Refrigerator/Heat pump

Transcription

1 UMEÅ UNIVERSITY Department of Physics Leif Hassmyr Updates versions Joakim Ekspong Refrigerator/Heat pump

2 1 Task 1. Understand the principle of a refrigerator/heat pump and know its main components. 2. Determine the condenser, evaporator and compressor power. 3. Determine the coefficient of performance (c.o.p.) for: c.o.p. for cooling of cooling medium c.o.p. for heating of cooling medium real c.o.p. for the refrigerator real c.o.p. for the heat pump c.o.p. for a Carnot refrigerator c.o.p. for a Carnot heat pump v, real k, coolmedium v. coolmedium k, real k, Carnot v, Carnot Literature Course literature 1. Introduction 1.1 The principle of cold generation In order to clarify the principle of cold generation a well-known phenomena should be reminded. If you take some drops of a liquid that boils easy (for example ether or alcohol) in the hand it feels cold. The sensation will last until nothing of the liquid is left. The explanation is that all vaporization consumes heat. This heat of vaporization has to be brought to the liquid from the surrounding. In this case it is the hand that will lose heat. The above phenomenon also takes place in a refrigerator/heat pump. A working medium (cooling medium) with such properties that it condenses at the higher temperature and higher pressure, and vaporizes at the lower temperature and lower pressure is frequently used at many cooling processes. The heat absorption (cooling) occurs by that the cooling medium at low temperatures absorbs its heat of vaporization from the surrounding. The temperature at this point, the vaporization temperature, keeps constant. The reverse process is occurring when heat is emitted from the cooling medium during the condensation at higher temperature. This temperature is also constant during condensation and is called the condensation temperature.

3 2 1.2 Compression refrigerator / heat pump construction A simple refrigerator/heat pump system is composed of an evaporator, a compressor, a condenser and an expansion valve, and refrigerant piping and the drive motor for the compressor. In general, external power has to be added to drive the compressor motor and heat is either released from the cooling medium at the condenser to heat e.g. water, or adsorbed by the cooling medium at the evaporator to cool e.g. water. In fig.1, compression heat pump/refrigerator system is schematically plotted. Additional information about heat pumps can be found in appendix A. Fig.1 A schematic of a compressor driven heat pump/refrigerator system

4 3 2. Experimental setup 2.1 Description of our refrigerator/heat pump construction In fig.2, it is shown how the device used in this experiment is built. Fig.2 An overview schematic of the device that is used in this experiment. A. Compressor B. Condenser C. Expansion valve D. Evaporator E. Cooling medium container F. Flow meter G. Dryer H. Viewing glass I. Circulation pump J. Electric heater (2*1kW) P1-P6 Pressure gauges connected to the points 1-6 in the system. T1-T6 Pt-100 sensors for measuring the temperatures in the points 1-6 in the system. T7-T10 Pt-100 sensors for measuring the incoming and outgoing temperature in the water at the condenser and evaporator.

5 4 Pressure and temperature sensors At the refrigerator/heat pump there is a board with 3 pressure gauges for the low-pressure side as well as for the highpressure side and a flow meter for determination of the cooling medium flow. There are also10 Pt-100 sensors in the system, connected to a box with an alteration switch, by which one gradually can make a four-wire resistance measurement of the Pt-100 sensors using the multimeter HP3478A. The temperature can now be determined from the relation: T=((R-100)/0.385) C Fig.3 An image of the pressure gauges and the flow meter. Fig.4 An overview image of the device. The water tank with heater is shown in the upper left and the pressure gauges is shown in the upper right next to the alternation box for measuring the temperature. In the bottom part the evaporator is shown to the left and the condenser, the expansion valve as well as the compressor to the right. The switch for turning on the compressor is on the outside of the shielding grid. The bottom plate On the bottom plate there are a total hermetic electric compressor and a water-cooled coaxial condenser. The cooling water coil of the condenser is equipped with a pressure controlled water valve, by which the working temperature of the condenser can be controlled. On the high pressure side after the condenser there are dryers, viewing glass and a cooling medium container for the excess of cooling medium. On the bottom plate there is, in a green isolated tank, a coaxial evaporator, which cools the water that is pumped from the green isolated tank on the upper plate by a circulation pump. The upper tank is equipped with an electrical heater (2*1 kw) for desired heat load on the cooling circuit. The system is also equipped with a twinpressure control for continuous running at desired runningpressures/temperatures.

6 5 2.2 Detailed information about individual parts in the device The main parts of the construction are a total hermetic electric compressor, a water-cooled coaxial condenser, an expansion valve, a coaxial evaporator and a separate water tank with heater. On the high pressure side after the condenser there are a cooling medium container, dryer, viewing glass and a flow meter to measure the flow of the cooling medium Compressor Total hermetic compressor model CAJ4492A for Forane R134a. The compressor is connected to a twin-pressure control. Its high-pressure side should stop the compressor at a maximum pressure of 15 bar overpressure. (Setting: 14 bar overpressure.). Its low-pressure side should stop the compressor at a minimum pressure of 0 bar overpressure, in order to avoid the compressor to suck air into the system. Switch on can e.g. occur at switch off-pressure plus 1 bar. (Setting: Switch off=2 bar overpressure. (in order to avoid the temperature in the water-cooling system of the vaporizer to be less than 0 C) and difference-switch on=switch off + 1 bar) Condenser COAX-2050H water-cooled coaxial condenser. A water valve on the cooling water pipe controls the cooling of the condenser. By setting this, the temperature of the cooling water of the condenser can be adjusted and the construction can be used as a heat pump. The setting for Forane R134a corresponds to a water temperature of about 40 C Expansion valve To the character of the thermostatic expansion valve is that it maintains a constant overheating in the evaporator even when the vaporization pressure is varying. (Usually 5 C) overheating. The thermostatic expansion valve works as follows: The sensor (2) is attached after the vaporizer (1) and thus senses the temperature of the outgoing low-pressure pipe (3). The sensor, which is filled with some medium that boils easy, is through the capillary tube (4) connected to the expansion valve and a pressure P 1, which magnitude is determined by the sensor-temperature, thus acts on the upper side of the membrane (5). The vaporization pressure P 2 (first part of the evaporator) and the spring pressure P 3 acts on the lower side of the membrane. At equilibrium: P 1 =P 2 +P 3 Fig. 5 A schematic of the expansion valve used in the device.

7 Evaporator WKV1 water-cooled coaxial evaporator. Cooling medium volume 0.5 dm 3 and cooling water volume.4 dm Water tank The heat absorbed from the water in the coaxial evaporator at the vaporization of the cooling medium is replaced by that water from an isolated water tank, equipped with a 2*1 kw heater, which is circulated through the coaxial evaporator using a circulation pump. Supplied electric heat is measured by a clip-on ammeter/voltmeter. The water volume of the tank = 30 liter Dryer Next after the condenser is a dryer containing silica gel and molecular sieve. Its purpose is to absorb existing water from the cooling medium and to collect pollution particles Viewing glass After the drying filter is a viewing glass through which one can observe the cooling medium while the system is running. During normal running conditions one should have a uniform flow of the fluid with no bubbles. If bubbles are observed it can be due to for example lack of cooling medium, a cut down in the fluid pipe or a fall of pressure of abnormal size Flow meter For measuring of the flow of cooling medium a flow meter Brooks instrument model BR is installed. Fig.6 The flow meter for the cooling medium. Reading of the flow value is done at the upper edge of the floating body. 100% turn of the scale corresponds to a cooling medium flow = 128 kg/h.

8 7 2.3 Typical failures, technical problems Normally a refrigerator-and heat pump construction would work without any operation-technical problem. Next some examples of problems that can occur are given. Moisture in the system can occur if air leaks into the system. Moisture can lead to that the expansion valve freezes during operation and prevents the cooling medium (Forane R134a) from circulating normally. Usually the moisture is removed by changing the dryer. The most common problem with refrigerators is that leakage occurs. This can be detected by leak-searching equipment. An example of such equipment is Ion-pump detector CPS model L-780. It detects a cooling medium leakage by sucking in gas from the leaking area between two high-voltage electrodes, which ionizes the cooling medium gas, and electrical charge carriers are created. The instrument can detect the current that now occurs. In handling with cooling mediums of all types caution should be exercised when these appear in the form of liquid. If cooling medium liquid leaks out it will at its vaporization immediately adopt a temperature corresponding to the pressure of the atmosphere. In the cooling medium diagram for Forane R134a a temperature of -25 C is read. If drops of cooling medium hit the eyes, frostbite immediately occurs. If there is risk of cooling medium to jet out protective goggles should be used. 2.4 Cooling medium diagram-pressure/enthalpy diagram Liquid Critical point Liquid+Vapor Vapor A common diagram to use for refrigerators/heat pumps is the log(p) vs. h diagram. The abscissa = enthalpy (h). The ordinate = log pressure (p). From this diagram one can read off the pressure and change in enthalpy between different states of the cooling medium. Besides pressure-and enthalpy values one can also, from the diagram, read off temperature-, specific volume and entropy values of the cooling medium. Temp. lines Entropy lines Spec. vol. lines The x-marked lines in the diagram indicate how much of the cooling medium that is vapour. Ex. x=0.6 60%vapour, 40% liquid X-lines

9 8 log P bar T c h A h B h kj/kg The compressor The compressor is sucking in dry saturated Forane R134a-vapour with temperature T c, heat content h A and specific volume V A. The point A represents the low-pressure vapor that is sucked in. If now the compression of the vapor in the compressor could be reversible and adiabatic, ds=0, it would be represented by the line A-B. The point B with temperature T B and enthalpy h B represents the state of the cooling medium just before the condenser. log P bar P h T c T h The condenser The compression is followed by the condensation, which occurs at constant pressure P h. The line B-C thus represents the condensation. The point C with temperature T h and enthalpy h C is thus the state of the cooling medium just before the expansion valve. h C h A h B h kj/kg log P bar P h P c T h T c h C h A h B kj/kg h The expansion valve In the expansion valve the pressure of the cooling medium is decreased from P h to P c. This pressure decrease takes place without a change in enthalpy, in other words the line C- D represents the process. The point D with temperature T c and enthalpy h C thus represents the state of the cooling medium just before the vaporizer..

10 9 log P bar P h P c T c T h The vaporizer Finally, vaporization occurs in the evaporator at constant pressure P c, which is represented by the line D-A. When the cooling medium reaches the point A it is totally vaporized and it is then again sucked into the compressor for another compression etc. h C h A h B kj/kg h log P bar P h T h In order to get better efficiency and to avoid hammering in the compressor one usually supercool the cooling medium in the condenser and overheats it in the vaporizer. This gives the cyclic-process shown in the figure to the left. P c T c h C h A h B kj/kg h log P bar The processes shown above should be considered as ideal, while the real vaporization process gets a cycle in the diagram that is deformed due to the pressure drops in the cooling medium-circuit, heat exchange with the surroundings etc. h kj/kg

11 10 3. Questions to the laboratory exercise refrigerator/heat pump. Should be shown to the supervisor before the laboratory exercise begins. 1. Low temperatures are generated in a cooling battery (evaporator) by: a) a substance that boils easily is evaporated b) warm liquid is cooled in the expansion valve c) heat is removed from the condenser 2. The task of the compressor is to: a) separate low pressure side from high pressure side b) circulate the cooling medium c) overheat the vapour before the condenser 3. The task of the condenser is to: a) absorb heat from the cooling water b) emit heat to the cooling water c) keep up the pressure after the compressor 4. The task of the expansion valve is to keep the cooling medium: a) overheated before the compressor b) supercooled before the compressor c) overheated after the expansion valve 5. Outflowing cooling medium must be avoided because: a) it is extremely toxic b) the smell is annoying c) frost-bite results if the cooling medium jets out in liquid form and hits the eyes or the skin 6. The cooling medium diagram is split in three parts from left: a) liquid, overheated area, moisture area b) liquid, moisture area, overheated area c) moisture area, liquid, overheated area 7. The Carnot coefficient of performance is always: a) greater than the real b) smaller than the real c) equal to the real

12 11 4. Experimental procedure 1. Check the water hoses to the condenser are connected to a cold-water tap. Open the tap. (Note! The condenser-thermostat opens at T8>40 C.) 2. Check water hoses are connected between the vaporizer and the upper water tank, and that the three-way valves are in correct position for circulation between the upper water tank and the evaporator. 3. Start the circulation pump (compressor) so the water starts circulating between evaporator and upper water tank. 4. Turn on the electrical heater at desired heating power (given by the supervisor) and start the heat pump by turning the current-switch to position I. (If heavy vibrations occurs in the compressor, the heat pump should immediately be turned off by turning the currentswitch to position 0. The supervisor should then be informed.) The heating power is determined by a clip-on ammeter/voltmeter. 5. Let the heat pump run until the system is stabilized. Study for example the change in temperature in T1 (the temperature after the compressor). Discuss with the supervisor when the system is stabilized. When the system is stabilized current and voltage to the electric heater in upper water tank and to compressor motor is measured. Then the pressureand temperature-sensors and cooling medium flow meter are read. The flow of water is determined using a graduated measuring glass and a stopwatch. All measure results are kept in a record.

13 12 5. Results Power of electric heater Pressure of cooling medium 1. After compressor 2. After condenser 3. Before expansion valve 4. Before evaporator 5. After evaporator 6. Before compressor Temperature of cooling medium 1. After compressor 2. After condenser 3. Before expansion valve 4. Before evaporator 5. After evaporator 6. Before compressor Temperature of water 7. Water temperature condenser in 8. Water temperature condenser out 9. Water temperature evaporator in 10. Water temperature evaporator out Flows 1. Flow meter (cooling medium) 2. Water condenser 3. Water evaporator Compressor data Current to compressor Voltage to compressor (U*I=) W +1 bar = +1 bar = +1 bar = +1 bar = +1 bar = +1 bar = C C C C C C C C C C Kg/h l/s l/s A V

14 Power determinations 1. Condenser The output power of the condenser for heating the water is determined by measuring the mass flow of cooling water! ( H 2O) and the temperature increase over the condenser. P cond m cond ( T ( out ) T ( )) ( H 2O) = m! cond ( H 2O) CH in 2O cond cond W ( C H2 O = Specific heat capacity of water) At the cooling medium side the heat power in the condenser is determined by measuring the flow of cooling medium m!( R134a) and the change in enthalpy Δh cond. P ( R134a) = m! ( R134a) Δ h W cond 2. Evaporator cond Supplied heat power in the water tank is determined by measuring current and voltage to the electric heater using a clip-on ammeter/voltmeter. The power output for the vaporizer to cool the water is determined by measuring the temperature difference over the vaporizer and circulated flow of water! ( H 2O). m vap P heatpower U I vap ( ) = W P!"# H! O = m!"# (H! O) C!!! T!"# out T!"# (in) W At the cooling medium side the cooling power in the vaporizer is determined by measuring the flow of cooling medium m!( R134 a), and the change in enthalpy Δh vap. P ( R134a) = m!( R134a) Δ h W vap 3. Compressor vap At the cooling medium side the power of the compressor is determined from: P ( R134a) = P ( R134a) P ( R134a) W comp cond vap In order to determine the real coefficients of performance for the system one also has to consider losses between motor and compressor. The power of the compressor should then be replaced by the power of the motor, which is determined by using a clip-on ammeter and the measured voltage to the electric heater in the water tank. P motor = U I cos f W (cos f = 0.65)

15 Determination of the coefficient of performance Coefficient of performance for the cooling of the cooling medium k, coolmedium = P P vap comp ( R134a) ( R134a) = Δh Δh cond vap Δh vap Coefficient of performance for the heating of the cooling medium P ( R134a) Δh = ( R134a) Δh cond cond v, coolmedium = = k, coolmedium + Pcomp cond Δhvap Real coefficient of performance for the refrigerator 1 k, real = P O) vap ( H 2 P motor Real coefficient of performance for the heat pump v, real = P O) cond ( H 2 P motor Coefficient of performance for a Carnot refrigerator k, Carnot Tc = T T h c Coefficient of performance for a Carnot heat pump v, Carnot Th = T T h c

16 15 Refrigerator / heat pump CONTENTS: 1. Figure over the principle of refrigerator/heat pump with main parts included and an explanation of the function of the included parts. 2. Pressure/Enthalpy - diagram for the process. (Page 17) 3. Table over the power of condenser, evaporator and compressor. 4. Give a) c.o.p. for cooling of cooling medium k, coolmedium 5. Discussion of the result. c.o.p. for heating of cooling medium b) real c.o.p. for the refrigerator k, real real c.o.p. for the heat pump v, real c) c.o.p. for a Carnot refrigerator k, Carnot c.o.p. for a Carnot heat pump v, Carnot v. coolmedium

17 16

18 17 Appendix A The heat pump The heat pump is not a newly invented phenomenon, but in practice has been around for over 100 years. Heat pumps are made in different sizes and varying designs among which one could mention: 1. Heat pumps only for water heating. 2. Heat pumps only for heating of air. 3. Heat pumps only for heating of water heaters. 4. Heat pumps only for heating of hot tap water and water heaters. Using a heat pump one can thus transfer heat from a lower temperature (outdoor) to a higher temperature (indoor). The energy used to operate the heat pump will also generate some heat, which can be used in heating purpose. Different heating sources can be used as reservoir for the heat pump. For example: Solar energy The solar energy is utilized in solar collectors and accumulated in the water, which is circulated to a heat exchanger in the heat pump, and emits the absorption heat. Waste heat In cases where there is sufficient waste heat, the waste heat can be utilized and the temperature level through the heat pump to be raised to the extent necessary.

19 18 Waste water The waste water contains large amounts of low temperature energy. After the water has been purified some of its heat can be utilized and through a heat pump it can be distributed at a more usable temperature level. Heat pumps for this purpose becomes very large and distribution to the plant's surrounding buildings require substantial investment. Water Lake water and ground water can also be utilized as a heat source. By pumping it through the heat exchanger of the heat pump the water temperature is decreased and the heat absorbed by the heat pump can then be transferred to, for example, water heaters. An interesting point with regard to lake water is ice accumulation since the heat of fusion represents 334 kj/kg. That is the same amount of energy released when 1 kg water cools from 80 C to 0 C. Ground By placing tubes in the ground and circulate such as low temperature mixture of glycol and water the liquid absorbs heat from the ground. This heat can then be transferred to the heat pump system.

20 19 Air The heat content of outdoor air can also be transferred to the heat pump system. This is done in such a way that the air blown through a cooling coil whose temperature is about 10 C under air temperature. Upon passing through the battery the air leaves heat to the heat pump system An example of a commercial air heat pump system for hot tap water and water heaters. 1. The warm ventilation air is drawn into the heat pump. 2. The ventilation air supplied by a blower to the evaporator where the air is cooled to about 0 C. 3. The cold air leaves the house through the exhaust duct of the heat pump. 4. The heat energy absorbed from the ventilation air by the evaporator is transported by the compressor to a condenser, which transfers the heat energy to the hot water. 5. The heat energy in the hot water is transported through a circulator to the boiler where the energy is released as heat and hot water.