Temperature control in trailers The operation of the refrigeration system

depends on its construction and control system. It is therefore important that the handling agent is familiar with the direc-tions supplied by the manufacturer.

Temperature control systems

Set-point. The set-point is the temperature at which the controller is set. However, the temperature in the cargo space depends on where the temperature sensor for the controller is placed. Refrigeration units in trailers are controlled by a temperature sensor located in the return air, i.e. the air coming back to the refrigeration unit after absorbing heat from the cargo space. This is called return air control.

Delivery air control, see section 3.3 above, is seldom used in trailers.

It must be emphasized that the set-point temperature should not be confused with the product temperature. In warm cli-mates heat will be coming through the in-sulation, the door if it is not sealed and in the case of respiring cargoes such as fruit and vegetables from the product, conse-quently the air warms up as it moves through the cargo space, and the temper-ature of the return air will be higher than the temperature of the delivery air. Thus, to maintain the required average tem-perature of the food in the vehicle, the temperature on a return air controller must be set higher than on a delivery air controller. If the required temperature of the load should be around 4°C, a delivery air controller should be set at about 3°C, while a return air controller should be set

PRIMARY DISTRIBUTION VEHICLES (TRAILERS)

at about 5°C. However in arctic climes heat may be escaping through the insula-tion and arctic cold air entering through any gaps in the door. In this case the set point must be adjusted the other way.

Control systems. Some units operate in an on/off mode. When the temperature of the return air falls to the set-point, the controller switches the refrigeration unit off. To avoid short-cycling, i.e. switching on and off too frequently, the refrigeration unit is not switched on again until the air temperature is 1-2°C warmer than the set point temperature. The delivery air tem-perature will cycle more than that, some-times 3-6°C. In most cases the

evapora-tor fans run continuously to provide a constant air change in the cargo space.

Most newer refrigeration units are driven by a diesel engine running continuously.

The compressor, usually a four cylinder compressor, runs at two speeds, high speed (HS) which is about 2200 rpm, and low speed (LS) which is about 1450 rpm.

Naturally, low speed operation saves en-ergy and hence fuel.

In many refrigeration units, a 4-stage or 6-stage thermostat control system auto-matically regulates the unit. A typical 4-stage control system operates in the following way: When the thermostat

sen-PRIMARY DISTRIBUTION VEHICLES (TRAILERS)

65 Temperature

Control modes in 4-stage system

Control modes in 6-stage system

a

b

d

e

c

Set-point HS, Cooling

LS, Cooling

LS, Heating

HS, Heating

HS, Cooling

LS, Cooling

LS, Cooling, Unloading

LS, Heating, Unloading

LS, Heating

HS, Heating

Decreasing Temperature

Decreasing Temperature Increasing

Temperature

Increasing Temperature

sor in the return air measures a tempera-ture some degrees (often about 3.5°C) warmer than the set-point, the unit cools at high speed. When the return air tem-perature gets below point a in the figure above, the unit runs at low speed, still cooling. This continues until the return air gets colder than the set-point (point c), or, more correctly, colder than point d, which is about 0.8°C colder than the set-point.

The unit then is switched into low speed heating. When the return air gets warmer than point b (about 0.8°C warmer than the set-point) the unit is switched to low speed, cooling.

High speed heating is used when chilled foods are transported through very cold regions, e.g. ambient temperatures below -10°C, as this could cause freezing of some foods.

However, heating of the air when the re-turn air becomes some degrees colder than the set-point may have some disad-vantages, for example when deep frozen foods are loaded with a temperature of -25°C and the thermostat is set at -18°C.

As frozen foods should be stored and transported as cold as possible, it is a waste of energy and has a negative (al-though very small) influence on product quality to heat the air and the food pro-ducts. The thermostat should be set a little colder, or there should be a system where high speed heating cannot be in-troduced when the thermostat is set below -13°C. This system is used by some manufacturers of refrigeration units.

Other control systems include a “heat lock-out” option.

In some modern units, a 6-stage control system is used, see the figure above.

Here, the refrigeration capacity is reduced by means of cylinder unloading, further reducing the fuel consumption. The unit

will usually start at HS, cooling and change to LS,cooling. When the return air temperature gets below point b in the fig-ure above) two of the four cylinders are unloaded, and in many cases the unit can operate with low speed, cylinder unload-ing, cooling or heatunload-ing, most of the time.

This saves energy and also results in a more even temperature in the cargo space.

Such control systems normally allow a time lapse of some minutes (e.g. 6-10 minutes) from the return air temperature increasing to a level where more refrigeration capacity is indicated, until cylinder unloading is finished and low speed, cooling is started.

The evaporator fans normally run with a speed proportional to the compressor. In some units, a constant air flow option is incorporated in the control system as there is a special or separate electrical evaporator fan motor. Thus, high speed air circulation can be maintained all the time, even though the compressor may run at low speed 70% of the time. Higher air circulation should result in an improv-ed air distribution, minimizing temperature variations.

It is recommended that the air circulation should be at least 60-80 times the body volume per hour for all frozen food car-goes as well as for sensitive loads at about 0°C, e.g. fresh meat.

At low speed, 40-50 times the body vol-ume per hour may be sufficient.

As mentioned in section 3.3 above, the effectiveness of temperature control is judged by:

• Constant (and correct) temperature at the thermostat sensor.

PRIMARY DISTRIBUTION VEHICLES (TRAILERS)

• The temperature spread across the width.

• The temperature range through the cargo. Greater awareness of what is a normal and reasonable temperature dis-tribution is needed, especially as there are now increased demands on and specifications for the temperature of chilled and (deep) frozen foods. The dif-ference between delivery and return air can be below 2°C, but is sometimes higher. The temperature difference be-tween the coldest and the warmest foods can be about 2-3°C.

4.3 Temperature recording in trailers