In reality, the pressure drops that occur in the evaporator, condenser and piping must be considered. There are also mechanical and electrical losses in the compressor. The consequences are increased operational and maintenance expenses. However, some measures can be taken to minimize the costs. The main ones are discussed below.
Superheating of the refrigerant gas
In the fundamental process, the gas entering the compressor was assumed to be dry and saturated. In reality, the gas is overheated as shown in Figure 2.9 (point 1.2). The overheating is the difference between the temperatures at points 1.1 and 1.2 in the figure and is created in the end of the evaporator. It is a practical necessity to allow the refrigerant vapor to become superheated to prevent the carry-over of liquid refrigerant into the compressor, where it may cause severe damage due to its incompressibility. It may also contaminate the lubricants. The level of superheating should be kept to a minimum to minimize both the work to be done by the compressor and the necessary heat transfer surface in the evaporator.
Sub-cooling of the refrigerant liquid
In the fundamental process, the liquid leaving the condenser was just on the saturation line for liquids. The pressure drop in the pipes, filters, etc., before the expansion valve is negligible, but still causes "flash gas", i.e. vaporization of a small part of the liquid. The condensed liquid is therefore sub-cooled to a temperature below that of the saturation temperature corresponding to the condenser pressure, for two reasons: the cooling capacity of the refrigeration process is increased and the risk of gas bubbles in the flow fed to the expansion valve is avoided. (Gas bubbles in the inlet flow to the expansion valve disrupt the regulation mechanism.) The sub-cooling is the difference between the temperatures at points 3.1 and 3.2 in Figure 2.9, and is generated in the condenser or in a separate heat exchanger after the condenser.
The compression process
In the real refrigeration process, compression does not follow the lines of entropy (see Figure 2.9) as it does in the ideal, fundamental process. This means that the compression work increases. The ratio of the theoretical to the real compression work is called the isentropic efficiency.
The basic compressor-driven refrigeration cycle consists of one compressor, two heat exchangers (a condenser and an evaporator) and a throttling device (expansion valve). These components form the circuit in which the refrigerant circulates. The cycle operates between the two pressure levels p1 and p2, and the temperatures T1 and T2, where T1>T2 (see Figure 2.10).
The refrigerant receives energy in the cold chamber at a temperature below that of the surroundings. The energy at rejection is of a higher quality than in the cold chamber, because of its higher temperature. This energy can be used for heating purposes. Plants designed entirely for this purpose are called heat pumps. The term "heat pump" is appropriate because energy is transferred against a natural temperature gradient from a low temperature to a higher one. It is analogous to the pumping of water from a low level to a higher one against the natural gradient of gravitational force. Both actions require an input of energy for their accomplishment. There is no difference in operation between a refrigerator and a heat pump. However, in a refrigerator the desired effect is the removal of energy from the cold chamber, represented by Q2 (refrigerating effect) in Figure 2.11. In a heat pump, it is the energy to be rejected by the refrigerant, Q1, for heating purposes (see Figure 2.11) that is desired. Both actions also follow the first law of thermodynamics:
"When a system undergoes a thermodynamic cycle, the net heat supplied to the system from its surroundings plus the network input to the system from its surroundings is equal to zero."
Applying the first law of thermodynamics to the refrigerant system in Figure 2.11 gives the following equations:
The power input, W, is important because it is the quantity that has to be paid for and constitutes the main item of the running costs. Refrigerator and heat pump performances are defined by means of the coefficient of performance, COP, which is defined as: