WO1999020958A1

WO1999020958A1 - Method for supplying make-up water from a source to the receptacle tank of an evaporative condenser and/or a cooling tower

Info

Publication number: WO1999020958A1
Application number: PCT/NL1998/000609
Authority: WO
Inventors: Cornelis Doomernik
Original assignee: York Refrigeration Aps
Priority date: 1997-10-23
Filing date: 1998-10-23
Publication date: 1999-04-29
Also published as: ES2175805T3; BR9813258A; ATE217410T1; EP1025404B1; AU9766698A; NL1007346C2; DE69805319T2; EP1025404A1; DE69805319D1; DK1025404T3

Abstract

Method for operating a refrigeration system (20) by means of a refrigerant circuit, in which to condensate the refrigerant in said circuit a cooling water circuit is used, incorporating an evaporative condenser (6), evaporation losses from said condenser (6) being topped up with the aid of make-up water being supplied to the condenser (6) via a heat exchanger (21) with the aid of which the liquid refrigerant of the refrigeration circuit (20) is subcooled.

Description

METHOD FOR SUPPLYING MAKE-UP WATER FROM A SOURCE TO THE RECEPTACLE TANK OF AN EVAPORATIVE CONDENSER AND/OR A COOLING TOWER

The present invention relates to a method for operating a refrigeration system

5 by means of a refrigerant circuit, in which to condensate the refrigerant in said circuit a cooling water circuit is used, incorporating an evaporative condenser, evaporation losses from said condenser being topped up with the aid of make-up water being supplied to the condenser via a heat exchanger with the aid of which the liquid refrigerant of the refrigeration circuit is subcooled.

10 Currently, modern industrial cooling plants are generally equipped with evaporative condensers or with a combination of cooling towers and water-cooled condensers, for the purpose of dissipating the heat of condensation. In cooling systems of this nature, the cooling circuit normally incorporates a liquid vessel from which refrigerant is supplied to an evaporator. In the latter, the refrigerant evaporates

15 and extracts the heat from the surrounding environment. The evaporated refrigerant is then supplied, by means of a compressor, to an evaporative condenser in which the refrigerant is condensed. Finally, the refrigerant is fed back to the liquid vessel. As has been stated, the evaporative condenser may also comprise a combination of a cooling tower and a water-cooled condenser.

20 A method in accordance with the type described in the preamble is known from Swiss Patent 392,576. According to this known method, cooling is carried out using a refrigerant which flows into a closed cooling circuit which includes an expansion device and a compressor and a heat exchanger. Cooling water which is heated in the heat exchanger and gives off the heat to an evaporative condenser also

25 flows through the heat exchanger.

In order to maintain the water level in the evaporative condenser used, makeup water is metered more or less continuously into the receptacle tank of the condenser. The make-up water is brought into the water circulation circuit via a recirculation pump.

30 The make-up water is generally fed in at a temperature of from 10 to 15°C

(springwater or tap water). This temperature is lower than the temperature of the water in the water receptacle tank. Since the mass ratio between the make-up water and the circuit water is generally at least 1:50, the relatively low temperature of the make-up water has scarcely any perceptible influence on the thermodynamic performance of the condenser. The fact that the make-up water is relatively cold is utilized according to the abovementioned Swiss patent by the fact that the make-up water is supplied from a source to the receptacle tank of the evaporative condenser via a heat exchanger in which the refrigerant, which flows in the cooling circuit from the liquid vessel to the evaporator, is cooled by the make-up water, which in this process is heated up.

The known method utilizes the relatively cold make-up water in the high-grade section of a cooling circuit. The actual heat exchange which takes place between the relatively cold make-up water and the refrigerant in the cooling circuit is not measured.

The object of the present invention is to provide a method in which the heat exchange which takes place between the relatively cold make-up water and the refrigerant can be used to gain more information about the performance and efficiency of the cooling system.

This object is achieved in the present invention by the fact that the method comprises the following steps:

- measuring the volume of make-up water supplied to the receptacle tank of the evaporative condenser and/or the cooling tower, - measuring the temperature difference in the make-up water before it is used to cool the refrigerant and after it has been used to cool the refrigerant,

- calculating the amount of heat which has been supplied to the water,

- measuring the difference in temperature of the refrigerant before it is cooled with make-up water and after it has been cooled with make-up water, - determining the refrigerant mass flow on the basis of the calculated amount of heat supplied to the water and the measured temperature difference in the refrigerant,

- measuring the suction pressure (Po) and the condenser pressure (Pc), respectively, of the refrigerant,

- using the measured values for the suction pressure (Po) and the condenser pressure (Pc) and the refrigerant mass flow to determine the instantaneous cooling capacity

(Qo),

- and displaying this instantaneous cooling capacity (Qo) as required.

In this case, it is advantageous that the make-up water from the source is fed to the condenser via the heat exchanger.

Therefore, using the method according to the present invention, the cooling capacity Qo can be calculated using a flow meter for the make-up water and by measuring the temperatures of the make-up water. Both the flow measurement of the make-up water and the temperature measurements can be carried out with a very high level of accuracy. This also means that the instantaneous cooling capacity Qo can be calculated very accurately. In devices according to the prior art, the cooling capacity has to be calculated on the basis of a measurement of the flow of refrigerant which flows from the liquid vessel to the evaporator. The fact that a meter has to be placed in this line means an additional restriction in this line. Moreover, it may be that refrigerant flows through this line not only in the liquid phase but also in the gas phase. Therefore, in practice, accurate measurement of this flow of refrigerant is difficult to carry out accurately. Therefore, the measures according to the present invention replace the complex, expensive and inaccurate measurement of the refrigerant flow with an accurate measurement, which is easy to carry out, of the make-up water flow and the temperature change of the make-up water and the refrigerant.

Furthermore, it is possible according to the invention for the method to comprise the following steps: - measuring the electric power (Pe) consumed for the purpose of operating the compressor(s) from the cooling system,

- calculating the instantaneous performance (COP, Coefficient Of Performance) of the cooling system by dividing the calculated instantaneous cooling capacity (Qo) by the value measured for the electric power (Pe) consumed, - displaying this instantaneous performance (COP, Coefficient Of Performance) as required.

The electric power (Pe) consumed can also be measured with a high level of accuracy. This means that the instantaneous performance, i.e. the COP, of the cooling system can be calculated in a simple manner. This in turn means that a user always has information about the instantaneous performance of the cooling plant.

Moreover, according to the present invention it is possible for this method to comprise the following steps:

- calculating the instantaneous performance (COP) of the cooling system, - changing process variables of the cooling system or the configuration of the cooling system,

- recalculating the instantaneous performance (COP) of the cooling system,

- repeating the above method steps as required. This makes it possible to adjust and regulate the plant iteratively. This is because the corresponding COP can be calculated after each change in the process variables or the configuration. If this COP is more favourable than the COP before the changes were carried out, the new configuration or the new process variables can be maintained. If the COP measured is less favourable, it is possible to return to the previous setting. These method steps can be repeated until an optimum COP is reached.

By calculating the COP, it is also possible to monitor the performance of the cooling system. If a specific COP is expected and the calculated COP differs considerably from this, a user is able to look for possible faults. Furthermore, it is possible, with the aid of the present invention, for the method to comprise the following steps:

- determining a maximum desired thickening factor for the water in the receptacle tank,

- using the value for Po, Pc and the refrigerant weight to determine the load of the condenser (Qc),

- determining the correct volume of make-up water on the basis of the calculated load of the condenser (Qc) and the predetermined thickening factor,

- and adjusting the volume of make-up water which flows to the receptacle tank per unit time as required. The major advantage of this method is that it optimizes the water consumption.

In cooling installations according to the prior art, it is generally the case that a flow of make-up water is continuously supplied to the receptacle tank of the evaporative condenser irrespective of the volume of make-up water which is actually required. In order to limit to a sufficient extent the maximum permissible increase in the quantity of salts in the water in the water receptacle tank (the so-called thickening factor), therefore, a continuous volume of waste water also flows from the receptacle tank to the sewer. By using the method according to the present invention as mentioned above, only the required volume of make-up water, which is adapted to the instantaneous performance of the cooling plant, is supplied to the water receptacle tank. As a result, excess and unnecessary water consumption is avoided.

Moreover, it is possible for the refrigerant to be injected from the liquid vessel to the said heat exchanger in a modulating manner. This is because the make-up water is fed to the water receptacle tank virtually continuously. If the refrigerant is then introduced from the liquid vessel into the cooling circuit via a line which is alternately open and closed, no refrigerant flows through the heat exchanger when the line is closed, and at those moments the possible cooling potential of the make-up water supplied is still lost. The present invention moreover relates to a cooling system intended to carry out the method according to the present invention.

The present invention is explained further with reference to the appended drawings, in which: Figure 1 shows an overview of an industrial cooling plant according to the prior art. Figure 2 shows a diagrammatic overview of a cooling plant according to the present invention.

Figure 3 shows the log P-H diagram of a possible cooling system according to the present invention in which NH₃ is used as the refrigerant.

Figure 1 diagrammatically depicts a cooling plant 1 which is much used in the prior art. This cooling system comprises a cooling medium circuit including a liquid vessel 2, an evaporator 3 and a screw-type compressor 4 and an oil cooler 5. The refrigerant is supplied to an evaporative condenser 6 with the aid of a screw-type compressor. This evaporative condenser is fed with the aid of water from a water receptacle tank 7. In order to maintain the water level in this water receptacle tank 7, make-up water is supplied, with the aid of a line 8, from a source (not shown).

In the evaporative condenser, there is generally a certain level of subcooling of the liquid refrigerant. As a result, this liquid flows into the liquid vessel of the cooling liquid at a few degrees below the condensation temperature.

In most modern cooling plants, the compression step in the refrigerant circuit is carried out by means of screw-type compressors. These are cooled with the aid of oil coolers to which liquid refrigerant is regularly supplied from the liquid vessel using a thermosyphon system. Part of the refrigerant will evaporate as a result of heat exchange with the oil coolers. The heated refrigerant is then returned to the liquid vessel.

Figure 2 shows a cooling system 20 according to the present invention. In addition to the components which have already been discussed in Figure 1, the cooling device 20 comprises a heat exchanger 21. The heat exchanger 21 is connected, on the one hand, to the feed line for springwater or tap water 22 and is connected, on the other hand, to the outlet line 23 from the liquid vessel 2. In the heat exchanger 21, the refrigerant will be cooled by the relatively cold make-up water before it is delivered to the evaporator 3. As a result of this measure, the cooling capacity of the cooling system 20 will increase. The relatively cold make-up water is used in the relatively "high-grade" section of the cooling circuit. This is because when a refrigerant is injected from the liquid vessel 2 into the evaporator 3, the pressure of the refrigerant will fall from the relatively high condenser pressure to the lower evaporator pressure. As a result, part of the refrigerant evaporates before it can contribute to the actual cooling process. That part of the refrigerant which evaporates in this phase is also known as the flash vapour. By now using the make-up water to cool the refrigerant which is flowing from the liquid vessel 2 to the evaporator, the amount of flash vapour will be reduced, so that the cooling potential of the refrigerant increases without employing additional energy other than the cooling potential of the make-up water. The contribution of the relatively cold make-up water in the section of the cooling process between the liquid vessel and the evaporator is much higher than if the make-up water were to be supplied directly to the water receptacle tank 7 of the condenser. This results from the fact that the mass ratio of the make-up water and the circulation water, which is generally at least 1:50, means that the relatively cold make-up water will have scarcely any perceptible influence on the thermodynamic performance of the condenser 6.

Moreover, it is possible for the refrigerant to be injected from the liquid vessel to the said heat exchanger in a modulating manner.

This is because the make-up water is fed to the water receptacle tank virtually continuously. If the refrigerant is then introduced from the liquid vessel into the cooling circuit via a line which is alternately open and closed, no refrigerant flows through the heat exchanger when the line is closed, and at those moments the possible cooling capacity of the make-up water supplied is still lost. It can be seen in Figure 2 that the cooling system 20 is equipped with two compressors. It is clear that the system may also comprise more compressors. Each of the compressors is provided with a measuring element 24, with the aid of which the electric power consumed by the compressors can be measured. The evaporative condenser 6 is also provided with a measuring element 25, in order to be able to measure the electric power consumed by the fan of the evaporative condenser 6.

Using the method according to the present invention, it is possible to measure the volume of make-up water which is supplied to the water receptacle tank 7 through the line 8. Moreover, the temperature of the make-up water is measured before the make-up water in the line 8 flows into the heat exchanger and after the make-up water has flowed out of the heat exchanger 21. These temperature measurements, as well as the flow measurement of the make-up water, together provide the total amount of heat supplied to the water. Moreover, in the line 23 it is possible to measure the difference in temperature of the refrigerant before the refrigerant in the line 23 is cooled by the make-up water and after it has been cooled with the make-up water. The refrigerant mass flow can be determined on the basis of these measurements and the calculated amount of heat supplied to the water.

Then, the suction pressure Po and the condenser pressure Pc in the cooling system are respectively measured. The instantaneous cooling capacity Qo can be determined using the measured value for the suction pressure Po and the condenser pressure Pc and the refrigerant mass flow determined. This therefore means that a cooling capacity of the cooling system 20 is known at all times. This instantaneous cooling capacity Qo can be displayed as required, for example on a control panel.

By additionally measuring the electric power Pe consumed which is required to operate the cooling system 20, it is moreover possible to determine the instantaneous performance, i.e. the COP, of the cooling plant. This COP is defined by dividing the instantaneous cooling capacity Qo by the value for the electric power Pe consumed. This instantaneous performance, i.e. the COP, can also be displayed as required.

In practice, having the instantaneous COP at the disposal of the operators is an important instrument for optimal control of the cooling system 20. It is possible, for example, to change the process variables of the cooling system. This can be achieved, for example, by switching one of the compressors 4 used in the cooling system 20 from full load to a part load. After these new process variables have been set, the COP of the cooling system can be determined again. Depending on the calculated value, it is possible to establish whether the COP has risen or fallen. If the COP has in fact risen, other process variables can then be changed. It is also possible, for example, to change the configuration by switching off one or more compressors. Process variables and/or the configuration of the cooling system can be varied until it is found that a higher COP can no longer be achieved. In other words, the present nvention makes it possible to carry out iterative control until an optimum COP is reached.

The present invention can also be used to optimize the supply of make-up water to the evaporative condenser. This is achieved as follows: on the basis of a one-off hardness measurement of the make-up water, a desired thickening factor is determined, for example 2. The thickening factor is the maximum permissible increase in the quantity of salts in the water which is situated in the water receptacle tank. During use of the system 20, the volume of water in the water receptacle tank

7 is measured continuously. Moreover, the difference in temperature of the water which flows via the feed line 22 to the heat exchanger 21 is measured before this water reaches the heat exchanger and after the water has flowed out of the heat exchanger 21. The amount of heat supplied to the water is calculated on the basis of this temperature measurement.

Then, the temperature difference in the refrigerant before it flows into the heat exchanger and after it has flowed out of the heat exchanger is measured. The refrigerant mass flow is determined using the calculated amount of heat which is supplied to the water in the heat exchanger 21.

Then, the suction pressure Po and the condenser pressure Pc of the refrigerant are respectively measured.

Using the values for Po, Pc and the refrigerant weight, the instantaneous cooling capacity Qo and the load of the condenser Qc are respectively determined. Then, the correct volume of make-up water is determined on the basis of the

Qc calculated and the predetermined thickening factor.

If there is any reason to do so, the volume of make-up water to be supplied which flows to the receptacle tank per unit time is adjusted on the basis of the calculated volume of make-up water.

The abovementioned actions may, of course, take place continuously, with the result that the correct volume of water is constantly adjusted and the liquid optimally subcooled. In order to explain the operation and advantage of the present cooling system, the following calculation example is given with reference to Figure 3: Assume that the cooling system 20 according to the present invention operates with NH₃, with the following parameters:

Qo = 1000 kW to = -10°C (evaporative temperature) tc = +35°C (condensation temperature).

The appropriate cooling circuit is illustrated in the log P-H diagram as represented in Figure 3. The refrigerant mass flow which circulates per hour is:

1000 x 3600 = 3312 kg/hr

(1449-362)

3312 (1680-362) Condensation heat to be dissipated = 1212 kW thermal

3600

According to the "rule of thumb" that approximately 3 kg of make-up water is consumed per kWh of condenser heat to be dissipated (thickening factor « 2), it follows that the make-up water consumption is 1212 x 3 = 3636 kg/h.

Suppose that the make-up water is heated from 12°C to 32°C, then it is possible to dissipate

3636 (32-12) 4.2 = 84.8 kW 3600 subcooling heat, i.e., per kg of circulating refrigerant,

84.8 x 3600 = 92 kJ, i.e. the refrigerant is subcooled to 15°C. 3312

The cooling capacity Qo increases from 1000 kW to (1000 + 84.8) = 1084.8 kW. This means that the cooling capacity increases by a good 8% without using additional energy apart from the cooling potential of the make-up water. Another possible advantageous application is in water-cooled cooling systems for air-conditioning purposes. These systems are generally combined with cooling towers. By positioning a liquid subcooler between the condenser and the evaporator upstream of the injection component (thermostatic expansion valve, high-pressure float or throttling port), it is possible to achieve the same resultant as that described above and in practice to achieve increases in capacity of from 8 to 10%. Naturally, the control would have to take place in the same way as that described above.

Claims

1. Method for operating a refrigeration system by means of a refrigerant circuit, in which to condensate the refrigerant in said circuit a cooling water circuit is used, incorporating an evaporative condenser, evaporation losses from said condenser being topped up with the aid of make-up water being supplied to the condenser via a heat exchanger with the aid of which the liquid refrigerant of the refrigeration circuit is subcooled, characterized in that the method comprises the following steps:

- measuring the mass flow of water supplied through the heat exchanger,

- measuring the temperature difference in the make-up water before it is used to cool the refrigerant and after it has been used to cool the refrigerant,

- calculating the amount of heat which has been supplied to the water,

- measuring the difference in temperature of the refrigerant before it is cooled with make-up water and after it has been cooled with make-up water,

- determining the refrigerant mass flow on the basis of the calculated amount of heat supplied to the water and the measured temperature difference in the refrigerant,

- using the measured values for the suction pressure (Po) and the condenser pressure (Pc) and the refrigerant mass flow to determine the instantaneous cooling capacity (Qo),

- and displaying this instantaneous cooling capacity (Qo) as required.

2. Method according to Claim 1, characterized in that make-up water from the source is fed to the condenser via the heat exchanger.

3. Method according to Claim 1 or 2, characterized in that the method comprises the following steps:

- measuring the electric power (Pe) consumed for the purpose of operating the cooling system,

- calculating the instantaneous performance (COP) of the cooling system by dividing the calculated instantaneous cooling capacity (Qo) by the value measured for the electric power (Pe) consumed,

- displaying this instantaneous performance (COP) as required.

4. Method according to Claim 1, 2 or 3, characterized in that the method comprises the following steps: - calculating the instantaneous performance (COP) of the cooling system,

- changing process variables of the cooling system or the configuration of the cooling system,

- recalculating the instantaneous performance (COP) of the cooling system, - repeating the above method steps as required.

5. Method according to one of the preceding claims, characterized in that this method comprises the following steps:

- determining a maximum desired thickening factor for the water in the receptacle tank, - using the value for Po, Pc and the refrigerant weight to determine the load of the condenser (Qc),

- and adjusting the volume of make-up water which flows to the receptacle tank per unit time as required.

6. Method according to one of the preceding claims, characterized in that the refrigerant is injected from the liquid vessel to the heat exchanger in a modulating manner.

7. Cooling system intended to carry out the method according to one of the preceding claims.