Overview of possible types of refrigeration

At this point we can only give a very general overview. For further information, see the relevant specialist literature. For example, a detailed overview can be found in Cube et al 1997 or Jungnickel et al 1990.

A basic distinction can be made between refrigeration systems with mechanical power und those with thermal power.

3.1.1 Refrigeration systems with mechanical power

Refrigeration systems with mechanical power are vapour compression systems (compressor systems). Such systems are used throughout almost the entire spectrum of applications of refrigeration and air-conditioning technology. Their detailed design, especially the compressor type, depends in particular on the performance required. The systems are filled with a refrigerant which passes through the various components of the system (compressor, condenser, expansion valve, evaporator), changing its physical state in the process: First of all, the refrigerant is compressed from evaporation pressure to condenser pressure by the compressor, with input of work (W). Then the refrigerant is liquefied in the condenser and gives off the heat flow Q. This is followed by expansion of the refrigerant from condenser pressure to evaporation pressure, and the evaporation of the refrigerant, taking in the heat flow. Figure 3.1 gives a schematic diagram of the refrigerant cycle.

Fig. 3.1: Schematic diagram of refrigerant cycle of a vapour compression refrigeration system. After: [Grage, Pareidt 2000].

In simplified terms, the evaluation criterion for the cycle process is the ratio of effort (work/energy input) to benefits (refrigeration capacity): the coefficient of performance (COP).

Refrigeration systems by compressor type: reciprocating, screw, scroll or turbo compressors.

They may be designed as open, semi-hermetic or hermetic compressors [Grage, Pareidt 2000].

Whereas the case of a hermetic compressor is welded, semi-hermetic compressors have removable assembly flaps. These permit repairs to the compressor. Both types have neither a shaft nor a shaft seal passing to the outside. In an open compressor, by contrast, the driven shaft passes through the case (DIN EN 378 [DIN 2008]). Only hermetic compressors are described as “permanently closed”, because of their welded case. Unlike semi-hermetic and open compressors, their use gives rise to very small refrigerant losses. However, other components/joints (refrigerant pipes, offtake points etc.) usually play a more important role in the refrigerant losses of an entire system – especially in large direct evaporation systems (see Harnisch et al 2008).

In reciprocating compressors, compression and displacement is by means of reciprocating pistons running in enclosed cylinders. Valves connect these alternately with the suction and pressure pipes of the compressor. Here again, a distinction can be made between hermetic (fully welded case, for low refrigerating capacity) and semi-hermetic motor compressors (for medium capacity), and also open compressors (high capacity). Reciprocating compressors are used for a wide range of applications. They are also suitable for part-load operation.

Screw compressors operate with rotary movements only. Their characteristic features are a minimum of moving parts, robustness, compactness, great reliability (short service intervals) and long life. The screw compressor draws the gas for compression into the working chamber, which is then closed and reduced in size to compress the gas (displacement principle).

Scroll compressors have a very simple design. The type of compression (also displacement principle) means that the noise level created by the compressor is very low. Their fields of application are mainly in air-conditioning. They can achieve very high coefficients of performance [Grage, Pareidt 2000].

Turbo compressors (centrifugal compressors) are not displacement machines, but fluid flow machines. Turbo refrigerating units (liquid chilling packages, cold-water units with turbo compressors) are built as compact systems for a wide performance range and used mainly in air-conditioning (of buildings) and for process units with cold-water or brine circulation [FKW 1998a]. They are often used in cases where high refrigeration performance is needed with frequent part-load operation, because compared with screw compressors their energy efficiency is greater under part-load conditions [Axima 2003b]. Other advantages of turbo compressors over reciprocating compressors (in high performance ranges) are their smaller size and simpler structure. These aspects mean they take up less space and are less prone to faults (simpler servicing). At high discharge rates their efficiency is no less than reciprocating compressor systems [FKW 1998a].

Turbo compressors, like centrifugal pumps, work on the dynamic principle. In other words they generate the static pressure by converting kinetic energy into static pressure energy.

Their power transmission element is impellers rotating at high speed. Depending on the impeller design, a distinction is made between radial compressors and axial compressors.

Turbo compressors with a radial impeller are suitable for medium gas flow rates. They supply a uniform, oil-free flow of pressurised gas. If several impellers in succession are mounted on a drive shaft, it is possible to achieve medium and very high pressures. Turbo compressors with axial impellers deliver very high flow volumes of up to 1,000,000 m³/h. They are used in natural gas liquefaction plants in the chemical industry [Grage, Pareidt 2000].

3.1.2 Refrigeration systems with thermal power

Refrigeration systems with thermal power are supplied with a “heating energy flow” as their driving force. One advantage is that this heating energy flow can be taken from thermal energy that is otherwise not very usable (waste heat, waste steam, hot water, solar energy).

Energy assessment of refrigeration systems with thermal power is by means of the heat ratio (as the “efficiency” of refrigeration systems with thermal power). The equipment required, especially in the case of absorption refrigeration systems, is more complicated than for refrigeration systems with mechanical power. But absorption refrigeration systems have virtually no moving parts. Wear is therefore low, and service life high despite low maintenance requirements.

A distinction is made between absorption, adsorption and steam jet refrigeration systems.

Fig. 3.2: Schematic diagram of refrigerant cycle in an absorption refrigeration system.

After: [Grage, Pareidt 2000].

Using an absorption refrigeration system is the best-known and most widespread means of thermal refrigeration. Here liquids / gases are taken up at low temperature and low pressure and delivered at high temperature and high pressure. Absorption systems have a compressor which operates not mechanically, but thermally. The refrigerant cycle outlined in Figure 3.2 is, apart from the compressor, identical to the cycle in a mechanical system – which here comprises an absorber, a pump (for the fluid (solution)), the boiler and the throttle valve.

In the thermal compressor, the evaporated refrigerant first enters the absorber, where it is absorbed by the fluid (solution). The resulting heat of solution has to be removed from the absorber. The fluid (solution) containing the refrigerant passes through a pump which brings it to a higher pressure level. Then heat is input into the boiler, which boils off the refrigerant from the fluid (solution). The boiled-off refrigerant vapour arrives at the condenser, from where it passes through the circuit to the evaporator. The solution from which the refrigerant has been boiled off passes through the throttle valve and back into the absorber.

The operating materials used are water / lithium bromide (air-conditioning, water cooling) or ammonia / water [Grage, Pareidt 2000].

Refrigeration is also possible by means of adsorption refrigeration systems with solid sorbent material. Figure 3.3 shows a schematic diagram of this thermally powered refrigeration system.

Fig. 3.3: Schematic diagram of refrigerant cycle in an adsorption refrigeration system.

After: [SorTech AG 2009].

Such systems use silica gel as sorbent material, for example, and water as refrigerant. The process is discontinuous and takes place in two periods. The refrigerant (steam) is adsorbed on the surface of the sorbent material. This releases heat of combination. As the deposition increases, the heat of combination tends towards zero. The evaporation of the refrigerant removes heat of evaporation from the surroundings. Desorption (second period) and pressure generation for condensation take place at very low drive temperatures, making this technology particularly suitable for the use of solar energy [Jacob 2002]. Quasi continuous refrigeration can be achieved by operating several units staggered in time [Hesse et al 1992]. Continuous operation is also possible by using liquid sorption systems. A further advantage of this system is the buffer effect of the adsorbent substance. If refrigeration requirements are continuous, but there is no solar energy available at the time (clouds combined with sultry heat), the

“used” solution can be temporarily stored for subsequent regeneration.

Other refrigeration processes are the steam jet refrigeration system and the DEC process (desiccative and evaporative cooling). The structure of a steam jet refrigeration system is identical to that of a compressor system. The mechanical compressor is simply replaced by a steam injection unit. These two processes are not described in any detail here.