Date of last update: May-12 Ref: : Application Engineering Europe ENHANCED VAPOUR INJECTION (EVI) FOR ZH*KVE SCROLL COMPRESSORS 1 Introduction... 2 2 Principle of operation... 2 3 Capacity effect... 3 4 Operating envelope with vapour Injection for models ZH09-48KVE... 4 5 Multiple compressors configuration... 4 6 Economizer selection and application... 5 7 Liquid receiver... 8 8 Expansion valve and solenoid valve selection for injection line... 8 9 Injection expansion valve setting... 9 10 Injection line diameter... 9 11 Expansion valve selection for main evaporator... 9 12 Main liquid line components... 9 13 Line lengths and insulation... 10 14 Additional application points... 10 14.1 Operation without vapour injection... 10 14.2 Discharge temperature protection... 10 14.3 Current sensing relay... 10 15 Reference List of Copeland Related Technical Information... 10 1/10
1 Introduction This Technical Information describes the principle of operation and the system design of the Enhanced Vapour Injection for ZH*KVE Heat Pump optimized Copeland Scroll compressors, whatever they are used in single or tandem configuration. The vapour injection scroll compressor is for use with an economized vapour compression cycle heat pump. This cycle offers the advantages of more heat delivered and a better COP than with a conventional cycle. Both the heating capacity and the COP improvement are proportional to the temperature lift and this technology offers best results at high condensing operation where capacity and efficiency are most needed. Due to this increase in capacity, it is possible to specify a smaller displacement compressor for a given heating load. Additionally the cooling provided by injection inter-stage allows the operation of the compressor over a larger envelope compared to a conventional single stage model, providing higher heat delivery temperatures at low evaporating temperatures. 2 Principle of operation As shown in Figure 1, a portion of the condensed liquid, i, is expanded through an expansion valve into a counter flow plate heat exchanger, HX, which acts as a sub-cooler. Superheated vapour is then injected into the intermediate vapour injection port in the scroll compressor. The additional subcooling increases the evaporator capacity by reducing the temperature of the liquid from T LI to T LO, thus reducing its enthalpy. The additional condenser mass flow, i, increases the heating capacity by the same amount. Figure 1: Circuit diagrams showing the main circuit, mass flow rate m, and the economizer circuit, mass flow rate i Efficiency with vapour injection scroll compressor cycle is higher than that of a conventional single-stage scroll delivering the same capacity because the added capacity is achieved with less power. The incremental vapour created in the subcooling process is compressed only from the higher inter-stage pressure rather than from the lower suction pressure. The cooling effect is achieved by the addition of vapour to the compression process at the intermediate pressure and at temperature T VO, controlled by the HX expansion valve. Superheated vapour is injected into the scroll set at the intermediate point of the compression process, via two symmetrically positioned ports as shown on the left in Figure 2. The size and position of these ports have been optimized to ensure maximum COP and capacity benefit at typical operating conditions. The superheated vapour enters the compressor via an additional inlet connection on the compressor shell and flows to the injection ports of the fixed scroll via a tube as shown on the right. The tubing characteristics flexibility ensures that axial compliance is maintained. 2/10
Figure 2: Position of the injection ports in the scroll set and the internal tubing connecting the injection inlet with the scroll set 3 Capacity effect The vapour injection (VI) scroll is particularly suited to Air Source Heat Pumps (ASHP). Figure 3: Capacity variation and heat load characteristics The blue line in Figure 3 representing the VI scroll characteristic has a lower gradient than that of a conventional compressor. At low ambient conditions, a bigger capacity is available. On the other side, when the outdoor air temperature is above the design point, less capacity is delivered by the VI scroll compressor, resulting in less cycling than with a conventional compressor. The vapour injection scroll offers advantages particularly in air/water heating applications where the water temperatures need to be high and where domestic hot water is needed. Capacity can be further reduced if needed by switching off the vapour injection (see chapter 12.1). 3/10
4 Operating envelope with vapour Injection for models ZH09-48KVE Vapour injection + 10K Suction Superheat + 5K Economiser Superheat Vapour injection + 10K Suction Superheat + 5K Economiser Superheat Limited Operation < 2000 hours on compressor lifetime Figure 4: Operating envelope for vapour injection and standard Scroll compressors (considering condenser subcooling and temperature approach at the economizer of 5 K) Figure 4 shows the comparison of the compressor operating envelope for the standard and the VI scrolls, illustrating clearly the additional high temperature capability at evaporating temperatures below 5 C. For air-to-water heat pump application, an envelope extension may be required for high temperature water production by low outdoor temperature. This can be carried out by the use of wet vapour injection. For further information about wet injection, contact the Application Engineering department of Emerson Climate Technologies. 5 Multiple compressors configuration Multiple Vapour Injection compressors can share one economizer (HX) and one expansion valve (Figure 5). Due to the larger range of capacity brought by such configuration, an electronic expansion valve is recommended. Solenoid valves must be fitted to the individual vapour injection lines to close when the compressor is switched off, in order to avoid excessive quantities of liquid entering the compressor during standstill. Figure 5: Multiple compressors configuration when one injection expansion valve is sufficient. For further information about multiple ZH compressors possibilities, refer to the technical information C071703 Paralleling of ZH Copeland scroll compressors for heat pump applications. 4/10
6 Economizer selection and application For applications with single or multiple ZH heat pump scroll compressors, plate heat exchanger recommendations are available in Tables 1, 2 & 3. Details about required economizer capacity and saturated injection temperature T SI are provided in Copeland brand products Selection Software to enable independent sizing of the economizer when required, as shown in Figure 6 below. The input subcooling is the natural condenser subcooling, not the subcooling at expansion valve inlet. Natural subcooling Additional information on vapour injection Figure 6: Selection Software Version 7 screen details The values of the temperatures T LO (temperature liquid out), T LI (temperature liquid in) and T VO (temperature vapour out) are also provided. It should be noted that the benefit of natural subcooling, i.e. condenser subcooling, is limited to a maximum of approximately 5K. Additional natural subcooling leads to little further reduction in liquid temperature T LO. The compressor performances published in the Select Software are based on a temperature difference THX = T LO -T SI (T SI in bubble) of 5K at the economizer and an injection superheat of 5K. Downstream extraction shown in Figure 7(b) refers to taking the liquid for the economizer expansion device from the economizer liquid exit as shown. This method is sometimes proposed to ensure good sub cooling at the TXV inlet. While there is no overall heat gain or loss compared to the usual upstream extraction, it does mean that the injected mass flow, i, is passing through the economizer twice and incurring extra pressure drop on the liquid cooling side. This may result in the need for a larger economizer. Also, downstream extraction requires more connections and tubing on the sub-cooled liquid side, all of which need to be insulated to ensure minimal heat gain. For these reasons, downstream extraction is less preferable than upstream (Figure 7(a)). 5/10
Figure 7: (a) upstream liquid extraction (highly recommended) - (b) downstream The performance data published in Select Software are for the system design (a). With other designs the injected mass flow is changing and with this the performance of the compressor. The economizer must be installed vertically with the vapour entry at the bottom. The expansion valve should be positioned at a distance between 150 and 200 mm from the entry expansion and at a position not lower than inlet connection. Figure 8: Position of the expansion valve and bulb when using a plate heat exchanger The bulb of the expansion valve should be located 400 600 mm from the vapour outlet, preferably after a bend and on the inside as shown on the left in Figure 8. The position of the bulb relative to the section of the tube is illustrated. It should not be attached underneath the tube. External equalisation is not essential. The tubing between the expansion valve and the economizer entry may either be straight or include a bend as shown on the right in Figure 8. It is most important that the connection diameter at the entry to the economizer is small enough to induce the turbulence required for uniform distribution and evaporation. 6/10
The following table gives the recommended economizer size and inner diameter at the economizer ports. Table 1: SWEP heat exchanger recommendation for single compressor configuration Table 2: SWEP heat exchanger recommendation for tandem compressor configuration Vapour injection Liquid Alfa Laval Inlet Outlet Inlet Outlet S3 S4 S1 S2 ZH09KVE AC30-8EQ 1/4" H28 3/8" H27 1/4" H28 1/4" H28 ZH13KVE AC30-8EQ 3/8" H27 3/8" H27 3/8" H27 3/8" H27 ZH18KVE AC30-10EQ 3/8" H27 3/8" H27 3/8" H27 3/8" H27 ZH24KVE AC30-10EQ 1/2" H62 5/8" H22 1/2" H24 1/2" H24 ZH33KVE AC30-14EQ 1/2" H62 5/8" H22 1/2" H24 1/2" H24 ZH40KVE AC30-14EQ 5/8" H64 5/8" H22 5/8" H22 5/8" H22 ZH48KVE AC30-20EQ 5/8" H65 5/8" H22 5/8" H22 5/8" H22 Table 3: Alfa Laval heat exchanger recommendation for single compressor configuration The recommendations above give the best economizer performance with the lowest number of economizer models. If the diameter of the pipe work is not fitting to the economizer ports, use reducers into the economizer at the inlet and outlet. Use Emerson Climate Technologies recommended pipe work sizes (see below). If heat exchangers with recommended port sizes are not available, order the same economizer with larger ports and use reducers to fit required port diameters. 7/10
7 Liquid receiver A liquid receiver may be necessary to accommodate charge variations over the operating condition range and limit the condenser subcooling. It should always be fitted in the condenser outlet liquid line and not in the heat exchanger outlet liquid line (Figure 9). This is because vapour phase may be present in a receiver, and this is only possible with saturated liquid. Figure 9: Position of receiver 8 Expansion valve and solenoid valve selection for injection line The injection expansion device used in conjunction with the economizer will be preferably electronic to operate in the almost whole wide range of capacity that such EVI compressor models allow. A solenoid valve should be added if the expansion device does not completely close. This is to avoid liquid migration to the compressor during the off cycle. In single configuration, the solenoid valve may be in the liquid line or the vapour line. A liquid line position is usually preferred because the valve is smaller. When the compressor is rotating in reverse direction during the first seconds of the off cycle, the valve is to be installed in the vapour line. With multiple compressors used in parallel with one economizer it is necessary to install a solenoid valve in the single vapour line of each compressor. The following table may be used as a guide for selection of the injection expansion valve and liquid line solenoid valve in single configuration. A thermostatic expansion valve, due to its inherent smaller capacity range than an electronic valve will only operate adequately in a limited area of the envelope. The zone for which the TXV is recommended is specified below the table. Table 4: Electronic (EXV), thermostatic expansion valve (TXV) and liquid solenoid valve recommendations for the injection (single configuration only) If an electronic expansion valve is used, it should be a variable orifice type. Pulse modulating types are unsuitable because the internal volume of the economizer is low, and they will give rise to unstable operating conditions which may result in an unacceptable quantity of liquid overfeed. 8/10
9 Injection expansion valve setting The injection expansion valve will be set to maintain the injection superheat around 5K throughout the operating envelope. This is a good compromise not to inject any liquid refrigerant into the scroll and to have a sufficient cooling of the compressed gas and therefore limit the discharge temperature. 10 Injection line diameter Model Vapour injection line diameter The injection line diameter should be correctly sized for vapour with minimal pressure drops if optimum performance is to be maintained. The size should follow the size of the compressor injection connection: ZH09 KVE 3/8" ZH13 KVE 3/8" ZH18 KVE 3/8" ZH24 KVE 3/8" ZH33 KVE 1/2" ZH40 KVE 1/2" ZH48 KVE 1/2" Table 5: Vapour injection line diameter 11 Expansion valve selection for main evaporator When using a thermostatic expansion valve (TXV), it must be sized taking into account the appropriate liquid subcooling, and this will result in a smaller valve than would normally be the case. Air/Water application requires a broad range valve of the TCLE type. The Table 6 shows recommended electronic and thermostatic expansion valves for Air/Water (A/W) and Brine/Water (B/W) systems. Table 6: Electronic and thermostatic expansion valve recommendations for main evaporator (single application only) 12 Main liquid line components Main liquid line components also need to be sized taking into account the appropriate liquid subcooling, and recommendations are shown in the following tables. Where reversible systems are used either for defrost or for air conditioning, bi-flow filter driers should be installed. Model Liquid line diameter Liquid solenoid valve Filter drier Table 7: Main liquid line components selection (single application only) Filter drier (bi-flow) Sight glass ZH09KVE 1/4" 110RBT T2 ADK082S BFK052S MIA014 ZH13KVE 3/8" 200RB3 T3 ADK083S BFL163S MIA038 ZH18KVE 3/8" 200RB3 T3 ADK083S BFK163S MIA038 ZH24KVE 1/2" 200RB4 T3 ADK164S BFK164S MIA012 ZH33KVE 1/2" 200RB4 T3 ADK164S BFK164S MIA012 ZH40KVE 5/8" 200RB6 T5 ADK305S BFK305S MIA058 ZH48KVE 5/8" 200RB6 T5 ADK305S BFK305S MIA058 9/10
13 Line lengths and insulation Vapour lines between both expansion devices and the heat exchangers (evaporator and economizer) need to be kept as short as practical and well insulated. The economizer should also be insulated. 14 Additional application points 14.1 Operation without vapour injection The ZH KVE models may be operated without vapour injection when the evaporating temperature is above 5 C to give limited capacity control. Figure 10 shows the approximate percentage of heating capacity reduction obtained at various condensing temperatures when the vapour injection is shut off. The capacity reduction effect is limited, particularly at lower condensing temperatures, and since the COP is always higher with vapour injection it is usually preferable to maintain this mode of operation in all conditions. Figure 10: Approximate % heating capacity reduction obtained if changing to operation without VI 14.2 Discharge temperature protection High discharge temperature protection should always be fitted to each compressor as detailed in the application guideline C060209 scroll compressors for heat pump applications ZH12K4E to ZH11M4E, ZH09KVE to ZH48KVE. This ensures the compressor cannot overheat in the event of loss of vapour injection cooling. It may be incorporated into the heat pump control circuit using an appropriate sensor. Larger models (ZH24KVE to ZH48KVE) have built-in protection in which case it may be desirable to install an external sensor integrated with the control circuit to avoid tripping the motor protector. 14.3 Current sensing relay It is recommended that compressors with internal motor protection (ZH09/13/18) are fitted with a current sensing relay which is connected to close the solenoid valve in the event of motor trip. 15 Reference List of Copeland Related Technical Information C071703 Paralleling of ZH Copeland scroll compressors for heat pump applications C070806 Discharge temperature protection with ZH compressors 10/10