no matter the state of loading. If the stacking pattern requirements are all met and the temperature still remains too high in certain positions, the reason is most probably an insufficient supply of cold air. This can be overcome with higher air velocity flows.
It is difficult to establish an actual value for the air circulation speed or the flow to be moved by the fans to achieve an adequate distribution in the cold room. A theoretical coefficient has been defined to be used as a guide for chamber draft design. The “rate of air change” is the ratio between the air volume passed through the cooler every hour and the total volume of the empty chamber.
For chilled storage rooms a rate of air change between 20 and 30 can be used as a reference, and from 40 to 100 for chilling chambers. In frozen storage values from 40 to 60 are suggested, 50 being a normal value: the smaller the chamber the higher the coefficient.
The rate of air change is inversely related to the temperature differences between air entering and air leaving the cooler, so in freezing chambers where temperature differences are as low as 1°C sometimes 200 recirculations per hour are possible.
For chilling and freezing tunnels the mean air speed in the zone where the product is placed is the characteristic factor which defines air distribution. In quick chilling tunnels the air speed must be about 2 m/s, measured in the empty section. For freezing tunnels the speeds are slightly higher.
The air velocity through the evaporator should be about 3m/s to achieve high overall heat exchange coefficients.
The rate of air is not controlled inside storage rooms as air distribution should always be the same. Chambers used both for chilling and chilled storage will need two different rates of air circulation, so several two-speed or manually operated fans should be installed. As air circulation must be effective in any situation, short circuits of air should be prevented by furnishing the fans with shutters that open when the fans start operation.
When a system of forced convection produces air velocities below 0.25 m/s natural convection tends to take over.
In storage rooms fans must be in operation when the refrigeration machinery is running, even though during off periods a natural circulation pattern is established; this takes some time to overcome the fan action. Fan control is of the on-off type, activated by a thermostat controlling the room temperature.
In chilling rooms and tunnels air circulation must be permanently maintained as a sufficient supply of cold air to the product surface is necessary to sustain adequate chilling or freezing rates, independently of air or evaporator temperatures.
Whenever the evaporator operates at temperatures below 0°C a layer of frost is deposited on the exchanger surface, increasing its thickness with time. Frost, a solid phase, is the result of moisture condensation and solidification which is deposited by the air circulating in the store space.
Moisture originates from produce water evaporation, from atmospheric moisture load as the outside air has a higher moisture content (closing and/ or sealing devices must be periodically and thoroughly checked), and from personnel working in the cold store, though this load is negligible.
Frosting-up of the evaporator is inevitable despite all the normal precautions (adequate air circulation rate to minimize desiccation of produce; efficient thermal insulation and water vapour barrier; good quality gaskets on doors; short periods of door opening and personnel handling in the store; cooled air locks correctly designed for humid climates).
Frost deposit on the evaporator first reduces the overall heat exchange coefficient because of the thermal resistance of ice and secondly hinders air circulation. Both cause a deterioration in the performance of the evaporator and of the refrigerating installation. The decrease in the heat exchange makes the compressor work for longer periods, and also lowers the refrigerant boiling temperature. Both factors increase energy consumption. Further, the temperature of the frosted surfaces diminishes, increasing the relative humidity in the chamber.
Regular defrosting of the evaporator is necessary because equipment performance deteriorates with increasing thickness of ice, primarily affecting the free passage of air through the evaporator coils. First the method to supply heat to the evaporator to melt the ice has to be decided, then the optimum defrosting frequency must be found.
There are several ways to defrost an evaporator. Occasionally more than one can be used simultaneously.
Hot gas defrosting is used for direct expansion systems. High-pressure hot refrigerant vapour discharged by the compressor is circulated through the evaporator, which acts as a condenser, and the latent heat of the vapour is absorbed by the melting frost. The refrigeration circuit is inverted. The system is complicated but often used because of its efficiency; it is recommended for relatively high-capacity installations. The compressor must be in operation but the fans are stopped.
Defrosting by hot liquid refrigerant is employed for industrial refrigeration installations with several evaporators. The evaporators are defrosted in succession by the hot pressure liquid which is subcooled before entering another evaporator. Only sensible heat is involved in this operation, and liquid temperature cannot be lowered below 0°C. Heat exchange is low, demanding long defrosting periods. Melting ice heat is recovered.
Hot brine defrosting, once employed for secondary circulation systems, is no longer in use.
Air defrosting can be used only in chilled chambers at temperatures above zero, 2°C or higher. The temperature in the room must be allowed to rise to 3–4.5°C but this sometimes represents too high a fluctuation of storage temperature. Fan operation is continued to accelerate heat exchange. The method is simple but seldom used in industrial installations as it presents serious difficulties: products close to the evaporator receive a direct flow of humid air, frequently carrying water droplets; air circulation is maintained for long periods and this leads to mass losses; the efficiency of defrosting is very low.
Water defrosting is achieved by spraying water on to the evaporator by means of a grid of tubes placed above it. Not only melted ice but some flakes may drop on the collecting tray, so care must be taken to ensure the drain is not blocked. Pipes and surfaces should be correctly sloped down, avoiding bending the pipes which could form a siphon. The water feeding pipe must not be in direct contact with the evaporator. This is a simple method, mainly used in industrial installations operating at temperatures close to or slightly below 0°C. However, there are some drawbacks. Water consumption is high (some 8–10 kg of water per kg of ice; warm water from the condenser is sometimes used to reduce this quantity) and defrosting heat is not recovered.
Electric heat defrosting, though included in external heating methods, supplies heat by electric resistances located around the evaporator tubes or even inside them, provided that enough electric insulation and tightness is assured. The simplest method consists in placing electric resistances close to the evaporator tubes.
Electric defrosting has the advantage of being simple and is readily controlled automatically. The installation cost is not high but it is costly in energy. The main inconveniences of the system are that energy is not recovered and that the quality of the electrical insulation of the equipment must be very good. It is suitable for all types of installation but it should be restricted to small-capacity equipment.
Antifreeze liquid defrosting utilizes water solutions of ether or propylene glycol to keep the evaporator surface continuously free of frost. This method has the advantage of maintaining a high and constant overall heat transfer coefficient. As the antifreeze solution absorbs moisture its concentration will gradually decrease; to regenerate it heating is necessary. The heating of the solution means a considerable increase in the heat load. Other inconveniences are the possibility of solution being splashed on produce at the rear (this can be prevented by installing dampers) and pollution from the solution, which demands a periodic change. This method is recommended for low temperature evaporators which must be constantly preserved without frost, such as in continuous freezing equipment.
Defrosting is an expensive operation which uses the energy incorporated in the cold store, and the lower the storage temperature the more the heat load of the store is affected. Nevertheless, as equipment performance deteriorates with increasing ice thickness it is necessary to defrost periodically.
An optimum defrosting frequency must be established. If the frequency is too low the heat transfer coefficient and air circulation deteriorate and equipment efficiency decreases. If the frequency is very high the thermal load increases and the total efficiency of the system is reduced. Optimum frequency must be established by trial. One easy way is to measure the duration of the cut-in of the thermostat; when the evaporator has no frost this time is minimal. Defrosting is necessary when the cut-in time increases about one third, all other conditions being equal. The trial should be conducted when the room is in thermal equilibrium, for instance when the plant is closed.
The periods between defrosting may be relatively long in dry climates but they will be much shorter in equatorial regions. They are also longer in summer than in winter.
Natural convection evaporators need to be defrosted only once a day, starting the defrost cycle usually around midnight and continuing for several hours. Forced convection units with finned coils should be defrosted at least once every three to six hours.
Defrosting can be conducted automatically or manually when the frequency is low. Automatic defrosting is controlled by a clock-timer or a programmer that functions during fixed periods at regular intervals. The clock-timer method is simple and accurate, the only inconvenience being that the frequency has to be modified according to external conditions.
To reduce the heat load during defrosting the cooling unit should be completely isolated from the cold room during the operation (body covered by 4–5 cm insulant; suitable shutters on the air intake and delivery openings). Careful consideration must be given to the location, trapping and heating of drain lines. Pipes and traps should be insulated and lagged with heating lead.
Condensers are basically heat exchangers in which the refrigerant vapour is cooled and liquefied after compression. The evaporator heat load plus the heat of compression are released into the atmosphere via the condenser by means of a fluid (normally water) or by air: condensers can be cooled by water or air.
The choice of the cooling medium is difficult. The availability and nature of the water supply is of paramount importance in this decision, as water is an excellent heat transfer medium with a very high heat capacity.
Condensers can be classified into three main groups: water-cooled, evaporating and air-cooled.
Water-cooled condensers can work on an open-circuit basis. After passing through the condensers the warm water runs to waste. In a closed circuit the warm water is cooled and recycled through the condenser again.
Open-circuit condensers are used when the refrigerating plant is close to a source of abundant and suitable water. Sea water is appropriate as it rarely exceeds 18°C (in some areas it might reach 24°C). However, it carries a high biological load which may obstruct the piping, and special precautions have to be adopted to prevent corrosion of the equipment. Port water supply presents the additional risk of spilled oil pollution.
River water and to some extent lake water undergoes big temperature fluctuations, from freezing point up to 25°C in summer.
The most common type of water condenser used in an open circuit is the shell and tube condenser, but vertical multitubular condensers are recommended for countries where the water is very hard and cleaning is often necessary because they are much easier to descale and dismantling of the condenser is not required, nor is cycling-off of the refrigeration plant. The main drawback is their considerable height, and they are also prone to corrosion as, besides the problem of salinity if sea water is used, the tubes are in contact with the air.
Shell and tube condensers work at an optimum with a water velocity between 1 and 2 m/s, with a recommended maximum of 2.5 m/s, and with a water flow of about 100 litres per hour and kW of refrigeration load. This water flow must be distributed among the tubes, not exceeding the recommended speed, and employing about 22 litres per tube per minute.
Water condensers in an open circuit using water that is not hard and contains no sand, mud or biological load, do not need any servicing. If the water carries sand a large fine-mesh filter must be placed at the water inlet and it should be regularly cleaned. Hard water must be treated with a softener. The question of chemical treatement should be carefully considered, keeping in mind the convenience of adopting a closed-circuit system because the volume of water to be treated becomes important.
As water availability is a common problem, closed-circuit water cooling of the condenser is extensively used. The operation is based on cooling the warm water that leaves the condenser to a temperature low enough for it to be circulated again through the condenser. This cooling is achieved by giving off heat and moisture to the ambient air by a direct transfer from water to air in special devices.
Shell and tube condensers used with cooling towers and evaporative condensers are the two modes of closed-circuit operation. In the cooling tower the warm water is pumped from the condenser to the top of the tower and it cools as it falls or is sprayed down to the water basin. Cooling is achieved almost entirely by partial evaporation of the water (80 percent of cooling is latent heat removal through evaporation and 20 percent is sensible heat transmitted to the air flowing against the current).
The efficiency of cooling towers relies mainly on the wet bulb temperature of the entering air. They are not recommended in warm and humid climates but operate very satisfactorily in hot and arid zones. Cooling tower efficiency can be appraised by a coefficient called “tower approach”, which is the difference between the average temperature of the exit water and the wet bulb temperature of the entering air. Normally the approach ranges from 3° to 6°C. The higher the quantity of water circulated the lower the tower approach, but this quantity is economically limited because pumping power increases with water flow rate; there is an economic flow that balances the power demand of the compressor and the water pump, ranging from 150 to 180 litres per hour and kW of refrigeration load.
Cooling towers allow a considerable saving in water consumption.
Water evaporation raises the salt concentration in the circulating water.
This accumulation is controlled by a purge which may be continuous or intermittent, and can be evaluated as about two to four times the water evaporated. The line of purge must derive from the warm water line leaving the condenser, close to the top of the tower. Purge and evaporated water as well as water dispersed in the circulating air must be recovered by a contribution of fresh water.
Evaporating condensers are a combination of condenser and cooling tower in a single unit. Water is sprayed over the refrigerant coils and evaporates from the spray and the wet surface of the condenser into the air.
The operating principles for the cooling tower apply also to evaporative condensers. They are not recommended for warm zones with a high humidity.
The total water consumption of evaporative condensers, equivalent to the water needed to compensate evaporation, drift and purge, is about 4–5 litres per hour and kW of refrigeration load (the proportion of evaporated water is about 35–50 percent of this quantity). Condensers of this type are considerably more difficult to clean so they should not be used with hard water.
The water-cooled condenser should be sited close to the evaporator; it is not affected by wind or air recirculation, but should be protected from heat from the sun and from freezing damage in winter. Dirt is not important but should be avoided. It must be installed with sufficient space for end cover removal and tube replacement.
Air-cooled condensers normally operate under forced convection, using either axial flow fans, which are noisy, or a centrifugal blower, which is more costly, to move air at high speeds over the finned tube. The usual air speed is between 2.5 and 5 m/s. However, as power consumption increases with the square of air velocity, 3 m/s is considered a reasonable speed.
Air-cooled condensers have some advantages over water-cooled condensers. Water consumption is nil, which is vital where water is in short supply or not available. Their efficiency is indifferent to moisture content in the air, so they are appropriate for humid climates. They are relatively simple to install and require little maintenance because cleaning is simple and rapid.
However, they present some drawbacks, the most relevant being the high condensation temperature, which is usually 15–20°C higher than the entering air temperature. The optimum size of the condenser is computed on a condensation temperature of about 42–45°C; in hot areas these levels will be at ambient temperature so the condensation temperature will rise up to 60–65°C or higher, at least in the hottest periods of the day. As the heat transfer coefficients are low large exchange surfaces are necessary, with obvious consequences in size and cost of equipment. Power consumption is another drawback, though it can be considered comparable to the consumption of cooling towers or evaporative condensers, as they are also equipped with fans and pumps.
Good air circulation is essential for satisfactory operation, therefore confined spaces must be avoided. For this reason the general trend is to mount condensers at a high level, far from the compressor, and in a cool location that is clean, dry and well ventilated, protected from the heat of the sun to prevent air recirculation and sited to take advantage of prevailing winds, particularly during periods of maximum load. They should be close to the evaporator, and whenever possible slightly above rather than below it.
The condensers should be readily accessible for cleaning and maintenance, installed on antivibration mountings to control noise and be safe from damage. Their efficiency depends on the cleanliness of the water or air side of the exchanger surface. Therefore periodical cleaning is necessary. Air condensers are cleaned by washing the tube bank with a hose and stopping the fans no more than once a month for a very dirty atmosphere. Water-cooled condensers are descaled mechanically or by treatment with an acid solution. Frequency depends on the quality of the water closed-circuit systems must be washed with clear water at least twice a year.
Furred condensers raise the condensation pressure, reducing the performance of the installation. If the condenser becomes blocked there is a risk of breaking the refrigerating circuit.
The condenser should be cleaned whenever the difference in the temperatures of the condensing fluid and the water entering increases above the normal value, which is usually between 5° and 10°C. Efficiency also depends on the condition of the exchange surface on the refrigerant side, so precautions should be taken to prevent oil accumulation. Purging noncondensable gases which may accumulate in the circuit will improve the efficiency of the condenser. To ease systematic maintenance it is advisable to install more than one condensing unit or, if a single condenser is used, this should be divided into separate circuits. This will also help in case of breakdown.
Comparing water-and air-cooled condensers, it can be said that they are similar in initial cost and also in operating costs, except when the former work in an open-circuit system supplied with cheap water and have no environmental restrictions.
Condensation pressure is the parameter to control for efficient and safe operation in the high-pressure circuit. This pressure should not be excessively high, first for safety considerations and second because thermodynamically the refrigerating cycle operates less efficiently as the pressure rises, influencing energy consumption and raising operating costs.
The condenser is protected against high-pressure failure by the highpressure pressostat, a device commanding compressor cycling which cycles-off whenever the pressure rises above an established level. High-pressure controlling devices are always desirable but they are indispensable when the condenser is water-cooled. It may also be necessary to regulate the condensation pressure to avoid it becoming too low, as this does not maintain a high enough pressure differential across the refrigerant expansion valve and the evaporator is fed with an insufficient refrigerant flow. To maintain a high condensation pressure, and corresponding high temperature, it is necessary to control the condenser capacity when the ambient temperature is low. This control is achieved either by reducing the flow of air or water circulated through the condenser or by diminishing the effective heat exchange surface area or condensing area.
The condensing surface is modulated by retaining liquid refrigerant in the lower part of the condenser, using a valve known as a pressure regulator.
The water flow in a water-cooled condenser is controlled by the water pressostatic valve, activated by the condenser pressure. When the pressure is low the valve will close in relation to the decrease, reducing the flow of water circulating through the condenser. This valve may be two-way or three-way. The latter is employed to bypass the condenser in water recirculating systems. It should be located in the water inlet pipe.
For air-cooled condensers the air flow control system may cycle-off some of the fans, when there are more than one, or the air flow can be reduced with pressostatic shutters activated in the same way as the water pressostatic valve.
Condensation pressure is again controlled by regulating the air flow over the condenser, using the same methods as for the air-cooled condensers. It is possible to cut water circulation and so reduce the heat exchange and consequently raise the condensation pressure, but it is not recommended because of surface scaling, unless it is done for a long period in cold weather before condenser cleaning.
Evaporator is the name given to any heat exchanger where the refrigerant is evaporated at low temperature and therefore at low pressure, but usually above atmospheric pressure to prevent gas and/or water vapour leakages into the low-pressure circuit. The evaporator is the element of the refrigerating circuit through which heat is absorbed from the environment that is being cooled. It can absorb the heat necessary for refrigerant vaporization either from the air (direct expansion systems), or from a liquid (usually water), or from a solution, which in the case of the meat industry is used as a secondary refrigerant, being circulated through the air cooler inside the refrigerated room.
Only the first mode of operation, direct air cooling through liquid refrigerant vaporization, will be described, as secondary refrigerant plants are installed when it is necessary to accumulate reserves of cold, as in multipurpose refrigerating installations. Direct expansion involves less initial investment and power consumption is generally lower, as temperature differences between primary refrigerant and air are less than in secondary fluid systems.
Air-cooling evaporators can operate under natural convection, where air movement is governed by differences in air density, or under forced or mechanical convection when fans or blowers are employed to expedite air movement over the cooler and to facilitate air distribution inside the cold room.
Natural convection systems have two main advantages: no energy is needed for air circulation and the desiccation of the produce is much less because air velocities are much lower and relative humidity is high. There are obviously some drawbacks, first, in their defrosting and, second, and more important, in their low overall heat exchange coefficient owing to low air velocities over the evaporator that lead to large exchange surface areas, making them bulky and very expensive. Their cost, including installation in the cold chamber, is from three to four times that of a forced convection evaporator because the exchange surface is larger, the tube manufacturing cost is high and erection in the chamber is difficult and time-consuming.
Natural convection grids can be uniformly distributed under the ceiling, leaving enough space for good air circulation (never less than 8 cm), and also along the chamber walls if there is not enough head space. Factory-built coil-and-baffle assemblies facilitate installation and can be used for almost any natural convection application.
Forced convection or forced draught evaporators have been developed because the higher overall heat transfer coefficient achieved by rapid air circulation over the coils permits a drastic reduction in evaporator surface areas. Further, they have great refrigerating capacity.
Air velocities through the evaporator must be in the range of 1.5 to 3 m/s to achieve a high heat-transfer coefficient, but it should be borne in mind that above the upper limit (3 m/s) the heat created by the electric fan motor will surpass the increase in cooling capacity. Also, velocities above 3 m/s will tend to carry moisture deposited on the coils toward the produce. The throw draught must be sufficiently far-reaching to achieve uniform air circulation and an even temperature distribution in the cold store.
Evaporator coils are finned tubes built to produce a large heat exchange area in a compact element. Good thermal contact between the tube and the fins must be assured (soldering, biting into the tube surface by tube expansion, securing by straightening fin flares). Fin size and spacing depend on the operation and fundamentally on the operating temperature.
Fin height should usually be equal to the diameter of the tube and the fin should be thick enough to provide mechanical resistance (0.7-1 mm). Fin spacing is between 6 and 10 mm or more for chilling rooms or 15 mm or more for freezer rooms, to minimize the risk of ice block. Sometimes fins are spaced wider at the coil air inlet to act as a frost catcher to avoid obstruction of the air flow.
Air conditioner/diffusers or unit coolers are a single unit consisting of finned coils, one or more fans and defrosting devices encased in metal housing. They reduce the installation cost and also obviate the need for air distribution ducts as they are mounted inside the cold room. This is also their major disadvantage because working conditions for repairs are very uncomfortable, particularly in large freezing rooms. They are ready for fitting with extra components such as two-speed fans, divisible exchanger batteries, electric heating resistances, etc.
Forced convection evaporators should be placed for easy access to the units themselves and also to ancillaries like the expansion valve, defrosting equipment and particularly the fan motor. The position is decided by the capacity of the cooler. High-capacity coolers are installed on the floor along one wall; the air is distributed through several nozzles pointing in various directions.
Medium-capacity units are suspended from the ceiling, distributed along the central axis of the room or along the walls. They can also be wallmounted. The flow and return of air must not be obstructed and they should be clear of the storage area. In large cold rooms several units may be mounted, one for each 500 m3 of room volume. They should be supported in such a way that noise and vibration from the fans is not transmitted and excessive heat is not conducted from outside through the supports. Corrosion of the supports from condensation should be minimized.
Evaporators fall into two categories, depending on the way they are fed with liquid refrigerant: dry or direct expansion and flooded.
In dry expansion operation the evaporator receives the fluid from thermostatic expansion as a mixture of liquid and a small proportion of vapour. The refrigerant leaves the evaporator totally evaporated and slightly superheated, about 2–4 °C above the evaporation temperature; this avoids any risk of liquid reaching the evaporator. Liquid refrigerant expansion takes place in the thermostatic expansion valve, a fluid-metering device fitted with a thermometric sensor that is able to maintain the superheating of a moderate level of refrigerant vapour.
The thermostatic expansion valve is the most commonly used device but there are others: capillary tubes for very small equipment, automatic expansors, and hand-operated valves or calibrated orifices for industrial plants operating in stable conditions. When thermostatic expansion valves are used in commercial and industrial equipment they are externally equilibrated.
The thermostatic expansion valve offers several advantages: it is simple to operate, it provides relatively high efficiency of the evaporator, the risk of liquid reaching the compressor is reduced (but it should be protected by an antislugging device such as a solenoid valve situated upstream of the expansion valve), it can be used with all kinds of refrigerant fluids, and also with all types of machines.
Its major drawbacks are that evaporation temperature is not constant but decreases with the temperature of the refrigerated space during the operation of the plant and this decreases relative humidity, and there is some risk of throbbing or faulty feeding of the evaporator, which is affected by the condensation pressure. The temperature difference between the air and the boiling refrigerant is usually kept in the range of 5° to 7°C, which should be used to calculate the correct surface area of the evaporator.
Flooded evaporators operate by a continuous pure liquid flow circulating through the evaporator, fed by gravity or by pump. The fluid leaves the evaporator as a mixture of liquid and vapour as only a part of the refrigerant volume delivered to it is evaporated. The mixture is directed to a liquid separator or surge drum from where the unevaporated liquid is fed back to the evaporator inlet. The high-pressure refrigerant is released into the liquid separator at low pressure in such a way that the level is kept fairly constant. The expansion of the fluid can be controlled by a low-pressure float-valve, a hand-operated valve, a constant flow rate or thermostatically, and today by an electronic expansion valve allowing easy adjustment of the liquid level.
One of the main advantages of the flooded system is a great heat-transfer coefficient because boiling liquid completely wets the internal surface of the evaporator; the lower the vapour content at the exit of the evaporator the greater this coefficient, so it is advisable to employ pump circulation to increase the liquid: vapour ratio.
The temperature difference between air and boiling refrigerant is kept in the range of 4° to 5°C, which is possible because the high heat-transfer coefficients and flooding operation assure high efficiency over the entire evaporator surface.
Flooded systems are usually employed in high-capacity ammonia plants and where a changeable thermal system is likely (e.g. refrigerated plants in a slaughterhouse). They cannot be used with halocarbon refrigerants because of oil problems, unless a rectifier is placed in the low-pressure circuit to ensure the return of oil to the compressor. When flooded evaporators are fed by gravity this can be achieved from a single liquid separator or accumulator that serves all the evaporators, which are run in parallel. The separator should be of the right size to avoid liquid carry-over and a vapour velocity under 0.4 m/s should be maintained.
Another gravity design has a liquid separator for every evaporator, so the number of expansion valves is equal to the number of separators. This system is used when a single separator is not enough for correct feeding to all the evaporators, as when pressure losses are excessive, and in large cold chambers and freezing and chilling tunnels. In all cases the pipes should be of adequate diameter to prevent undue pressure loss and counterslope. Pipes to and from the evaporator and separators must be thermally insulated.
It is necessary to isolate each evaporator (or evaporator-separator unit) with an automatic valve (solenoid or pilot-operated valves, though the latter are rarer) placed upstream or downstream to prevent refrigerant vaporization in cold rooms where the storage conditions are already established. This is particularly dangerous in rooms where storage temperature is kept around 0°C. The valves should be under evaporator thermostat control.
Flooded evaporators can be fed using a mechanical pump to circulate liquid refrigerant in the evaporator-separator circuit at a rate high enough to return the fluid to the separator in a wet vapour state. The flow rate recommended to achieve a high overall heat-transfer coefficient is in the range of three to eight times the rate of refrigerant vaporization, dependent on the heat load of the chamber.
The low-pressure liquid separator or surge drum is placed in the machine room and the pressure losses in the refrigerant circuit are counteracted by the pump, not by the compressor, which is useful when operating at low temperatures. All the evaporators in a single circuit should be fed in parallel using at least two pumps, one operational and one standby, isolating each evaporator with a solenoid valve under the control of the room thermostat. When these valves are closed the liquid refrigerant must be recirculated to the liquid separator.
The main disadvantages of flooded evaporators are that they are usually bulky, and they require a relatively high refrigerant charge.
Some care should be taken when designing a low-pressure circuit. The suction pressure should be increased as the evaporator temperature must be kept as high as possible. This is achieved by choosing the correct size for the evaporator transfer area, limiting pressure losses between evaporator and compressor by using the right pipe diameter and short pipes between them, and lastly, avoiding too frequent or too little defrosting; too little producing too thick a frost deposit.
When suction pressure is unduly low it may be because of poor feeding of the evaporator or that it is not fed at all. There are several reasons for both irregular situations: the expansion valve may be obstructed, the solenoid valve may not be in operation (closed when out of order), too high a compressor capacity for the evaporator capacity (the capacity control of the compressor is not in operation or there is too much frost on the evaporator), the constant pressure valve placed downstream to the evaporator may not be correctly adjusted, or the lower pressure limit for valve closing may be too high so that suction pressure is excessively reduced.
When the evaporators in the same circuit are operating at different temperatures several compressors or a single compressor provided with capacity control must be installed for their correct performance. The evaporators operating at higher temperatures must be equipped with constant pressure valves located downstream. These can be replaced by two temperature thermostatic valves providing the same control.
To prevent a delay in compressor cycling-in in relation to fluid refrigerant feeding of the evaporator it is necessary to set up an intake valve before the evaporator, served by the compressor operation. This valve obviates any risk of liquid dragging to the compressor.
The compressor is the active element of the refrigerating circuit. It has two functions: first, to reduce the pressure in the evaporator until the liquid refrigerant evaporates to the current temperature and maintain the pressure by drawing off the vapour produced through evaporation of the liquid refrigerant, and second, to compress the vapour by raising its temperature and pressure to the point at which the vapour can be condensed at the normal temperature of the condensing media.
Compressors can be reciprocating piston, rotary, screw or centrifugal. The first three are positive displacement machines and the last is a cinematic compressor. Reciprocating compressors are the most commonly used. Modern units are multicylinder, usually provided with two to 16 cylinders. This design minimizes their poor adaptation to large volume rates, which is one of their disadvantages. Moreover, they produce high compression ratios.
Rotary compressors of the sliding vane type are rarely used. They are sometimes employed as boosters. Screw and centrifugal compressors are installed in very large stores and when refrigerating capacities of over 1 000 kW are required. As reciprocating piston compressors are by far the most extensively used the following section deals mainly with this type.
Compressors are operated by an AC electric motor or a petrol or diesel engine, though the latter has no advantage except where there is no electric energy. When an electric motor is employed in an area with frequent power cuts, provision should be made for an electric generator capable of providing an intensity three to four times the running current.
The assembly compressor and drive motor are known as the compressor unit. There are three main types, depending on the way the unit is built.
Open-type compressors can be used in all commercial refrigeration plants except small ones. They are particularly useful when ammonia is the refrigerant fluid. Semi-hermetic compressors are recommended for medium-capacity installations and should operate with halocarbon refrigerants. Hermetic compressor systems should be used for very small refrigerating capacities. Open-type compressor units have several advantages over the other two types, making their installation advisable particularly in large refrigeration plants where skilled labour and maintenance personnel are employed. First, the electric motor is not in contact with the refrigerant vapour so the heat dissipated neither increases its temperature nor is it discharged by the compressor. Second, they are driven by standard electric motors which are easily replaced, and even the winding can be easily repaired. Third, problems with the electric motor do not contaminate or affect the refrigerating circuit. Lastly, they are less sensitive to system contamination so the erection of the refrigerating circuit does not demand extreme care and skilled operators.
To estimate the correct size of the compressors or compressor the size of the evaporators and condensers should be previously evaluated. No general rule for dividing the refrigerating compressor capacity can be established. However it is obvious that cost criteria should be followed. To help in deciding on the process some general factors must be borne in mind: compressors should be interchangeable and preferably of a similar type so the stock of spare parts can be reduced; the refrigerating plant should be provided with standby machines which are not used for any other purpose than replacing machines that are out of order; the number of compressors installed should be as small as possible but never less than three; one or two compressors should be equipped with capacity regulators (see later) and, whenever possible, with automatic regulation. In spite of all these requirements costs should be kept as low as possible.
Compressors, controls, condensers and their ancillaries are usually grouped in the machine room. The location should be carefully decided on to minimize pipe runs to all sections of the cold store and so as not to impede any future extension of the refrigerating plant and cold rooms. Care must be taken not to exceed the floor load limit. The position of the compressor in the machine room should allow easy access for maintenance, repairs and possible replacement. The environment should be cool and dry in any weather conditions and be well ventilated. The machines must remain clean and be safe from damage. Compressors must be mounted on cork mats or have antivibration mounting to avoid any transmission of noise. Thus, flexible connections must be provided on all refrigerant, oil and water pipes connecting the compressor.
To prevent high discharge temperatures and pressures (the higher the temperature the faster the rate of lubricating oil carbonization and the formation of acids), machines with water-cooled cylinder heads or cylinder water-jacketing should be installed, particularly in warm countries and/or when compressors operate with refrigerant fluids such as ammonia that have unusually high discharge temperatures. Lower rotational speeds should be used to keep down the discharge temperature and to increase the reliability of the machine. The current trend is to install compressors running at 950 rpm rather than 1 450 rpm. This leads to larger and costlier compressors, though maintenance and operating costs are reduced.
Reciprocating piston compressors have free suction and delivery valves that operate by pressure differences, and they must be protected against liquid slugging. The compressors also have service valves fitted to both the suction and delivery connection. These are used to isolate the compressor from the remainder of the circuit for repair and maintenance without losing the refrigerant. The delivery service valve must not be closed while the compressor is still running, otherwise some damage may occur. These valves provide a connection to the gauges that measure the operating pressures (suction and delivery), as well as a charging connection that adds refrigerant to the suction side and a purge connection on the delivery side to remove non-condensable gases and excess refrigerant.
When evaluating the total heat load of a refrigerating plant the operating conditions should be such that they lead to peak heat loads, i.e. the capacity during the cooling-down period should be two to four times as high as the capacity during storage under ordinary conditions. This means that if compressor capacity is higher than the heat load on the evaporator the system is not in equilibrium and will not perform satisfactorily.
To adjust the functioning of the compressor to the actual heat load it has to operate unloaded or under a reduced capacity; this operating mode reduces compressor efficiency. Some recommendations should be followed to avoid this situation. First, it is preferable to use small compressors at full capacity rather than a large one at partial capacity. Second, especially for centralized installations, compressors must be of a satisfactory size and work at full capacity for the expected heat loads.
In spite of previous planning and precautions it may be necessary to regulate the refrigerating capacity more accurately, which implies regulating the compressor capacity. The compressor capacity must be able to cope with peak heat load and adapt itself to any operating condition. There are several ways to regulate compressor capacity. The simplest is to start and stop the compressor by a control element, usually a thermostat or pressostats. This operation is conducted according to a pre-established programme and is of the on-off type. Other ways of regulating capacity control are:
Both ways of controlling compressor capacity are automatically governed by the action of a fluid under pressure, either an oil-or high-pressure refrigerant. When the suction pressure drops below a certain predetermined value, a solenoid valve activated by pressure control opens and allows discharge from one or more cylinders (bypass control) or admits highpressure vapour from the condenser by depressing the suction valves and holding them open. When the suction pressure rises the solenoid valve is deenergized and the compressor returns to full capacity operation.
In large plants with several multicylinder compressors, capacity can be controlled pneumatically. The system is simple and convenient, particularly if the compressor is designed to be operated by a capacity regulator. Efficiency diminishes very little and there is not much heating of the refrigerant fluid. The major drawback is the reduction of the power coefficient of the electric motor.
Another method also used is to unload compressor cylinders during compressor start-up by putting cylinders out of operation, thus reducing inrush current demand.
When the discharge vapours are bypassed into the suction line the proportion of recirculated flow is generally kept under 40 percent of the total discharge flow. The recirculated flow can be introduced anywhere into the low-pressure circuit, i.e. from the evaporator itself to the suction line just before the compressor.
Another way of controlling a compressor's capacity is by modifying its clearance volume. The volumetric efficiency of the compressor is inversely related to the clearance volume, so by increasing this volume the capacity is reduced. This control is generally achieved manually and is usually employed only for screw compressors.
Screw compressor capacity is also controlled by modifying the suction volume through the action of an oil-pressure circuit. This control is continuous from 100 to 10 percent, and the power requirements are proportional to the refrigeration capacity necessary in each situation. As well, there is no heating of the refrigerant fluid.
When refrigerating plants are operated at low temperature it is advisable and more economical to use a two-stage compression system, especially in high-capacity plants such as industrial freezers or large frozen storage chambers, despite the complexity in their installation and operation, as energy consumption is much less.
The fluid used to cool the thermal engines (petrol or diesel) driving the compressors and the heat recovered from their exhaust gases can be employed for different heat requirements in the refrigerating plant, for instance for sanitary water. Heat recovery should be carefully considered when designing and operating a slaughterhouse.