transport equipment usually operate with refrigerant R12 (Freon 12), and have reciprocating compressors mostly in the 5-8 hp range.
2.1 Principle of mechanical refrigera-tion
The refrigeration process, or the
refrigera-tion cycle, includes four stages, as shown schematically in fig. 2.3:
• Compression. In the compressor, the re-frigerant gas is compressed, i.e. the pres-sure and temperature of the gas is in-creased. Then the high pressure gas is discharged into the condenser.
• Condensation. In the condenser, the high temperature and high pressure gas is
Condenser Evaporator
Expansion Valve
Compressor
High Pressure Low Pressure
Fig. 2.3 Schematic diagram of a refrigeration cycle
MECHANICAL REFRIGERATION
cooled by means of air or water. The gas is changed into a liquid, still at high pres-sure.
• Expansion. The expansion valve controls the flow of refrigerant, which it receives from the condenser (possibly via a liquid receiver) so that the correct amount of re-frigerant passes to the evaporator.
• Evaporation. On entering the evaporator section the refrigerant passes from the high pressure side, through a small orific-e in thorific-e orific-expansion valvorific-e, to thorific-e low prorific-es- sure side of the system. The low pres-sure causes it to evaporate. The latent heat of evaporation is extracted from the surroundings, for example from air pass-ing the evaporator coils. The refrigerant gas is then drawn back to the compressor and the cycle is repeated.
Refrigeration capacity
Refrigeration capacity is the measure of cooling power available. Gross capacity is the total cooling done by the refrigeration unit. Net capacity (or effective capacity) is that available to the cargo space after re-moving the heat generated by evaporator fans and motors. The capacity is reduced as the evaporator temperature falls; the reduction is 3-4% per degree C at tem-peratures below 0°C.
The capacity is also reduced as the dif-ference in temperature between the in-side and the outin-side the vehicle increas-es.
Generally, every 2°C rise in ambient tem-perature means that the minimum achiev-able internal temperature becomes 1°C higher. If the minimum achievable tem-perature is -20°C at an ambient tempera-ture of 22°C, it will be -19°C at 24°C am-bient.
Capacity reduction.
In most modern transport equipment, the mechanical refrigeration unit has some sort of mechanism controlling the refriger-ation capacity, in order to achieve prac tically constant air temperatures. This is essential for many chilled foods, where precise control at the lowest temperature the goods can tolerate is necessary.
A common way of reducing the refrigera-tion capacity of the refrigerarefrigera-tion unit is hot-gas bypass, where a certain part of the hot refrigerant gas from the compres-sor is injected into the evaporator. There are several ways of controlling how much gas should bypass the condenser and is injected into the evaporator. The refriger-ation unit runs continuously, this gives a more accurate control over the tempera-ture but an increase in energy consump-tion compared to on/off control.
Other methods of capacity control are cylinder unloading (see section 4.2 below), suction throttling, where the flow of refrigerant to the compressor is regulat-ed, and control of the speed of the com-pressor motor. The latter method would result in lower energy consumption, but the system is still not fully developed.
In many trailers the compressor runs at high speed (HS) when the air is some de-grees C warmer than required, and at low speed (LS) when the air temperature is close to the required temperature, see section 4.2 below.
2.2 Transport refrigeration units A transport refrigeration unit is a conven-tional circuit consisting of a compressor, a condenser, an expansion valve, and an evaporator coil, with the thermal expan-sion valve providing the primary control to the circulating refrigerant.
MECHANICAL REFRIGERATION
51
It is necessary to minimize the space oc-cupied by the refrigeration unit, but at the same time securing the correct function-ing of the unit. A cutaway view of a typical layout of a container refrigeration unit is shown in fig. 2.4.
The internal air is circulated through the cargo space and the evaporator coils.
The direction of the air flow is shown by the arrows. This is known as bottom air delivery, see fig. 2.9. In earlier containers and most long distance trailers, the air
cir-culates in the reverse direction: top air de-livery, see figure 2.11.
The fans (1) force the air through the evaporator coil (2) which cools the air to the required temperature. The air then passes over the delivery air thermostat (4) used by the controller and out into the cargo space by way of the ducted floor (5). The most common form of ducted floor is known as T-bar floor, taking its name from the T-shaped cross section aluminium extrusions that form the floor, see fig. 2.9.
MECHANICAL REFRIGERATION
1 2
3 3
6
10
9
8
7 11
4
5
Fig 2.4 Cutaway view of refrigeration unit and container
The air returning to the fans (1) is warmer than the delivery air as it has absorbed heat from the cargo space.
The low pressure refrigerant in the evapo-rator (2) absorbs heat from (and cools) the air passing the evaporator coil, via the compressor (7) the refrigerant gas flows to the condenser (8) where the refrigerant gas is condensed by outside air forced through the condenser by the condenser fan (9). The now high pressure liquid re-frigerant returns to the evaporator via the thermal expansion valve (not shown). On passing through the expansion valve from the high pressure to the low pressure side, the liquid refrigerant expands to a mixture of liquid and gas, which reduces the temperature of the refrigerant. The re-frigerant in the evaporator coil again ab-sorbs heat from the air passing the evap-orator coil.
The temperature recorder (10) measures and records the internal air temperature by a separate sensor (6), shown here in the return air passage but more often lo-cated in the delivery air duct, or both places, see section 3.3 below.
2.3 Thermostats
In refrigerated transport equipment the temperature is maintained by a thermostat controlling the refrigeration machinery.
The thermostat sensor measures the temperature (practically always an air temperature) and sends a signal to the controller which adjusts the refrigeration system. Generally speaking, the refriger-ation system, if an on/off control type is switched on, or if a capacity controlled type is in it’s full capacity mode when the measured temperature rises to above the pre-set temperature. The refrigeration system is switched off or uses reduced capacity when the measured temperature falls below the pre-set temperature.
Controllers are described in sections 3.3
and 4.2 below, where the importance of the location of the thermostat sensor is also stressed.
2.4 Defrosting
During operation of the refrigeration unit, water vapour is transferred from the air and unwrapped food products, and the outer cases to the evaporator coils. If the temperature of the coils is below 0°C, then frost builds up, and the air circulation rate and refrigeration efficiency falls.
Many units are fitted with timers to give a defrost at set periods, commonly once or twice a day. In some units, a differential pressure controller starts the defrosting process when the resistance to air flow across the evaporator reaches a pre-set value. In some units, the defrosting pro-cess is initiated when the difference be-tween the evaporator temperature and the air temperature (return or delivery air) exceeds a pre-set value.
The defrosting process, i.e. heating of the evaporator coil is provided for by electrical resistance heaters or by hot gas from the compressor bypassing the condenser, going directly into the evaporator coil. The air circulation fans in containers are turn-ed off so that the heat is utilisturn-ed in melting the ice on the evaporator coils, and not in heating the cargo. In most trailers, the fans continue during defrosting, but by closing of air vents, the circulating air is kept inside the evaporator section.
The melt-water (melted ice) falls into a tray and then runs outside. By means of electric heaters, see (3) in fig. 2.4, the melt-water is kept above 0°C and can flow out.
When the outlet is blocked, water may ac-cumulate on the floor with potential dam-age to the outer cases and the food. The water on the floor may freeze and block the air flow leading to a rise in
temper-MECHANICAL REFRIGERATION
53
ature of the food products. Of course, blocked outlets should not occur, and the outlets should be properly cleaned before the transport begins.
During defrost, the temperature in the evaporator section rises, and this can usually be seen as a sharp rise and fall in temperature on the temperature recorder.
3. ISO INTERMODAL CONTAINERS During the last few years a large number of ISO intermodal containers have been
built. In 1990, about 180,000 refrigerated (thermal) containers are in operation, and the number of containers increases every year.
A container essentially consists of three parts:
• an insulated box
• a refrigeration system
• an air circulation and distribution system The work of international standardization