Produced in Association with
SERIES 22 / Module 03 Refrigeration
The key components of the system
are: ● Evaporator – this is a heat exchanger which absorbs the heat to be removed. One side of the exchanger will be the refrigerant, with the other either being air (e.g., an air conditioner, cold store) or a liquid (e.g., a water chiller). ● Condenser – again a heat exchanger – this exchanger rejects the heat removed. Condensers can be air cooled or water cooled. When water cooled the cooling water can be linked to an open or closed cooling tower. ● Compressor – this compresses the low-pressure vapour from the evaporator and passes it at high pressure to the condenser. ● Expansion valve/device – this is a restrictor that allows the high-pressure fluid to expand when it enters the evaporator.
Energy input The main energy input is the motive power for the compressor. This is normally an electric motor, although other motive power units can be used. For example, in an ICE (internal combustion engine) passenger car AC, the motive power is from the vehicle’s engine. However, with a battery electric vehicle (EV) we will again have an electric motor. Ancillary equipment also uses energy. On the evaporator side – this could be a fan (for a cold store or AC application) or a pump (chilled water). On the condenser side, motive power will be required to move the air or a fluid that carries the reject heat (fans, cooling towers, etc). In some systems energy is also required to defrost the evaporator.
System effi ciency As the quantity of heat moved by a refrigeration system exceeds the motive power input, a percentage statement of efficiency would always be in excess of 100%. For this reason, a Coefficient of Performance (COP) is used to assess performance. A system can have two coefficients
of performance. Coefficient of Performance (COP) which is calculated using only the power input to the compressor and Coefficient of System Performance (COSP) which takes account of the power input of the compressor and ancillaries. Both coefficients should be considered when reviewing a system. In practice COPs are typically in
COSP =
Cooling capacity (kW) Power Input (kW)
the range of 2 to 5 for refrigeration systems – which is about a tenth of the theoretical maximum, which suggests that there is scope for future improvements in the technology. When looking at the performance of
a refrigeration system it is important to understand the impact of what is called the temperature lift. This is the ∆T of the system where the relevant temperatures are the evaporating and condensing temperatures. Put simply, the greater the temperature lift the more energy input is required. To fully understand the energy requirement this needs to be coupled with the size of the cooling load. An analogy often used is that of lifting a weight. The larger the weight and the higher the lift, the more energy required (see Figure 2).
Reducing energy use There are two pathways for reducing cooling energy use. First is to reduce the cooling load, second is to improve
Fig. 2
the performance of the system. These pathways are not mutually exclusive and for the optimum outcome both need to be addressed. However, the starting point should always be to look at reducing or eliminating the need for cooling.
Load Reduction Typically for an occupied space it will be about limiting the amount of heat gain, this might be addressed by insulation, solar shading, etc. Equally, designing out cooling by use of building design, natural ventilation is valid. These so-called passive measures could reduce cooling loads by 24% with associated savings in investment costs, energy costs and GHG emissions. In the retail environment, load
reduction could be by using doors/ covers on display cabinets. For cold stores the use of LED lighting will reduce the load. Some aspects of load reduction are available at the design stage; others are operational issues.
For example, keeping cold store doors closed as much as possible. The next aspect is addressing the
temperature lift. This is achieved by looking at both the evaporating and condensing conditions. The most obvious is the cooling set point; in air conditioning, by aiming for as high a space temperature as acceptable. Added to this can be a ‘dead-zone’ to prevent rapid cycling between cooling and heating. For data centres which are a growing area of cooling requirement, optimising temperature control is critical. In the food sector there is ‘The Move to -15ºC’, an industry effort to move away from the traditional -18ºC for transportation and storage of food products. Adjusting the set point for cooling
may be obvious, but less attention is often paid to the condensing conditions. High condensing temperatures can arise from fouling of the heat exchanger and/or poor placement of the outdoor unit giving restricted airflow.
Improving system performance ● Compressors - can be reciprocating, screw, scroll and centrifugal. The size and type of system normally dictates the type of compressor. But as with all equipment some models are more energy efficient than others. When selecting new equipment look for compressors that have good part load efficiency – in practice systems operate more time at part load than at full load. A low-cost improvement can be the use of ‘floating head pressure control’. Most refrigeration systems operate at a higher pressure than necessary. Systems are available that allow head pressure and therefore condensing temperatures to float relative to ambient conditions. This typically requires a technician to implement and can save between 2-4% of compressor power for every 1ºC reduction.
● Condensers – for air-cooling, select a unit with wide fin spacing to minimise blockage and keep it clean. Savings of 5% can come from cleaning whilst increasing the condenser size by 30% might realise 10% savings. Typically, an increased size condenser, at design stage, will pay for itself in about two years. Another aspect for air-cooled condensers is the fan(s). A
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