Psychrometry

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The earth's atmosphere is a mixture of gases and water vapour. An understanding of physical and thermodynamic properties of air-water vapour mixtures (psychrometrics) is fundamental to the design of environmental control systems for plants, crops, animals or humans.

Properties of Moist Air

Pressure, volume, density and thermal properties are related by the use of the laws for a 'perfect gas'. For a mixture of dry air and water vapour this law can be used with only negligible error at the range of temperatures and pressures used for environmental control.

P = MRT/V where:

P = absolute pressure, Pa
M = mass, kg
R = gas constant, J/( kg.C)
T = temperature, K
V = volume, m

Dalton's Law each component in a mixture of gases exerts its own partial pressure, for a mixture of air (a) and water vapour (w).

P = Pa + Pa = (Ma x Ra x Ta) / Va + (Mw x Rw x Tw) / Vw

Assuming a uniform mixture:

P = T / V (MaRa + MwRw)

When the volume and temperature of the mixture are equal the following is true:

PW / Pa = MwRw / MaRa

Thus, if the total pressure and water vapour weight is known the partial pressures may be calculated.

Specific humidity (H) is the weight of water vapour in kg/ kg of dry air. It is sometimes called absolute humidity or humidity ratio. The base of one kilogram of dry air is constant for any change of condition, making calculations easier.

H = Mw / Ma = PwV / RwT = PaV / RaT = PWRa / PaRw = PWRa / (P-Pw)Rw

Relative humidity (RH) is the ratio of the actual water vapour pressure (Pw) to the vapour pressure of saturated air at the same temperature (Pwsat).

RH% = 100 Pw/Pwsat

The vapour pressure at saturation (Pwsat) is given in steam tables for different dry-bulb temperatures.

Specific volume is the volume of dry air per mass of dry air

Humid volume is the volume of an air-moisture mixture per mass of dry air. In ventilation calculations, the volume is in cubic metres of mixture (air + water vapour) per kg of dry air. The base of one kg of dry air is used because the kg of dry air entering and leaving the system in a given time will be constant once a steady-state flow is established. Humid volume increases as the temperature or water vapour content increases. The humid volume of air-water vapour mixtures is given in standard thermodynamic tables or may be read from a pschrometric chart.

Temperatures - air-water vapour mixtures can be described by the dry-bulb and either the wet bulb or dewpoint temperatures:

Enthalpy (h) is the heat energy content of an air-water vapour mixture. The energy is a combination of both sensible heat (indicated by dry-bulb temperature) and latent heat of vaporization (energy content of the water vapour). Enthalpy scales appear on pschrometric charts expressed as kJ/ kg of dry air.

Enthalpy can be calculated from the equation:

h=S x tdb+ H x hw where:

S = Specific heat of dry air, 1004 kJ/( kg.K)

tdb = dry bulb temperature

H = Specific humidity

hw = enthalpy of water vapour, kJ/ kg water vapour

Thus:

h = 1.004 x tdb + H(2454 + 1858 x tdb) kJ/ kg where:

2454 = latent heat of vaporization, kJ/ kg

1858 = Specific heat of water vapour, kJ/( kg.K)

Psychrometric Chart

A psychrometric chart (Figure 7.2 and appendix V:4-6) is a graphical representation of the thermodynamic properties of moist air. It is useful for solving engineering design problems. Charts for agricultural applications are usually corrected to standard atmospheric pressure of 101.325 kPa. However charts for other elevations are available. The following properties are shown on a psychrometric chart:

The intersection of any two property lines establishes a given state, and all other properties can be read from that point. The changes that take place between any two points are of particular use. The vertical lines show dry-bulb temperatures; the curved lines, relative humidity; the slant lines, wet bulb temperatures and enthalpy; the horizontal lines, dew point temperatures and specific humidity, and the steep slant lines, specific and humid volume.

The wet- and dry-bulb temperature for a building area may be read from a psychrometer and then used to establish a point of intersection on the chart. Psychrometers consist of two thermometers mounted close together, one of which has a wick on the bulb that is wet with a few drops of distilled water. Air movement is necessary. A sling psychrometer, which is actually swung in the air, is the simplest and least expensive. However, for locations with restricted space a motorized psychrometer must be used. The air movement in a ventilation duct is adequate to provide accurate readings from stationary temperature sensors.

Air-water Vapour Mixture Processes

Conditioning of air-water vapour mixtures involves heating, cooling, humidifying dehumidifying or some combination of these factors.

Sensible heat is the heat added to air without changing its specific humidity. Applications of sensible heating include heated-air grain drying and winter heating of room air in cool-climate homes.

Sensible cooling is the removal of heat at a constant specific humidity. An example is the air passing over a cooling coil having a surface temperature above the dew point of the air. The final temperature cannot be below the initial dew-point temperature or water vapour condenses and the process removes latent heat.

Sensible Heating

Sensible Cooling

Lines a and b are starting and ending dry-bulb air temperature lines in both processes. Lines c and d show starting and ending enthalpy values. The fact that line 1-2 is horizontal indicates that there was no change in specific humidity in either process. Lines e and f show that the relative humidity dropped in the heating process and rose in the cooling process.

Evaporative cooling is an adiabatic saturation process (no sensible heat gained or lost) and follows upward along a constant wet-bulb temperature line on the chart. Air to be cooled is brought into contact with water at a temperature equal to the wet-bulb temperature of the air. The sensible heat of the initial air evaporates the water, lowering the air's dry-bulb temperature. Sensible heat is converted to latent heat in the added vapour, so the process is adiabatic. Evaporative cooling is effective in hot dry climates where wet-bulb depression (the difference between dry-bulb and wet-bulb temperatures) is large and where the disadvantage of increased humidity is more than offset by a relatively large temperature drop.

Evaporative Cooling Process

Figure 7.2 Psychrometric Chart (By Courtesy of Carrier Corporation).

Evaporating moisture from a to b cools the air from c to d. As 1 and 2 are on the same enthalpy line, the process is adiabatic (no change in heat) the relative humidity rises from 1 to 2.

Heating and Humidifying Process

Point 2 has a higher temperature and specific humidity than 1. The heat added from a to b shows as sensible heat that caused a temperature rise from c to d end latent heat in the moisture that evaporated from e to f The relative humidity may or may not change.

Hearing and humidifying of ventilation air occurs as it moves through livestock buildings. Animals and poultry produce heat, vapour and water; both sensible heat and water vapour are added to ventilating air.

Cooling and dehumidifying is the lowering of both the dry-bulb temperature and the specific humidity. The process path depends on the type of equipment used. In summer air conditioning, air passes over a cold, finned type evaporator coil of a refrigeration unit. The air is cooled below the dew-point temperature and moisture condenses. Unless reheated or initially saturated, the final relative humidity of the moist air is always higher than at the start. Both sensible heat and latent heat are removed from the air in this process.

As airpasses the cooling coils of an evaporator, moisture will condense from a to b giving up latent heat. The air is also cooled from c to d giving up sensible heat. Relative humidity 2 will be at 100% (saturation) as the air leaves the evaporator.

Moisture transmission

As stated in Dalton's law, water vapour in the air exerts a separate pressure that is proportional to the amount of moisture present. This partial pressure is independent of the partial pressures exerted by other components of the air.

In as much as warm air can hold more moisture than cool air, it is typical for the vapour pressure to be higher on the warm side of a wall. Where ever a pressure difference exists, there is a tendency for moisture to permeate through the wall until the pressure equalizes. If in permeating through a wall a dew-point temperature is encountered, condensation will occur and free moisture will be left to reduce the effectiveness of insulation or cause deterioration in wood or metal. In cold climates, building walls should be designed with vapour barriers on the warm side of the wall in order to reduce moisture permeation. In all climates, but especially in warm, humid areas, it is essential to install a good vapour barrier on the warm side of a refrigerated storage wall.

To understand air-moisture movement and to make the calculations in a vapour-transmission problem, it is necessary to understand the following terms:

Vapour pressure is the partial pressure in the atmosphere due to the presence of vapourized moisture. It is measured in mmHg or Pa.

Permeability is the property of a material that allows the migration of water vapour. It is measured for 1 meter of thickness and the units are g/(24hr.m.Pa).

Permeance is the term chosen for the transfer of water vapour for a material in the thickness as used. The unit used is g/(24hr.m.Pa).

The permeability of a material may be determined by subjecting it to 100% relative humidity on one side and 50% on the other (wet-cup method) or to 0% relative humidity on one side and 50% on the other (dry-cup method). Of the two, the wet cup value is usually a little higher, but either value may be used for moisture-transfer calculations.

Moisture transmission may be calculated as follows:

W = M x A xT x Ap where:

W = total moisture (grams)
M = permeance (g/24 hr.m.Pa)
A = area unit (m)
T = time unit (24 hr)
AP = pressure difference (Pa)

As with heat transfer, only resistance may be added. Therefore if a wall has more than one vapour-resisting layer, the following equation is used:

1 / MT = 1 / M1 + 1 / M + 1 / Mn Where:

MT = the overall permeance of the wall.
M1 = permeance of a layer, etc.

Table 7.4 lists the permeability of several materials used in building construction.

Table 7.4 Moisture Permeability of Materials

Material Premeability/m thickness g/(24hr.m.Pa) x 10-3 Permeance Thickness as used g/(24hr.m.Pa) x 10-3
Air 15.3  
Exterior plywood 6mm   3.45
Pine timber 0.053 - 0.68  
Concrete 0.38  
Asphalt roofing   0.23
Aluminium paint   1.5 - 2.48
Latex paint   27.23
Polystyrene
Extruded 0. 15  
Bead 0.26 - 0.75  
Polyurethane 0.53 - 0.23  
Polyethene 0.1 mm   0.4
Polyethene 0.2mm   0.2

Vapor barriers

Any enclosed wall that has an appreciable temperature difference or humidity difference between the two sides for a substantial part of the time should have a vapour barrier installed on or near the warm or humid side. In cold climates this applies to the wall in any enclosed building that is heated or where the humidity is high. In warm climates it applies to air-conditioned or refrigerated buildings primarily.

Probably the most effective vapour barrier that is also reasonable in cost is polyethene sheet. The vapour barrier should be as continuous as possible. This can be achieved by using large sheets with well overlapped and sealed joints and as few nail holes as possible.

Condensation on Surfaces and Within Walls

If the insulation in the wall of a refrigerated storage is inadequate or if it has defective spots, the outside of the wall may be cool enough to be below the dew-point temperature. The result will be condensation on the outer wall surface. Remedies for this condition are:

Materials such as stone, concrete and brick are not affected by condensation.

Condensation within the wall is more serious and results from either the absence of a vapour barrier or a defective barrier. In that situation moisture moves into the wall from the warm side until it reaches an inner wall layer that is below the dew-point temperature. The resulting condensation soon reduces the effectiveness of the insulation and causes permanent damage. Remedies for this situation are:

Ventilation

Ventilation is one of several methods used to control the environment in farm buildings where it fulfills two main functions: the control of temperature and the control of moisture within a building. Ventilation may also be necessary to maintain adequate levels of oxygen and to remove generated gases, dust and odours.

There is a considerable range of ventilation requirements that depend on the local climatic conditions and the specific enterprise being served. The following examples will illustrate:

A great deal of research has been done to determine the ideal environmental conditions for various classes of livestock and types of plant and animal products. Within economic constraints, the nearer these ideal conditions can be maintained, the more successful the enterprise will be. That is, meat animals will gain faster and more efficiently, dairy cattle will produce more milk, and crop storages will maintain better quality and reduce losses.

Natural Ventilation

Thermal Convection or Stack Effect

Natural ventilation is provided from two sources - thermal convection and wind. Air which is heated with respect to the surrounding air is less dense and experiences - an upthrust due to thermal bouyancy.

Whenever a building contains livestock, the production of sensible metabolic energy is always available to warm the air entering from the outside. Similarly air may be heated in a greenhouse by incoming radiation. Provided there are two apertures with a height differential, convection currents will force the heated, less dense air out of the upper aperture to be replaced by an equal volume of cooler, denser air from outside. This is referred to as "Stack effect"

Hence natural ventilation by stack effect can provide the minimum ventilation requirement under winter conditions. While this system may be less expensive than a mechanical system, it will also be less positive in action and more difficult to control.

A building that is open on one side may be ventilated naturally by leaving the ridge open for an outlet and a slot along the rear for an inlet. An enclosed building may be more positively ventilated with stack outlets and correctly sized inlets.

To determine the inlet and outlet areas required to provide a given ventilation rate by thermal convection, the following equation based on stack effect theory can be used:

where:

Aj = inlet (m)
Ao = outlet area (m)
g = acceleration due to gravity (9.76 m/s2)
h = height difference, inlet to outlet (m)
Hp = heat supplied to building (W)
Tp = absolute temperature in building ( K = 273 C)
r = density of air in building ( kg/m), 1. 175 at 25C
S = specific heat of air (1005 J/ kgC)
V = ventilation rate (m/s)
W = heat loss through building shell (W/C)

The values in Figure 7.3a and b were developed using this equation. The values in (a) are for a solar-flue drier, while those in (b) more closely fit the conditions in a building.

Natural ventilating systems may be non-adjustable, manually adjustable, or automatically controlled. In as much as natural systems are likely to be chosen for economy reasons where conditions are not severe, manual adjustment should be the method of choice in most cases.

Ventilation Due to Wind

As the wind flows around a building, gusts and lulls create regions in which the static pressure is above or below the atmospheric pressure in the free air stream. In general, these pressures are positive on the windward side, resulting in an inflow of air, and negative on the leeward side, resulting in an outflow of air. Pressures are generally negative over low-pitched roofs.

Mechanical Ventilation

Compared to natural ventilation, mechanical ventilation with the use of fans is more positive in its action, less affected by wind, and more easily controlled. Initial installation will usually cost more and there is the added cost of operation. However, in many cases the advantages of mechanical ventilation outweigh the added expense.

Exhaust vs Pressure Systems

There are two main types of mechanical ventilating systems, namely, pressure and exhaust. In a pressure system the fan blows air through inlet openings into the building creating a positive indoor pressure that pushes air out of the building through the outlet openings. In exhaust ventilation the fan expels air from the building creating a lower than atmospheric pressure inside the building. The pressure difference between outside and inside causes ventilation air to flow in through the inlets. For good air flow control is important that the building is tight.

The exhaust ventilation system is popular because it is easier to control the distribution of the incoming air and is generally less expensive and complex than a pressure systems. However, there are situations when the pressure system (one that forces air into the building) performs better. These includes:

Under some circumstances pressure systems may cause humid air to be forced into building walls and ceilings. This can result in condensation and damage to wood and other materials.

A mechanical ventilation system is made up of three main components: fans, air-distribution system and controls to regulate fans.

Fans and Blowers

Axial-flow fans are normally divided into propeller and tubeaxial types. They move air parallel to the shaft and are the types most widely used. Centrifugal (radial flow) fans (blowers) discharge air at right angles to the shaft and often operate at substantial pressures.

Propeller fans are the least expensive and the easiest to install. A propeller fan may have 2 to 6 or more blades. Generally the more blades, the greater the pressure the fan will develop. The best propeller fans have a close-fitting curved inlet shroud or inlet ring which improves the efficiency of the fan. Propeller fans are most suited to moving large volumes of air at pressures in the range of 30 to 50Pa (3 to 5mm of water) and they are the most commonly used in conventional farm building ventilation. Figure 7.5.

The tubeaxialfan is a more reafined version of the propeller fan (Figure 7.6). It has aerofoil-shaped fan blades on an impeller with a large hub all mounted in a close-fitting tube. Tubeaxial fans are capable of operating against higher static pressures than ordinary propeller fans and are made for ducted installations with high resistance to air flow. If it is necessary for a tubeaxial fan to operate under very considerable pressure, it may be designed with two impellers in tandem, described as a multi-stage model.

Centrifugal (radial-flow)fans are used for ducted installations or where air must be moved through a product such as grain or potatoes. The blades on the blower may be radial, e.g., straight from the shaft, curved forward in the direction of rotation, or curved backward opposite to the direction of rotation. The latter can achieve the highest efficiencies under high-pressure performance and are most suitable for agricultural applications. The most important attribute of the backward-curve blower is its nonoverloading characteristic. Both the radial and forwardcurved types require their greatest power input when air flow is cut off. An air blockage therefore, is likely to overload the motor and cause damage. Figure 7.7.

All but the smallest-sized fan should be powered by a capacitor-start motor that is enclosed for dust and moisture protection. It should be equipped with an overload protector and bearings with long lubrication life.

The fan should be enclosed with a wire safety guard. Shutters and hoods are necessary in cold climates but should not be needed in mild climates.

The type of fan selected is largely related to operating pressure. It is important to choose a fan with a high performance efficiency in the range of operating pressures in order to avoid unnecessarily high energy consumption.

Figure 7.3a Natural ventilation stack design (drier).

Figure 7.3b Natural ventilation stack design (barn).

Figure 7.4 A solar food dehydrator.

Figure 7.5 Propeller fan.

Figure 7.6 Tubeaxial-flow fan.

Figure 7.7 Centrifugal blower.

Figure 7.8 Simple Instruments to measure pressure and air velocity.

Static Pressure

When an exhaust fan is installed in the wall of a closed building, a lowered air pressure will develop inside, or if the fan blows air into the building, a slight pressure increase will occur. Manometers or draught gauges are two simple but dependable devices which can be used to measure the small pressure differences that exist. Figure 7.8. They are usually calibrated to read in millimeters of water. That is, if the two columns of water in a glass "U" tube are equal, and then a plastic tube is connected from one side of the U tube to a building with an operating fan, the columns will become unbalanced. The difference is the millimeters of static pressure.

Fan Ratings and Selection

A fan performance is usually related in terms of volume of air moved expressed in cubic metres per second (m/s) against a pressure or resistance to air flow expressed in Pa or mm of water static pressure (mmWG). Free-air delivery is nearly meaningless since that situation seldom exists. Performance curves, available from the manufacturer, outline the performance of fans at different operating pressures. These curves also illustrate the maximum or cut-off pressure, efficiency and sound levels at different rotation velocities (rpm) and blade angle settings, as well as the power requirements for various operating conditions. Most countries that manufacture fans have an organization that tests fans and certifies the performance curves.

Fan Laws

When fan blades are mounted directly on the motor shaft, it is assumed that the manufacturer has correctly matched the combination. However, some fans are belt-driven, allowing for the substitution in service of a motor of a different speed or pulleys of different sizes. A knowledge of the following basic fan laws can avoid trouble:

For example, assume a fan is belt-driven by a 300W output 1725rpm motor. If that motor is replaced by a 300W/ 3400rpm motor without changing pulleys, the following would occur: The volume discharged would be doubled, the cut-off pressure would be quadrupled (22) and the horsepower requirement would be increased eightfold (23). The result would be such a badly overloaded motor that it would burn out unless the overload protector stopped the motor before damage was done.

The mild climate of East and South-east Africa greatly simplifies the housing requirements for most animals and some plant products. However, it seems worthwhile to discuss several factors of ventilation that apply primarily to cooler climates.


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