An introduction to commercial refrigeration
Energy consumption and prevention of refrigerant loss are highly important aspects of refrigeration – and the main items in total running expenses.
Regular maintenance can help contain these costs and improve operating margins for many food retailers, restaurants, etc.
The basic requirements of refrigerants are:
- Normal boiling point below 0°C to ¬–40°C
- Easily detectable in case of leaks
- Large latent heat of evaporation
- Easy to recycle or dispose of after use
- Low environmental impact if leaked.
Refrigerants are denominated by an alphanumeric code, R-xyz, where R stands for refrigerant. The rule for the following figures is as follows: z indicates the number of fluorine atoms (F), y is the number of hydrogen atoms plus one (H), and x is the number of carbon atoms minus one (C).
Thus the common refrigerant R-134 has four fluorine, two hydrogen and two carbon atoms in a molecule. It has the formula CH2FCF3, and a boiling point of −26.3°C at atmospheric pressure.
The R-134 refrigerant is an example of the hydrofluorocarbon (HFC) compounds of which there are many. HFCs are not banned, whereas the HCFC (chlorinated compounds) are now generally prohibited because of their destructive effects on the earth’s ozone layer. The refrigerant R133 is an example (note: the number of chlorine atoms is not indicated – its formula is C2 H2 Cl F3).
To round out the description of refrigerants, there are those based on ethane (C2 H6), as described above and methane (C H4). The refrigerant R-23, for example, is methane based. Using the rules described above, there are three fluorine atoms, one hydrogen atom, and one carbon atom (ie: we could have written R-023). Its boiling point is –82.1C.
Refrigeration cycle in detail
This section provides basic information on cooling capacity. You will see the use of the concept of enthalpy. The internal energy of the refrigerant is increased by virtue of it absorbing heat, described by the property ‘latent heat of vaporisation’.
Refrigerant vapour and pressure also represent energy (imagine a piston being pushed by vapour pressure, constituting mechanical work and requiring more heat). Adding the two forms of energy yields ‘enthalpy’, and its usual measurement unit is kilojoules per kilogram.
By adjusting the high and low pressures, condensing and evaporating temperatures can be selected as required.
The high pressure is determined by the temperature of cooling water (in the case of a water-cooled condenser) or by dry-bulb thermometer for air-cooled condensers. The evaporating pressure is determined by the low temperature required for the product or by the rate of cooling or freezing needed.
Low evaporating temperatures mean higher power requirements for compression and greater volumes of low-pressure vapour to be handled, therefore demanding larger compressors.
On the basis of refrigeration demand, the weight of refrigerant to be circulated can be calculated. Each kg/s extracts a given quantity of heat according to the difference in enthalpies (h2 – h1); refer to the refrigerant enthalpy-pressure diagram.
From the volume of refrigerant, the compressor displacement can be calculated. In the case of a piston-driven compressor not all the vapour can be displaced, as there must be a clearance volume. The ratio of the amount taken in to the theoretical volume displacement is called the volumetric efficiency of the compressor, and this applies to piston compressors and other types.
With increasing heat load the temperature tends to rise, causing an increased amount of refrigerant to boil off. In processing this increase, the compressor will have increased pressure on the suction side with the net result that the evaporation rate is reduced and the temperature increases.
Therefore, increased compression capacity is called for in order to provide more cooling.
Refrigeration compressors come in many types and sizes.
Hermetically sealed, piston-type compressors are commonly found in smaller units.
For larger systems – for example, long supermarket display units – vane compressors are often used. These must handle large volumes of refrigerant vapour and operate at low temperatures to cater for open displays that require a curtain of cold air.
In still larger refrigeration systems, multi-stage centrifugal compressors are used.
The advantage of hermetically sealed units is that the risk of refrigerant loss is very low. All other systems have the compressor drive shaft emerging, thus relying on shaft seals to prevent refrigerant leakage.
Energy consumption is mainly due to the compressor, although air-curtain fans in open display units also consume considerable energy.
Raising the suction pressure (less suction) reduces energy consumption because the compression ratio is lower. Of course, higher inlet pressure (less suction) raises temperature and conversely a drop in temperature with increased suction. Approximately 5% more compressor capacity is required for a drop in temperature of 1°C.
Refrigeration is not rocket science, but some things are not immediately apparent.
Returning to energy consumption, it seems that selection of a smaller-capacity compressor will reduce input power. However, as mentioned, the increase in saturated vapour pressure of the refrigerant will reduce the temperature differential in the evaporator.
This will necessitate a larger surface area evaporator to preserve the heat flow from the contents of the system, or increased cycling of interior fans to facilitate that heat flow. The latter solution therefore increases power draw, so there may be no net gain – and perhaps increased power consumption.
Reciprocating compressor capacity control is provided for via cylinder unloading, by disabling the compression cycles. The suction valve is kept open as the cylinder moves to the top position and the charge is routed back to the suction line.
Other forms of compressor have more gradual ways of adjusting capacity. The screw compressor, for example, has a slide valve that allows for adjustment along the axis of the rotor. The valve positions the suction port along the rotor, thus increasing or decreasing compression volume.
The choice of compressor is dictated by the type of service. Refrigerated display cases are subject to frequent opening – and supermarket customers leaving doors open. Therefore rapid pull-down of temperature requires compressors that load and unload linearly (percent capacity responding to percent load necessary).
In this regard multi-cylinder compressors are better than screw compressors, and compressors powered by variable-frequency drives provide excellent performance.
The refrigeration systems described so far are based on mechanical compressors. Other forms of cooling are sometimes used in air-conditioning, and they can be applied to refrigeration.
Vapour absorption chillers work on a different basis, using an absorbent (often lithium bromide) and a refrigerant (usually water).
The absorbent acts like the suction side of a compressor, drawing in water vapour and consequently cooling the surroundings.
Vapour absorption chillers are a standard feature of tri-state electricity generators: waste energy from the gas turbine exhaust heats the absorbent so that it releases water vapour and allows the cycle to continue.
Some mechanical energy is needed to pump the absorbent to a heat exchanger, but the total mechanical energy is 90% less than that of a mechanical compression cycle.
Air-cooled condensers are typical for self-contained refrigeration units. Their construction, other than for domestic refrigerators, involves a fan or fans to provide cooling air flow over radiator fins.
As mentioned, cool-sink temperature is determined by a dry-bulb thermometer.
For larger systems, such as cool rooms, evaporative condensers are used. A recirculation pump sprays water over the condenser fins, and a wet-bulb thermometer determines the cool-sink temperature, which is now lower than for the air-cooled case.
The saturated vapour-condensing pressure for air-cooled condensers is higher than for evaporative condensers, so greater demand is placed on the compressor. The efficiency of condensers is therefore crucial in the overall energy performance of refrigeration systems.
For very large cool rooms, condensers cooled by water towers are sometimes used when there is a plentiful and cheap supply of water. As a rule, the water temperature will be higher than the wet-bulb temperature, so condensing pressure will fall somewhere between that of evaporative and air-cooled condensers.
To review briefly, the cooling process relies on the latent heat of vaporisation of the refrigerant as it is transformed from liquid to vapour (boils).
Direct expansion evaporators allow the refrigerant to expand immediately as it passes through the throttling valve. Liquid recirculation or ‘overfeed’ evaporators separate the expansion process from the low-pressure refrigerant liquid, providing more efficient heat transfer than the direct expansion method.
‘Flooded’ evaporators fill the entire heat exchanger with refrigerant liquid, thus using the latent heat of vaporisation to maximum extent.
Heat exchange can be facilitated by an air stream, shell and tube heat exchangers (often used for cooling brine), and by way of plate heat exchangers (with alternating refrigerant coils).
Frost formation on evaporators severely affects heat transfer, causing the compressor to work much harder.
The problem is particularly noticeable in systems where the suction pressure isotherm is below the freezing point of water and below the dew point temperature, so that condensation and frost forms.
Defrosting methods include:
- hot refrigerant gas circulating on an intermittent basis through evaporator coils;
- air defrost, allowing fans to move air at approximately 2C over coils while evaporators are momentarily turned off; and
- electric resistance defrost (although this method, rapid as it is, increases the heat load of the volume being refrigerated).
Compressor control is probably the most critical part of a refrigeration control scheme.
It is crucial to prevent liquid refrigerant entry into the compressor, as this can effectively destroy it.
At the suction side this is achieved by ‘superheating’ the refrigerant (see the enthalpy diagram for R134a). Practically, it is achieved through ensuring complete vaporisation via extra heat absorption in the suction line. At the high-pressure end a check valve is installed to prevent reverse flow of refrigerant.
Condenser control is achieved via fan or cooling water control, depending on the type of system. For example, in larger installations, control of a separate liquid refrigerant pump may be necessary.
Evaporation control is primarily provided by trimming the throttling valve. One device uses a thermostatic expansion valve that monitors ‘superheat’ (suction side of the compressor).