15.1 Compressors
15.2 Condensers
15.3
General notes on refrigeration plant
15.4 Power requirements
Most of the mechanical refrigeration plant used for freezing and cold storage of fish is of the vapour compression type basically consisting of compressor, condenser, expansion valve and evaporator (cooler). In simple terms, a refrigeration system takes in heat at a low temperature and rejects it at a higher temperature. The following is a brief description of the major components. Reference should be made to the appropriate text books for a more detailed description.
The selection of a compressor to suit a particular installation is better left to a qualified person. Detailed information on compressor design cannot be given in a document such as this. As a general rule, small freezer and cold store installations should not share the same refrigeration machinery. Load fluctuations brought about by the freezer being loaded and unloaded, could result in temperature fluctuations within the cold store. In addition, when only the cold store only is in operation, a refrigeration compressor of large capacity will be used for what is a relatively small refrigeration load. Apart from being uneconomic, this will result in problems with capacity control. On the other hand large installations usually have multiple compressor systems and are maintained and controlled by competent engineers.
Table 28 gives some indication of the water requirement for various types of condenser.
Table 28 Condenser water requirement (t/h)
Type of condenser |
100 kg/h freezer |
1000 m3 cold store |
Shell and tube (water rejected) |
5 to 7 |
10 to 14 |
Shell and tube with water recooling |
0.03 to 0.06 |
0. 06 to 0. 12 |
Evaporative |
0.03 to 0.06 |
0.06 to 0. 12 |
Selection of a condenser must take into consideration many factors relating both to the system used and to the climatic conditions. Selection must again be left to a qualified person who is aware of all the relevant information about the project.
Duplication of cold store plant
The value of frozen product in a cold store can be high and precautions must be taken to ensure that the contents are not damaged in the event of a major breakdown of the plant. Cooling by multiple units, each with a separate condensing unit, is one way of ensuring that sufficient refrigeration effect is available to maintain the store at the operating temperature or slightly higher should a unit break down. Another method is to cross-connect the cold store and freezer refrigeration pipework. This allows the freezer refrigeration machinery to be used to cool the store in an emergency. With normal operation, the two would be isolated and only a competent person would be allowed to make the cross-connection.
Centralised plant
Centralised machinery enables one operator to take care of all the refrigeration equipment. Care should be taken in arranging plant layout such that the refrigeration lines to and from the cooler are not too long, as this can give rise to a number of difficulties. Plant operation and economics are considerations that also have to be taken into account.
Standardisation of plant
Standardisation of equipment is another good policy to follow especially in remote areas. Parts can be interchangeable and stocks of spares will be kept low. If possible, the same refrigerant should be selected for each installation and similar machinery made by one manufacturer should be specified. The size of individual units should also be standardised
whenever possible even if it means that some adjustment has to be made in their capacity to suit each requirement.
Simplicity and reliability
Small plants seldom justify a full time engineer in attendance; therefore, simplicity and reliability should be major considerations when selecting the equipment particularly in a developing country. The plant and all auxiliaries should also be well tried and tested. Although these requirements apply particularly when plant is unattended, conditions in most developing countries are such that they should be applied there as a general rule. Whatever incentive there may be to purchase plant that is new and offers potential economic or other benefits, the purchaser should place a good deal of importance on reliability.
The power required for the operation of a refrigerated warehouse is high-voltage main electricity. A transformer unit must supply low-voltage mains in the establishment via a general switchboard. The regulations in force in most countries imply that refrigerated premises act as good conductors due to moisture condensation risks and consequently impose a certain number of precautions. Those precautions include earthing of motors and all equipment through a special circuit connected to an earth-point that is independent of the high voltage earthing connector. Equipment should be waterproof with water-sealed cables, including those of lighting circuits. The supply for mobile equipment should be low voltage. The mains circuit within the store must include a conductor to earth the frames of fans, coolers, small machine tools and other equipment. An outdoor main contactor to switch off the entire installation except the engine room extractor fans should be provided for emergency purposes. With the increasing use of electrical trucks, provision for battery charging is required. Further plug-in facilities for refrigerated vehicles may be necessary. All work concerning the power supply must be carried out by specialists.
It is obviously difficult to quote general figures for either freezing or cold storage requirements, especially when both the installed power and the peak power requirement figures are needed to plan for the connection of a suitable electrical supply. The examples that follow are therefore hypothetical and merely illustrate the calculations that may be made at the planning stage before details of the actual equipment to be used is known.
Freezer power requirements
The requirements in Table 29 are based on a heat extraction figure of 110 kcal/kg of fish frozen which includes the heat to be extracted in reducing the fish from +5°C to -30°C, fan power, insulation heat leaks, heat from trays, trolleys and so on.
Table 29 Compressor power (kW) requirements to freeze 100 kg/h
Condensing temperature |
Evaporating temperature |
||
(°C) |
- 35°C |
- 40°C |
- 45°C |
20 |
6 |
7 |
9 |
30 |
7 |
8 |
10 |
40 |
8 |
9 |
10 |
Additional power requirements that may be added are:
Condenser water pump and fan
0.5 kW
Electrical defrost (2 x 8 kW in sequence)
8 kW
Door heaters etc.
0.5 kW
The total power to be installed for an air blast freezer operating at 30°C Condensing temperature, -40°C evaporating temperature and capable of freezing 100 kg/h of fish from +5°C to -30°C will therefore be 17 kW. An electrical defrost is not activated while the compressor is running or the cooler fan in operation; therefore, the maximum power requirement will not exceed 12 kW.
Cold store power requirements
A cold store of 1000m3 capacity keeping frozen fish at -30°C, maximum ambient temperature of 35°C, would require a refrigeration capacity of 30,000 kcal/h. If the operating conditions are 30°C condensing temperature and -35°C evaporating temperature, the compressor power requirement will be 20 kW. Additional power requirements may be:
Condenser pump and fan
0.6 kW
Door and underfloor heating
0.5 kW
Mechanical handling equipment
1.5 kW
The total power required would therefore be
22.6 kW
The above examples for freezer and cold store power requirements illustrate the type of calculations that have to be made to determine the power supply required for a project. Other factors and the application of safety margins may, however, increase these calculated values and expert advice should be taken on this aspect of planning.
