Compression refrigeration machine
The compression refrigeration machine is a refrigeration machine that uses the physical effect of the heat of vaporization when the physical state changes from liquid to gaseous; this very common design is used in most refrigerators .
A refrigerant that is moved in a closed circuit experiences various changes in its physical state one after the other . The gaseous refrigerant is first compressed (compressed) by a compressor . In the following heat exchanger ( condenser ) condenses (liquefies), releasing heat. The liquid refrigerant is then expanded due to the change in pressure via a throttle, for example an expansion valve or a capillary tube . In the downstream second heat exchanger (evaporator), the refrigerant evaporates while absorbing heat at a low temperature (evaporative cooling ). The cycle can now start over. The process must be kept going from outside by supplying mechanical work (drive power) via the compressor.
The refrigerant absorbs heat at a low temperature level (e.g. 5 ° C refrigerator interior) and releases it to the environment with the addition of mechanical work at a higher temperature level (e.g. 35 ° C condensing temperature in the heat exchanger on the rear of the refrigerator).
The Carnot process is the cold process with the highest performance factor; it can only be approximately achieved in real compression refrigeration systems, since the thermodynamic changes in state of the Carnot process can only be approximately technically implemented in real systems.
The range of compression refrigeration systems ranges from relatively simple refrigerator compression refrigeration systems with cooling capacities of a few 100 W to large systems for cold stores or for air conditioning mines with cooling capacities of over 10 MW.
In 1834, the American Jacob Perkins (1766–1849) built the first compression refrigeration machine with the refrigerant ether , which he had patented on August 14, 1835 under the name Äthereismaschine . The disadvantage of the refrigerant ether, however, is that it forms highly explosive peroxides with atmospheric oxygen and the ether ice machines sometimes exploded.
Cooling systems with a compressor that is directly connected to the working volume are of the Stirling type, those with high and low pressure tanks and a distributor valve are of the Gifford-McMahon type.
With regard to the choice of refrigerant, the following aspects must be taken into account:
- Vapor pressure of the refrigerant
- The vapor pressure of the refrigerant in the working area between the liquefaction and evaporation state should be in a technically controllable range. In the high pressure area, excessive wall thicknesses should not be required for compressors, pressure vessels and pipelines; on the other hand, the cross-sections of the apparatus and pipelines in the low pressure area should not be too large, since the density of the steam and the enthalpy of vaporization determine their dimensions. If there are negative pressures on the low-pressure side, ventilation devices must be installed in order to remove the air that inevitably diffuses in.
- Thermodynamic properties
- In addition to the vapor pressure in the refrigerant application, the enthalpy of vaporization and the isentropic exponent are decisive for the design of the refrigerant circuit. The enthalpy of evaporation and the vapor pressure at the evaporation temperature determine the volumetric cooling capacity and thus the dimensions of the apparatus. The temperature rise of the refrigerant during compression is mainly determined by the isentropic exponent.
- Environmental aspects
- From an environmental point of view, natural refrigerants such as ammonia (NH 3 ), R290 propane (C 3 H 8 ), R744 carbon dioxide (CO 2 ) are preferable.
- Ammonia has the disadvantages that it is poisonous and not compatible with non-ferrous metals. However, even small leaks are noticeable due to the low odor threshold (5 ppm).
- Propane and butane are highly flammable gases, so that explosion protection must be observed with this refrigerant .
- Carbon dioxide has a high vapor pressure, so that the components of the refrigeration circuit must be dimensioned for higher pressures. Due to the high density and volumetric cooling capacity, the components have a smaller volume. Under ambient conditions, carbon dioxide often condenses in the supercritical range, so that the heat transfer is poor; When throttling, a relatively high proportion of the liquid phase evaporates. The use of CO 2 in single-stage systems is falling. The use of the refrigerant in car air conditioning systems has been abandoned for the time being, as refrigerants (e.g. HFO 1234 yf ) that have a very low greenhouse effect are now available under the statutory framework conditions . However, CO 2 is increasingly being used as a low-temperature refrigerant in two-stage systems. Natural substances (ammonia, propane) or fluorinated hydrocarbons are used as refrigerants for the high pressure stage.
Common refrigerants are fluorocarbons (HFC) and partially halogenated fluorocarbons (HFC) such as R134a (tetrafluoroethane) , their mixtures such as R 507, R 407C and R 404A, as well as ammonia (NH 3 ), carbon dioxide (CO 2 ), hydrocarbons ( for example propylene , isobutane and propane). Ammonia is widely used as a natural refrigerant for industrial refrigeration systems with high performance. In the field of commercial refrigeration and air conditioning, PFCs are mostly used, but because of their considerable global warming potential, they are the subject of a political discussion aimed at restricting their use. The use of chlorofluorocarbons (CFCs) that used to be common has already been severely restricted in the EU because of their ozone depletion potential ; their use for new systems is no longer permitted. In addition to single-component refrigerants, refrigerant mixtures can also be used. A distinction azeotropic mixtures of such with temperature glide.
The simple refrigerant circuit consists of the four components compressor (K), condenser (C), throttle device (E) and evaporator (V). In the single-stage refrigeration system, a distinction is made between high-pressure and low-pressure side. The high pressure side (HD for short) extends from the pressure side of the compressor to the throttle element. The low pressure side (short: LP) comprises the refrigeration circuit behind the throttling up to the compressor inlet.
The condenser is either air or water cooled. The air-cooled condenser is a pipe arrangement whose surface is enlarged by shrunk-on copper or aluminum fins. A fan conveys the cooling air through the lamella pack. Small cooling devices such as refrigerators are limited to natural convection so that the fan can be omitted. The condensing temperature and thus also the coefficient of performance of the refrigeration system depends on the cooling air temperature. Evaporative condensers are technically more complex and require more maintenance, as very low condensing temperatures are possible here due to the partial evaporation of the water. Due to the additional evaporation of the water, a very low condensing temperature can be achieved, which in the limit case corresponds to the wet bulb temperature . If the air is dry, the condensing temperature can be below the ambient temperature. However, the evaporated cooling water must be replaced and treated. In the case of water-cooled condensers, the condensation heat is first transferred to the heat carrier water or a water-brine mixture. The cooling water is conveyed to the cooling tower via a centrifugal pump . If an open cooling water circuit is used, evaporative cooling can also be used here by trickling the water. In the cooling tower, the water is sprayed downwards through nozzle sticks. Air is forced through the cooling tower in countercurrent. There is an exchange of heat and part of the cooling water evaporates. Droplet separators are arranged at the air outlet in order to divert water droplets back into the cooling tower cup.
In refrigeration technology, flooded evaporators or heat exchangers with refrigerant injection (dry expansion) are used.
Dry expansion evaporator
Evaporators with dry expansion are equipped with a throttle device at the inlet, which is designed so that the refrigerant is completely evaporated and superheated at the outlet. The refrigerant is injected into the evaporator in droplet form.
Capillary tubes are used as throttling devices in small refrigerators ( refrigerators ), which are unregulated and therefore have a constant pressure loss coefficient . Capillary tubes can therefore only be used in devices whose cooling performance is almost constant. The refrigerant charge must be matched to the operating conditions so that no liquid, non-evaporated refrigerant is sucked in by the compressor.
In other cooling devices with dry expansion, only thermostatic expansion valves - often with external pressure compensation - are used. Mechanically working expansion valves work without auxiliary energy. A capillary sensor is attached to the pipeline at the outlet of the evaporator. The stroke of the injection valve and thus the injection mass flow are regulated via the pressure of the enclosed capillary liquid. The overheating of the escaping refrigerant is used as a control variable. Recently, electronic expansion valves have been used more and more, which enable a finer adjustment of the control behavior and improve the coefficient of performance of the refrigeration system.
Systems with flooded operation have a refrigerant tank on the low-pressure side, the so-called separator, in which liquid refrigerant is stored with a gas blanket under saturated steam conditions. The fill level of the separator is either regulated by a mechanical float regulator (high or low pressure float ) or a fill level control with a fill level probe and a control valve is installed in the liquid feed line. The pressure in the separator is kept at a certain value, which is determined by the required temperatures at the cooling points. The refrigerant compressors suck the gas out of the upper area of the separator, whereby the saturated steam equilibrium is maintained in the separator at the required temperature.
Ammonia is mostly used as the refrigerant . The use of the refrigerant CO 2 has so far been limited to special cases.
A variant of the flooded system is the thermosiphon refrigeration system. It can be used when a coolant circuit such as a cold water or brine circuit is used. A plate heat exchanger is installed below the separator . The lower inlet of the plate heat exchanger is connected to the separator and liquid refrigerant is applied. The refrigerant evaporates through the heat transfer from the brine and the refrigerant vapor reaches the upper space of the separator. Due to the plate heat exchanger used and a small reservoir volume in the separator, these refrigeration systems have low fill quantities (approx. 100 kg fill mass with a refrigeration capacity of 500 kW).
In refrigeration systems with many connected evaporators and with longer flow paths with load-dependent pressure losses, no satisfactory control can be achieved on the individual evaporators due to these disturbance variables. Pump systems are then used and the evaporators are then operated with flooding.
The pumps suck in refrigerant from the lower part of the separator (liquid phase) and increase the pressure of the refrigerant by typically 2 bar. The pressure is above the saturated steam pressure, which prevents the refrigerant from evaporating in the pipes to the cooling points. The refrigerant evaporated at the cooling points is returned to the separator.
Typical areas of application for ammonia plants with pump operation are:
The refrigerant charge in ammonia refrigeration systems with pump operation can be very large; they are between one and 200 tons. Systems with a filling quantity of more than 3 t are subject to the Federal Immission Control Act (BImSchG) and, due to the properties of hazardous substances, require special tests by approved monitoring bodies .
A notable advantage is the low price of ammonia compared to other refrigerants. Furthermore, the refrigerant relaxed in the separator represents cold energy stored, so that peaks can be covered and a failure of the compressor can be bridged for a certain time. Furthermore, depending on the system design, there is the option of operating the compressors in periods that are favorable in terms of the electricity tariff.
Even in systems with a low-pressure separator, facilities must be available to protect the compressor from liquid hammers (overfill protection, separator container on the suction side).
Depending on the conception of the system, additional facilities are necessary, which result from the structure and the mode of operation. Since air-cooled condensers have a small volume, a high-pressure collector is practically always installed in these systems. The filling quantities in the condenser and evaporator change under different operating conditions; these changes and creeping refrigerant leaks are compensated for by the accumulator effect. CFC / HFC-filled refrigerant circuits are sensitive to residual water content. In particular, water can freeze in the injection valves and endanger the functioning of the refrigeration system. For this reason, dryers filled with water-absorbing zeolites are often used .
Piston compressors , in particular, must not suck in liquid refrigerant, as liquid refrigerant can destroy the piston or cylinder head (seals) when it is compressed. If the supply of refrigerant drops to the compressor cannot be ruled out, liquid separators are installed on the suction side.
Optimization of the compression refrigeration process
To optimize refrigeration systems, the specific properties of the refrigerants must be taken into account. These are essentially:
Compared to the refrigerant ammonia (h v = 1369 kJ / kg at 0 ° C) the enthalpy of vaporization of the other common refrigerants is significantly lower. In particular, the new fluorinated refrigerants R404a (h v = 171 kJ / kg at 0 ° C) and R410a (h v = 221 kJ / kg at 0 ° C) have significantly lower enthalpies of vaporization. As a result, a large proportion of the refrigerant evaporates at the expansion valve. This effect becomes stronger the lower the evaporation temperature is set. An economiser is installed as a measure in the refrigerant circuit . The liquefied refrigerant is fed to a heat exchanger on the high pressure side . A partial flow of the refrigerant is relaxed and is conducted to the other side of the heat exchanger and thus cools the main flow of the refrigerant. In this way a strong subcooling of the refrigerant is achieved. The relaxed refrigerant is sucked in at an intermediate pressure connection of the compressor. In terms of design, screw or scroll compressors are suitable for this ; Piston compressors are not suitable for this circuit. A plate heat exchanger can be used as the heat exchanger .
The refrigerants propane (R290), R404a and R410 have an isentropic exponent that is close to 1. This has the advantage that the compression is approximately isentropic . In order to optimize the process, these refrigerants can be fitted with an internal heat exchanger, which acts as a subcooler for the liquid refrigerant and a superheater for the suction gas. The suction gas is heated by approx. 20 K. With this refrigeration system, a desuperheater can also be installed on the high pressure side in order to use the compression heat thermally. Without an internal heat exchanger, the compression temperature is too low for waste heat recovery. This circuit is independent of the type of compressor, but the heat exchanger must be designed for the different heat transfer coefficients on the liquid and suction gas side.
A further optimization stage is the combination of internal heat exchanger and economiser. The liquid refrigerant is first cooled by the suction gas. The economiser is used for further subcooling.
With the measures described, savings of 30 to 45% (variant with heat exchanger and economiser) should be possible.
Carbon dioxide as a refrigerant (R744) has almost the same isentropic exponent as ammonia and R22, so that the use of an internal heat exchanger is not necessary, as the compression end temperatures then become too high.
Despite the environmentally hazardous properties of the fluorinated hydrocarbons PFC ( greenhouse effect ), these refrigerants will be usable in the medium to long term under the current legal framework. Taking into account the energy costs related to the service life of the refrigeration system, the additional costs for more energy-efficient refrigeration system circuits should be weighed up. The optimal interconnection is strongly dependent on the thermodynamic properties of the refrigerant used.
Two-stage and multi-stage systems
Compression chillers are designed in several stages if the difference between condensing and evaporation pressure is too great or different evaporation temperatures are required.
Please note: single stage means 1 compressor, clearly separated high and low pressure sides.
If there is a very large difference between the condensing and evaporation pressures in a system (for example in a deep-freeze store), the final compression temperature increases rapidly. This has particularly negative effects on the service life of the compressor.
In order to relieve the compressor, the gaseous refrigerant is sucked in by a second compressor - the so-called booster or low-pressure compressor - and compressed from the evaporation pressure p 0 to a mean pressure p m . The “pre-compressed” refrigerant is now fed to the first compressor (high-pressure compressor) and completely compressed by it to the condensing pressure p c .
The refrigerant of the medium pressure p m expelled by the low-pressure compressor can also be cooled in between in order to lower the compression end temperature again. However, precondensation must be avoided here in order to protect the high-pressure compressor.
Two compressors are not absolutely necessary for this process. For example, in a six-cylinder piston compressor, four cylinders can be used as a low-pressure stage and two as a high-pressure stage. However, this makes separate suction chambers necessary for the high pressure stage.
Two-stage separator system
If different cooling temperature levels are required in a flooded system, it is energetically favorable to operate two separators with different evaporation temperatures. The lower pressure separator (low pressure separator) is fed with refrigerant from the higher pressure separator (medium pressure separator ) by means of a low pressure float or solenoid valve , so that a certain fill level is maintained in the LP separator. The housing of the LP float is connected to the LP separator in a communicating manner. Instead of the low-pressure float, a high-pressure float can be attached to the medium-pressure tank so that it communicates, so that it always maintains a certain level in the high-pressure separator.
The compressor, which is supposed to maintain the pressure in the low-pressure separator, pumps the refrigerant directly into the high-pressure separator. The two-stage circuit has the advantage that the compression end temperatures are significantly lower and thus the compressors in each stage are operated with a lower pressure ratio than with a single-stage compression. Due to the use of the refrigerant at different temperature levels, a higher coefficient of performance can be achieved in the medium pressure stage.
The fill level in the MD separator is regulated by a high pressure float. If there is refrigerant in the float housing, the float ball is raised and the liquid refrigerant expands to the mean pressure and flows into the separator. If several condensers are arranged in parallel, a high-pressure float should be installed for each.
The pressure in the medium pressure separator is maintained by the regulation of the HP compressor.
- Use of heat exchangers, energy efficiency reserves of refrigeration systems, Die Kälte- und Klimatechnik 3/2008
- Systems with pistons or displacers in the working volume are known as Stirling or Gifford-McMahon coolers.
- Systems that use valves and tanks instead of a piston / displacer are called pulse tube coolers or pulse tube coolers of the Stirling or Gifford-McMahon type.