DE19620105A1 - Operation of refrigerating plant - Google Patents

Abstract

A procedure for operating a refrigerating unit has two speed- controlled compressors(2) run constantly to maintain the pressure in front of them and at the output of the evaporators (4) at most 33% below the equilibrium vapour pressure of the coolant. This is related to the temperature at these locations. To be more precise, the pressure in front of the compressors is controlled to be a value between 20% and 5% below the equilibrium vapour pressure of the coolant flowing into the compressors from the evaporators. The coolant temperature is at most 10 degC below the set point temperature of the coldest place(3). The overheating of the coolant in the evaporators is no more than 4 degC.

Description

The invention relates to a method for operating a device for supply several cooling points equipped with evaporators with cooling liquid, the cooling liquid fed to the evaporators via valves independent of the temperature profile in the evaporator is and with the evaporators several compressors are connected downstream, which compress the steam and a condenser to regenerate Supply coolant and part of the compressor is speed controlled.

Such devices are used in particular to store several cooling points in Supermarkets, slaughterhouses, hospitals and the like are constantly on a desired one Maintain temperature, which can be different for the individual cooling points. It is For example, it is possible to close display cases and freezers with different target temperatures combine, the lowest target temperature being determined by the freezer and typically around -20 ° C.

The evaporators provided in the individual cooling points are currently inadequate utilized. It is namely provided that the evaporation of the coolant only in about the first Third takes place after the inlet valve. By overheating the gaseous coolant in the The remaining, larger part of the evaporator is intended to discharge liquid coolant from the Evaporators can be avoided with certainty because liquid coolant the downstream com would destroy pressors. If the overheating, as usual, is 6 to 8 ° C, is also at considerable pressure fluctuations at the evaporator outlet a complete transition of the ensures liquid coolant in the gaseous state.

So that there is sufficient cooling of the cooling point despite the partial use of the evaporator comes, there must be a large temperature difference between the evaporator and the surrounding Room temperature of the cooling point can be maintained. This is because the removed Amount of heat essentially proportional to the temperature difference and that of liquid Coolant wetted evaporator surface is.

A low temperature of the coolant in turn requires that the pressure at the outlet of the Evaporator is kept low, otherwise the very cold liquid will not evaporate is guaranteed. In this context, it must be borne in mind that with the Evaporator inlet temperature of the coolant also the equivalent weight vapor pressure drops, which must be undercut so that the coolant in the Evaporator actually changes its physical state.  

The invention has for its object the whole evaporator for evaporation and use the entire evaporator surface. Thereby the tempera door difference to the cooling point can be kept low. A higher coolant temperature then allows the pressure at the inlet of the compressor to be kept relatively high and the Reduce compressor performance and thus overall energy consumption. An essential one Difficulty in solving this problem is that with such an interpretation the evaporator a slight increase in pressure at the outlet of the evaporator causes Coolant gets into the compressor and destroys it.

In order to avoid this danger, evaporators with thermostatic have become known Control valves continuously change the amount of coolant flowing into the evaporator. As The controlled variable is the temperature measured at several points in the evaporator used. Such thermostatic control valves are very expensive and also prone to failure. Their use is particularly uneconomical in larger cooling systems, since each one Refrigeration point must be equipped with such a valve including measuring devices.

In the present invention, starting from a simple cooling device without elaborate thermostatic control valves the risk of incomplete evaporation of the Coolant avoided when using the entire evaporator surface in that a constantly speed-controlled compressor is in operation to the pressure upstream of the compressors or at the outlet the evaporator is at most 30% below the equilibrium vapor pressure of the coolant in relation to the temperature in front of the compressors or at the pull-out of the evaporators.

For the reasons stated, the pressure at the outlet of the evaporator should be as high as possible, to save energy, on the other hand so low that the evaporation takes place quickly and on in any case so low that accidental exceedances of the equilibrium vapor pressure, and so that the liquefaction of the steam can be avoided with certainty. Preferably therefore provided that the pressure upstream of the compressors be between 80 and 95% the equilibrium vapor pressure of the inflow into the compressor or out of the evaporators escaping coolant is regulated.

The invention allows the difference between the evaporator inlet temperature and the Lowering the temperature of the cooling point to around 5 to 7 ° C, compared to the systems currently used this difference is 17 ° C. At the same time it is achieved that the cooling liquid overheats maximum in the evaporators is 4 ° C. The amount of 4 ° C is the upper limit  see. In test runs already carried out, the overheating could even reach about 1 ° C be restricted.

A major advantage of the invention is also that the process is simple built cooling device can be performed. In particular, it is provided that at least two speed-controlled compressors are provided, of which at least one is constantly in operation. By regulating the speed of the two compressors, their performance can be reduced be continuously adapted to the respective needs.

For a structurally simple design of the evaporator it is further provided that the Evaporators have solenoid valves, which a further valve with during operation constant cross section is connected. To adapt to the respectively necessary Cooling capacity can be provided so that the constant cross section of the valves different values can be set.

According to a preferred embodiment, it is finally provided that the A speed-controlled fan is assigned to the condensing device. This results in another way to vary the cooling capacity of the system.

Further features and details of the method according to the invention and of the preferred method The proposed device is explained in more detail below with reference to the drawings.

Show it:

Fig. 1 shows the schematically simplified structure of a refrigeration system for performing the method according to the invention, and

Fig. 2 is a pressure-temperature diagram to illustrate various operating conditions.

Fig. 1 shows schematically the essential elements of a refrigeration system which is suitable for performing the method according to the invention. The refrigeration system has two compressors 1 a and 1 b connected in parallel, which draw in refrigerant from a composite suction line 13 . The two compressors 1 a and 1 b are speed-controlled via respectively assigned frequency converters 2 a and 2 b. The compressors are designed so that a continuous performance adjustment in the range between the required maximum or minimum capacity of the refrigeration system is possible.

The refrigerant compressed in the compressors 1 a and 1 b is fed via a common line to a condensing device 8 , in which it is liquefied in direct heat exchange with the ambient air. Air is applied to the outer surfaces of the condensing device 8 via a fan 9 , this fan 9 being speed-controlled via a frequency converter 10 in order to be able to adapt the amount of cooling air supplied to the respective requirements. The liquefied coolant is subsequently fed to a collector 7 , from which the refrigerant is fed to the respective consumers.

In Fig. 1 only two consumers are shown in the form of the cooling points 3 a and 3 b for simplification. The refrigeration system shown, which is normally found in supermarkets, slaughterhouses, hospitals and the like. Like. Is used, but generally has a large number of cooling consumers, which is in principle unlimited, as long as the maximum cooling capacity of the system is adapted to it.

The individual cooling points 3 a and 3 b each have a simple solenoid valve 5 a and 5 b on their feed line, the actuation of which can interrupt or release the supply of the coolant, depending on the cooling requirement. The solenoid valves 5 a and 5 b have two settings for "open" and "closed". In the flow direction behind the solenoid valves 5 a and 5 b there are additional valves 6 a and 6 b, the cross section of which can be changed, for example, using an adjusting screw. The cross-sectional adjustment is particularly important when commissioning the refrigeration system. By expanding or narrowing the flow cross-section, the amount of coolant flowing into the downstream evaporators 4 a and 4 b of the cooling points 3 a and 3 b can be adjusted such that the evaporators with the solenoid valves 5 a and 5 b open almost completely with at least partially liquid Coolant is filled. The setting is not changed briefly during operation.

The coolant expanded at the expansion valves 6 a and 6 b subsequently changes into the evaporator 4 a and 4 b due to the cooling air directed against the outer surfaces of the evaporator and evaporates. Via the common suction line 13 , the gaseous coolant is finally fed back to the two compressors 1 a and 1 b, whereby the refrigeration cycle is closed.

The central control unit 11 is also essential for operation. On the basis of the pressure measured by both pressure transducers 12 and the temperature in the cooling points 3 a and 3 b, the central control unit 11 calculates the instantaneous cooling capacity requirement, after which the frequency converters 2 a and 2 b and 10 for controlling the compressors 1 a and 1 b or the fan 9 can be controlled accordingly.

The method according to the invention is now characterized in that at least one of the two speed-controlled compressors 1 a and 1 b is in constant operation. In cooperation with the pressure transducers 12 and the central control unit 11 , an extremely precise control of the pressure in the common suction line 13 is possible by running through at least one compressor for 24 hours a day. The pressure transducers react to fluctuations in the order of magnitude of 0.05 bar, which ensures an extremely narrow range of pressure fluctuations in the suction line 13 . Due to the fact that one of the compressors 1 a and 1 b is constantly in operation, any slight deviation can be corrected immediately.

The stable management of the pressure in the common suction line 13 is the prerequisite for the evaporators 4 a and 4 b in the cooling points 3 a and 3 b to be used to a much greater extent than in the known prior art. In known systems constructed similar to FIG. 1, as already mentioned at the beginning, only about the first third of the evaporators was used, ie the evaporators were only filled with at least partially liquid coolant in this area. In order to still be able to dissipate the necessary amount of heat, the inlet temperature of the coolant into the cooling points had to be kept correspondingly low. The coolant was overheated in the remaining substantially larger part of the evaporator in the gaseous state, so that even with larger pressure fluctuations in the suction line 13, the escape of liquid coolant from the evaporators could be avoided with certainty. The reason for the pressure fluctuations in the suction line in the prior art is the fact that the compressors, which are not or only partially speed-regulated, do not allow continuous cooling capacity adjustment. Furthermore, only continuous operation of at least one compressor allows immediate reaction to pressure fluctuations at any time.

From the pressure-temperature diagram shown in FIG. 2, the significant improvement of the method according to the invention compared to the prior art can be explained particularly clearly. The starting point is a target temperature T₀ which is to be reached in the cold store. In this case, the coldest temperature, i.e. that of the freezer, with a temperature of around -20 to -23 ° C is decisive for a large network of several refrigeration points.

In the prior art, in which only a small part of the evaporator is used, the coolant inlet temperature T _E2 in the evaporator must be chosen to be correspondingly low in order to be able to dissipate the necessary amount of heat. The pressure p 2 in the suction line upstream of the compressors must also be selected to be correspondingly low, so that complete evaporation of the coolant can be ensured. The pressure p₂ is about 10% below the equilibrium vapor pressure based on the inlet temperature T _{E2 of} the coolant in the evaporator. In the case of a stable operating situation, the coolant in the prior art is overheated at 6 to 8 ° C. and leaves the evaporator in a state 2 , ie at a temperature T _A2 .

In contrast, if one considers the physical states of the coolant when carrying out the method according to the invention, the first difference lies in the evaporator inlet temperature T _E1, which is much closer to the target temperature of the coldest cooling point. The pressure in the suction line leading to the compressor is correspondingly higher. The pressure p 1 is about 10% below the equilibrium vapor pressure based on the evaporator inlet temperature T _E1 according to the recommendations of the coolant manufacturers. The condition of the coolant at the evaporator outlet is marked with 1 . The diagram makes it clear that the process according to the invention results in significantly less overheating of the gaseous coolant in the evaporator, advantageously around 1 ° C.

If one compares the known state of the art with the method according to the present invention and considers the situation at the evaporator outlet, which is decisive for the risk of possible liquid entry into the compressor, it is found that the method according to the invention has a substantially lower pressure difference Equilibrium vapor pressure at the evaporator outlet enables. The pressure difference from p₂ to p _DA2 is a multiple of the pressure difference from p₁ to p _DA1 due to the better utilization of the evaporator surface can also be worked overall at a lower temperature and thus pressure level. This reduces the subsequent compressor output and thus energy consumption.

Claims (9)

1.Method for operating a device for supplying a plurality of cooling points equipped with evaporators with cooling liquid, the cooling liquid being supplied to the evaporators via valves which are independent of the temperature profile in the evaporator, and the evaporators being connected in series with several compressors which compress the steam and a condensing device for renewed use Supply of coolant, and part of the compressor is speed-controlled, characterized in that a speed-controlled compressor ( 2 a, 2 b) is constantly in operation in order to control the pressure upstream of the compressors ( 2 a, 2 b) or at the outlet of the evaporator (4 a, 4 b) at most 30% below the equilibrium vapor pressure of the coolant based on the temperature before the Verdichtem (2 a, 2 b) and on the extract from the evaporator (4 a, 4 b) to keep. 2. The method according to claim 1, characterized in that the pressure upstream of the compressors ( 2 a, 2 b) to a value between 20 and 5% below the equilibrium vapor pressure of the inflowing into the compressor ( 2 a, 2 b) or from the Evaporators ( 4 a, 4 b) flowing coolant is regulated. 3. The method according to any one of claims 1 or 2, characterized in that the temperature of the cooling liquid is at most 10 ° C below the target temperature of the coldest cooling point ( 3 a, 3 b). 4. The method according to any one of claims 1 to 3, characterized in that the superheating of the cooling liquid in the evaporators ( 4 a, 4 b) is at most 4 ° C. 5. Device for performing the method according to one of claims 1 to 4, characterized in that at least two speed-controlled compressors ( 2 a, 2 b) are provided, at least one of which is in constant operation. 6. Device according to claim 5, characterized in that all compressors are speed controlled.   7. Device according to claim 5 or 6, characterized in that the evaporators ( 4 a, 4 b) have solenoid valves ( 5 a, 5 b), which is followed by a further valve ( 6 a, 6 b) with a constant cross section during operation is. 8. Device according to claim 7, characterized in that the constant cross section of the solenoid valves ( 5 a, 5 b) downstream valves ( 6 a, 6 b) is adjustable to different values. 9. Device according to one of claims 5 to 8, characterized in that the condensing device ( 8 ) is assigned a speed-controlled fan ( 9 ).