What is Refrigeration?
In simple terms it is the removal of heat from one location (in vehicles this is the insulated loadspace) and, by the changing states of a refrigerant gas, dissipating that heat somewhere else (outside of the load space).
Cold
When we talk of a cold area what we are actually talking about is a reduction in heat. A freezer compartment has less heat in it than a chill compartment. It is a sensation felt as a reduction in heat.
Insulation
All vehicles required to carry temperature controlled goods should be suitably insulated. Insulation material has the ability to inhibit the flow or transfer of heat. Industry standards have been adopted by the reputable converters and are quoted as K factors. The standards are 0.7K for chill applications and 0.4K for freeze applications. The lower the K factor the better the insulation properties of the load space. K = the Watts/M2/°C that enter the box without the doors being opened.
Direct Drive systems
Direct drive refrigeration systems take power from the vehicles engine to drive a refrigeration compressor that is bolted directly to the engine and has an electric clutch on the front so that when the load space reaches the desired temperature the compressor is disconnected.
Independent Diesel Drive systems
These box body systems have their own separate engine that runs independently from the vehicle engine. The compressor is coupled directly to this engine therefore it does not depend on the vehicle for its power source. Independent systems are generally favoured on large boxes where is impractical to get enough power from a direct drive compressor.
Refrigerant
Until concerns about depletion of the ozone layer arose in the 1980s, the most widely used refrigerants were R12 and R22 (fluorocarbons HCFC’s). These have now been replaced with ozone benign refrigerants. R134a is widely used in chill applications an R404A is used for low temperature (freezer) applications. These refrigerants have a much lower Global Warming Potential (GWP) and, when used in sealed systems post no significant risks.
Compressor
The compressor is the heart of any refrigeration system. It takes refrigerant vapor in from the low pressure side of a refrigeration circuit and discharges it at a much higher pressure into the high pressure side of the circuit and keeps the refrigerant flowing. The high pressure vapour then passes into the condenser.
Condenser
The hot refrigerant vapor discharged from the compressor passes through the condenser; cool air passes over the condenser coils and absorbs enough heat from the vapor to cause it to condense. Heat is dissipated in the condenser at this point. The high pressure liquid refrigerant will flow down the liquid line, through a filter drier that is designed to prevent contaminants from flowing through the system, and on to the evaporator.
Evaporator
When the high pressure liquid refrigerant passes into the evaporator, it enters it through an expansion valve that meters the flow of liquid refrigerant into the low pressure side of the system. Its pressure will drop and it will evaporate into a vapour. As this happens in the evaporator coil, air is passed over the coil (in the load space) and heat is absorbed from the air reducing the temperature to the desired set point. The low pressure refrigerant then returns to the compressor and the cycle continues.
So, when you turn on a refrigeration system, it is designed so that the evaporator coil will stay colder than the area it is cooling, and will continue to remove heat until the pre set temperature is achieved.
Standby
A facility designed to allow a transport refrigeration system to be plugged into a mains power source allowing the system to be used for storage or pre chilling without having to run the vehicles engine.
Defrost
A by product of cooling the load space is that the evaporator also collects any moisture that is in the air as frost as it passes over the coils. As the frost builds up it will reduce the efficiency of the system, reduce the airflow and eventually, the temperature will not be able to be maintained.
On basic chill only units, a timer in the system will disconnect the compressor whilst keeping the fan in the load space running. Air passing over the coils in a chill system will be above 0°C therefore it will melt the frost that has built up. After a set period of time (approx 5 minutes) the timer will be connected again and the refrigeration cycle will start again.
In freezer units, some companies use hot gas defrost. The timer will switch off the fan and change the flow of gas with electrically operated valves so that the hot vapor from the compressor is passed through the evaporator coils. Although favoured by a large number of companies as it is cheap and simple, the hot gas system is relatively slow therefore; the refrigeration system spends too much time heating and the load space temperature will rise.
Other companies use Reverse Cycle Defrost systems so that the timer will turn off the fan in the load space and reverse the flow of refrigerant so the system absorbs heat from the ambient air outside of the load space and dissipates that heat in the evaporator. They also have heated drain pans and drain lines to take the melted frost and ice outside of the load space. When the thermostat senses that the coil is at a pre determined temperature, the fridge is automatically switched back on and starts to cool again. A manual override is provided so that a defrost can be initiated at any time if required. Reverse cycle defrost is a very fast and efficient way of defrosting that keeps the air temperature within much closer tolerances and ensures that the fridge is cooling more often than its hot gas counterparts.
Strip Curtains
A series of plastic strips placed over the door aperture to slow down the ingress of warm air when the door is opened.
Drop rates
The number of times during a working day that the doors will be opened to make a deliver or drop.
Best Practice
When using a refrigeration system it is wise to follow some simple guidelines to ensure that you get the best from your system, simple things that will ensure that your products stays at the correct temperature include:
• Pre cool the refrigerated compartment prior to use
• Ensure the product is loaded at the correct temperature (the refrigeration system is designed to maintain the loaded temperature)
• Do not run the unit on engine or standby whilst loading the refrigerated compartment
• Keep the door opening times to a minimum
• Keep the engine running if it is safe (and legal) to do so
• Do not restrict air circulation around the load or the evaporator
• Keep the strip curtains in their correct position
• Carry out a manual defrost during the last return journey of each day
• Do not use leads that are in a poor condition i.e. showing exposed wires, or broken plug or sockets
• Do not plug a live lead into the refrigeration system, it can cause damage