SCHOOL OF ENGINEERING Year 2 Laboratories HPR Heat Pump/Refrigeration Cycle Lab MECH217: Thermodynamics Notation Symbol C COP COPR CoPHP H р Pin Pout P Qin Qout lc Qn RPM Tort Tc Th Tin Tout Tshaft Win W C g h mR mw S u Z ne 3 Meaning Specific heat of water Coefficient of Performance Coefficient of performance or refrigerator Coefficient of performance of heat pump. Enthalpy Pressure Pressure before throttle Pressure after throttle Power Heat into the system Heat out of the system Heat transfer rate in evaporator Heat transfer rate in condenser Revolutions per minute of compressor shaft Temperature Temperature of heat source Temperature of heat sink Water coolant inlet temperature Water coolant outlet temperature Torque on compressor shaft Work into system Work Speed of fluid Gravitational constant Specific enthalpy Mass flow rate of refrigerant Mass flow rate of water Specific entropy Specific internal energy Position along the vertical axis Isentropic efficiency of compressor Angular velocity of compressor shaft Unit J/kgK J Pa Pa Pa J/s J J J/s J/s K K K K K Nm J/s J m/s m/s² J/kg kg/s kg/s J/kgK J/kg m rad/s Abbreviations BDC HP R TDC 1 Introduction In thermodynamics we are commonly interested in the conversion of thermal to mechanical energy. The analysis of these conversion systems often involves thermodynamic cycles. Another important branch of thermodynamics is concerned with the conversion of mechanical to thermal energy and this takes place in reverse cycles. These systems are commonly known as refrigeration systems. Bottom dead centre Heat pump Refrigerator Top dead centre The Second Law of Thermodynamics states that, 'it is impossible to transfer heat from a region of low temperature to a region of high temperature without the aid of some external agency'. Heat pumps and refrigerators are devices that enable heat transfer to take place from a region of low temperature to one of high temperature, the external agency being compressor work in a vapour compression cycle (Win), see Figure 1. The refrigerator is a device for removing heat from a low temperature region. Its coefficient of performance, COPR is defined as the refrigeration rate (or duty) divided by the power input. The heat pump is a device whose prime duty is the delivery of heat to a higher temperature region. Its coefficient of performance CoPHP is defined as the rate at which heat is delivered divided by the power input. ● To avoid ambiguity, the power on which the above definitions for CoP are based must be clearly stated. The power may be: The electrical power required to drive the motor. The increase in enthalpy across the compressor The theoretical power of an ideal compressor. It is important to state clearly which method of energy measurement is being used. The equations to calculate the CoPs will be discussed in more detail in Section 2. 1.1 Background 1.1.1 The Vapour Compression Cycle As discussed in the thermodynamics lecture course, refrigerators and heat pumps operate on reversed vapour compression cycles, i.e. the working fluid (refrigerant) flows around the cycle in the opposite direction to that in a Heat Engine. The simplest thermodynamic cycle is the Ideal Vapour Compression Cycle shown in Figure 2 and Figure 3. Starting at the compressor inlet (1), with reference to Figure 2 and Figure 3, the saturated vapour refrigerant is compressed from a low pressure to a higher one and exits at (2). The high pressure and temperature (relatively) vapour enters the condenser at (2) and passes the condenser through where it gives up its heat and changes from a superheated vapour to a saturated liquid. The refrigerant exits the condenser at (3). The high pressure saturated liquid enters the throttle at (3), and is then expanded with constant enthalpy (no work or heat transfer) to a lower pressure and exiting the throttle at (4). Ambient R Cold Refrigerated Space 3 QH Throttle W in Warm Sink. TH QH Condenser Refrigerator Heat Pump Figure 1: Schematic of heat pump and refrigeration plant This is done so that a portion of the liquid evaporates causing the refrigerant to become colder due to part of its internal energy being used to provide the latent heat of vaporisation. The refrigerant then repeats the cycle when it enters the evaporator at (4), where the heat is transferred from the cold source and the refrigerant changes from a gas/liquid mixture to a saturated vapour. The terms 'condenser' and 'evaporator' are recognisable from their counterparts in a steam power cycle and are the technical terms used in refrigeration; it can be seen however that the evaporator is at the lower temperature in the cycle, and the condenser is at the higher. The energy transfers, as either heat or work, over the different process are listed below. It is important for the student to understand the direction of the energy transfer, so as to be able to make 'sanity checks' on the results. Evaporator Warm Space (e.g. house) 41 Heat transfer from a 'cold' source to refrigerant 12 Work done on fluid during compression 2 → 3 Heat transfer from refrigerant to a 'hot' sink in the condenser 3 →4 A constant enthalpy throttling process QH The Ideal Vapour Compression Cycle is characterised by the refrigerant entering the throttle as a saturated liquid, entering the compressor as a saturated vapour, and an isentropic compression process (reversible and adiabatic). It is also assumed there are no additional heat losses/gains around the cycle and no pressure losses. Qc Cold -Source. Te HP Qc 2 Constant Compressor Enthalpy EFTER 3 Cool/warm Source 1 Win Figure 2: The ideal vapour compression cycle 4 QH Гос 2 1 Win Entropy s Pressure p ● ● 3 Win Figure 3: Pressure - Enthalpy diagram for the ideal vapour cycle It is more convenient and normal practice to represent the cycle on a pressure vs. enthalpy (p-h) diagram as shown in figure 3. Showing the cycle this way makes it easy to see the energy transfers (Q, W). 4 Qc 3 QH 4 1.1.2 Practical vapour cycle The ideal vapour cycle is not a practical cycle for the following reasons: Compression is neither adiabatic nor reversible (non-isentropic). Vapour leaving the evaporator is usually made slightly superheated. Liquid leaving the condenser is usually slightly sub-cooled. 1.2 Aims and Objectives The objectives of the experiment are as follows: Subcooling at Throttle inlet 1 2 Figure 4 shows the p-h diagram for the practical cycle. A higher heat input and output found in practical vapour cycles, is shown on the graph as longer horizontal lines for processes 4-1 and 2-3. It also results in a higher work input, which reduces the CoP of the cycle. Constant Entropy The refrigerant is sub cooled in the condenser to make sure that the refrigerant has become completely fluid. Similarly, the refrigerant is superheated in the evaporator to make sure it is all gas. Refrigerant going into the compressor as part liquid and part gas, would damage the compressor. 1 Enthalpy h 2s 2 Non-isentropic compression Superheat at Compressor inlet Figure 4 Practical vapour cycle plotted on a p-h diagram