AE4-1381May 2011
ZW21 to ZW61KAE and ZW30 to ZW61KSE
Copeland Scroll® Water Heating Compressors
TABLE OF CONTENTS
Section Page Section Page
Introduction (2)
ZW**KA Application (2)
ZW**KS Application
Vapour Injection - Theory of Operation (2)
Heat Exchanger and Expansion Device Sizing (3)
Flash Tank Application (3)
Intermediate Pressure and Vapour Injection Superheat (3)
Application Considerations
High Pressure Cut-Out (4)
Low Pressure Cut-Out (4)
Discharge Temperature Protection (4)
Discharge Temperature Control (4)
Discharge Mufflers (4)
Oil Dilution and Compressor Cooling (4)
Electrical Considerations (5)
Brazing and Vapour Injection Line (5)
Low Ambient Cut-Out (5)
Internal Pressure Relief Valve (5)
Internal Temperature Protection (5)
Quiet Shutdown (5)
Discharge Check Valve (5)
Motor Protector (5)
Accumulators (5)
Screens (6)
Crankcase Heat-Single Phase (6)
Crankcase Heat-Three Phase (6)
Pump Down Cycle (6)
Minimum Run Time (6)
Reversing Valves (6)
Oil Type (7)
System Noise & Vibration (7)
Single Phase Starting Characteristics (7)
PTC Start Components (7)
Electrical Connections (7)
Deep Vacuum Operation (7)
Shell Temperature (7)
Suction & Discharge Fittings (7)
Three Phase Scroll Compressors (8)
Brief Power Interruptions ..........................................8Assembly Line Procedures
Installing the Compressor (8)
Assembly Line Brazing Procedure (8)
Pressure Testing (8)
Assembly Line System Charging Procedure (8)
High Potential (AC Hipot) Testing (9)
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Unbrazing System Components (9) Service Procedures
Copeland Scroll Functional Check (9)
Compressor Replacement After Motor Burn (10)
Start Up of a New or Replacement Compressor (10)
Figures
Brief Product Overview (11)
ZW21KAE Envelope (R-134a) (11)
ZWKAE Envelope (R-407C, Dew Point) (12)
ZWKA Envelope (R-22) (12)
ZWKS Envelope (R-22) (13)
ZWKSE Envelope (R-407C, Dew Point) (13)
Heat Pump with Vapour Injection – EXV Control (14)
Heat Exchanger Schematic (14)
Heat Pump with Flash Tank (15)
Possible Flash Tank Configuration (15)
Oil Dilution Chart (16)
Crankcase Heater (17)
Compressor Electrical Connection (17)
Scroll Tube Brazing (17)
How a Scroll Works (18)
Introduction
The ZW**KA and ZW**KS Copeland Scroll®compressors are designed for use in vapour compressio
n heat pump water heating applications. Typical model numbers include ZW30KA-PFS and ZW61KSE-TFP. This bulletin addresses the specifics of water heating in the early part and deals with the common characteristics and general application guidelines for Copeland Scroll compressors in the later sections. Operating principles of the scroll compressor are described in Figure 15 at the end of this bulletin.
As the drive for energy efficiency intensifies, water heating by fossil-fueled boilers and electric elements is being displaced by vapour compression heat pumps. Emerson Climate Technologies has developed two lines of special water heater compressors to meet the requirements of this demanding application. ZW**KA compressors are designed for lighter duty applications where the ambient temperature does not fall below 0°C and where lower water temperatures can be accepted as the ambient temperature falls. ZW**KS compressors are equipped with a vapour injection cycle which allows reliable operation in cold climates with significantly enhanced heating capacity, higher efficiency, and minimal requirement to reduce water outlet temperatures. Figure 1gives a brief product overview.
Water heating is characterized by long operating hours at both high load and high compression ratios. Demand for hot water is at its highest when ambients are low and when conventional heat pu
mp capacity falls off. On the positive side, the system refrigerant charge is usually small, so the risk to the compressor from dilution and flooded starts will usually be lower than in split type air-to-air heat pumps.
Water heaters must operate in a wide range of ambient temperatures, and many systems will require some method of defrost. Some systems such as Direct Heating, Top Down Heating or Single Pass Heating operate at a constant water outlet temperature with variable water flow. Others such as Recirculation Heating, Cyclic Heating or Multipass Heating use constant water flow with the water outlet and inlet temperatures both rising slowly as the storage tank heats up. Both system types need to cope with reheating a tank where the hot water has been partially used, and reheating to the setpoint temperature is required. More complex systems deliver water at relatively low temperatures for under-floor heating circuits and are switched over to sanitary water heating a few times per day to provide higher temperature water for sanitary use. In addition, some countries have specific water temperature requirements for legionella control.
ZW**KA Application
The application envelopes for ZW**KA compressors are shown in Figures 2 - 4.
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Appropriate system hardware and control logic must be employed to ensure that the compressor is always operating within the envelope. Small short-term excursions outside the envelope are acceptable at the time of defrost when the load on the compressor is low. Operation with suction superheat of 5 -10K is generally acceptable except at an evaporating tem-perature above 100C when a minimum superheat of 10K is required.
ZW**KS Application
The ZW**KS* vapour-injected scroll compressors differ from ZW**KA models in many important details:• Addition of vapour injection
• Significantly different application envelopes
• Some differences in locked rotor amps (LRA), maximum continuous current (MCC), and
maximum operating current (MOC) – see
nameplates
The application envelopes for ZW**KS compressors are shown in Figures 5 and 6.
Vapour Injection – Theory of Operation Operation with vapour injection increases the capacity of the outdoor coil and in turn the capacity and efficiency of the system – especially in low ambient temperatures. A typical schematic is shown in Figure 7. A heat exchanger is added to the liquid line and is used to cool the liquid being delivered to the heating expansion device. Part of the liquid refrigerant flow is flashed through an expansion valve on the evaporator side of the heat exchanger at an intermediate pressure and used to subcool the main flow of liquid to the main expansion device. Vapour from the liquid evaporating at intermediate pressure is fed to the vapour injection port on the ZW**KS compressor. This refrigerant is injected into the mid-compression cycle of the scroll compressor and compressed to discharge pressure. Heating capacity is increased, because low temperature liquid with lower specific enthalpy supplied to the outdoor coil increases the amount of heat that can be absorbed from the ambient air. Increased heat absorbed from the ambient increases the system condensing temperature and in turn the compressor power input. The increase in power input
also contributes to the improvement in the overall heating capacity.
Vapour Injection can be turned on and off by the addition of an optional solenoid valve on the vapour injection line on systems using a thermostatic expansion valve. Alternatively, an electronic expansion valve can be used to turn vapour injection on and off and to control the vapour injection superheat. A capillary tube is not suitable for controlling vapour injection.
The major advantage of the electronic expansion valve is that it can be used to optimise the performance of the system and at the same time control the discharge temperature by injecting “wet vapour” at extreme operating conditions.
The configurations and schematics shown are for reference only and are not applicable to every system. Please consult with your Emerson Application Engineer.
Heat Exchanger and Expansion Device Sizing Various heat exchanger designs have been used successfully as subcoolers. In general they should be sized so that the liquid outlet temperature is less than 5K above the saturated injection temperature at the customer low temperature rating point. At very high ambient temperatures, it will normally be beneficial to turn vapour injection off to limit the load on the compressor motor. Application Engineering Bulletin AE4-1327 and Emerson Climate Technologies Product Selection Software can be used to help size the subcooling heat exchanger an
d thermal expansion valves, but selection and proper operation must be checked during development testing. Plate type subcoolers must be installed vertically with the injection expansion device connected at the bottom through a straight tube at least 150mm long to ensure good liquid distribution. See the schematic in Figure 8. Flash Tank Application
A possible flash tank configuration is shown in Figure
9. This particular configuration is arranged to have flow through the flash tank and expansion devices in heating, and it bypasses the tank in defrost mode. The flash tank system works by taking liquid from the condenser and metering it into a vessel through a high-to-medium pressure expansion device. Part of the liquid boils off and is directed to the compressor vapour injection port. This refrigerant is injected into the mid-compression cycle of the scroll compressor and compressed to discharge pressure. The remaining liquid is cooled, exits from the bottom of the tank at intermediate pressure, and flows to the medium-to-low pressure expansion device which feeds the outdoor coil. Low temperature liquid with lower specific enthalpy increases the capacity of the evaporator without increasing mass flow and system pressure drops.
Recommended tank sizing for single compressor application in this size range is a minimum of 200
mm high by 75 mm in diameter with 3/8 in. (9.5mm) tubing connections, although it is possible to use a larger tank to combine the liquid/vapour separation and receiver functions in one vessel. A sight tube (liquid level gauge) should be added to the tank for observation of liquid levels during lab testing. See schematic diagram Figure 10 for clarification.
It is important to maintain a visible liquid refrigerant level in the tank under all operating conditions. Ideally the liquid level should be maintained in the 1/3 to 2/3 full range.
Under no circumstances should the level drop to empty or rise to a full tank. As the tank level rises, liquid droplets tend to be swept into the vapour line leading to “wet” vapour injection. Although this can be useful for cooling a hot compressor, the liquid quantity cannot be easily controlled. Compressor damage is possible if the tank overflows. If liquid injection is required for any reason, it can be arranged as shown in Figures 7 and 9.
Since liquid leaves the tank in a saturated state, any pressure drop or temperature rise in the line to the medium-to-low pressure expansion device will lead to bubble formation. Design or selection of the medium-to-low pressure expansion device requires careful attention due to the possible presence of bubbles at the inlet and the low pressure difference available to drive the liquid into the
evaporator. An electronic expansion valve is the preferred choice. Intermediate Pressure and Vapour Injection Superheat
dawson cdPressure in the flash tank cannot be set and is a complex function of the compressor inlet condition and liquid condition at the inlet of the high-to-medium pressure expansion device. However, liquid level can be adjusted, which in turn will vary the amount of liquid subcooling in the condenser (water to refrigerant heat exchanger) and vary the injection pressure. Systems with low condenser subcooling will derive the biggest gains by the addition of vapour injection. Systems operating with high pressure ratios will show the largest gains when vapour injection is applied. Such systems will have higher vapour pressure and higher injection
mass flow. Intermediate pressures in flash tank and heat exchanger systems should be very similar unless the subcooling heat exchanger is undersized and there is a large temperature difference between the evaporator and the liquid sides. Vapour exiting a flash tank will be saturated and may pi
ck up 1 - 2K superheat in the vapour line to the compressor. Vapour injection superheat cannot be adjusted on flash tank systems. Heat exchanger systems will be at their most efficient when the vapour injection superheat is maintained at approximately 5K.
APPLICATION CONSIDERATIONS
High Pressure Cut Out
If a high pressure control is used with these compressors, the recommended maximum cut out settings are listed in Figure 1. The high pressure control should have a manual reset feature for the highest level of system protection. It is not recommended to use the compressor to test the high pressure switch function during the assembly line test.
h2so4Although R-407C runs with higher discharge pressure than R-22, a common setting can be used. The cutout settings for R-134a are much lower, and the switches must be selected or adjusted accordingly.
Low Pressure Cut Out
A low pressure cut out is an effective protection against loss of charge or partial blockage in the syst
em. The cut out should not be set more than 3 - 5K equivalent suction pressure below the lowest operating point in the application envelope. Nuisance trips during defrost can be avoided by ignoring the switch until defrost is finished or by locating it in the line between the evaporator outlet and the reversing valve. This line will be at discharge pressure during defrost. Recommended settings are given in Figure 1. Discharge Temperature Protection
Although ZW compressors have an internal bi-metal Therm-O-Disc®(TOD) on the muffler plate, external discharge temperature protection is recommended for a higher level of protection and to enable monitoring and control of vapour injection on ZW**KS* models. The protection system should shut down the compressor when the discharge line temperature reaches 125°C. In low ambient operation, the temperature difference between the scroll center and the discharge line is significantly increased, so protection at a lower discharge temperature, e.g. 120°C when the ambient is below 0°C, will enhance system safety. For the highest level of system protection, the discharge temperature control should have a manual reset feature. The discharge sensor needs to be well insulated to ensure that the line temperature is accurately read. The insulation material must not deteriorate over the expected life of the unit.
Discharge Temperature Control
Some systems use an electronic expansion valve to control the vapour injection superheat and a thermistor to monitor the discharge temperature. This combination allows the system designer to inject a small quantity of liquid to keep the discharge temperature within safe limits and avoid an unnecessary trip. Liquid injection should begin at approximately 115°C and should be discontinued when the temperature falls to 105°C. Correct functioning of this system should be verified during system development. It is far preferable to use liquid injection into the vapour injection port to keep the compressor cool rather than inject liquid into the compressor suction which runs the risk of diluting the oil and washing the oil from the moving parts. If some operation mode requires liquid injection but without the added capacity associated with “wet” vapour injection, a liquid injection bypass circuit can be arranged as shown in Figures 7 and 9.
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期盼Caution: Although the discharge and oil temperature are within acceptable limits, the suction and discharge pressures cannot be ignored and must also fall within the approved application envelope.Discharge Mufflers
Discharge mufflers are not normally required in water heaters since the refrigerant does not circulate within the occupied space.
Oil Dilution and Compressor Cooling
The oil temperature diagram shown in Figure 11is commonly used to make a judgment about acceptable levels of floodback in heat pump operation. Systems operating with oil temperatures near the lower limit line are never at their most efficient. Low ambient heating capacity and efficiency will both be higher if floodback is eliminated and the system runs with 1 - 5K suction superheat. Discharge temperature can be controlled by vapour injection, “wet” vapour injection, or even liquid injection if necessary. In this situation, the oil temperature will rise well into the safe zone, and the compressor will not be at risk of failure from diluted oil. The oil circulation rate will also be reduced as crankcase foaming disappears. Special care needs to be taken at the end of defrost to ensure that the compressor oil is not unacceptably diluted. The system will resume heating very quickly and bearing loads will
access2000下载increase accordingly, so proper lubrication must be ensured.
Electrical Considerations
Motor configuration and protection are similar to those of standard Copeland Scroll compressors. In some cases, a larger motor is required in the ZW**KS* models to handle the load imposed by operating with vapour injection. Wiring and fuse sizes should be reviewed accordingly.
Brazing the Vapour Injection Line
The vapour injection connection is made from copper coated steel, and the techniques used for brazing the suction and discharge fittings apply to this fitting also. Low Ambient Cut-Out
A low ambient cut-out is not required to limit heat pump operation with ZW**KS compressors. Water heaters using ZW**KA compressors must not be allowed to run in low ambients since this configuration would run outside of the approved operating envelope causing overheating or excessive wear. A low ambient cut-out should be set at 0°C for ZW**KA models
In common with many Copeland Scroll compressors, ZW models include the features described below: Internal Pressure Relief (IPR) Valve
All ZW compressors contain an internal pressure relief valve that is located between the high side an
d the low side of the compressor. It is designed to open when the discharge-to-suction differential pressure exceeds 26 - 32 bar. When the valve opens, hot discharge gas is routed back into the area of the motor protector to cause a trip.
Internal Temperature Protection
The Therm-O-Disc® or TOD is a temperature-sensitive snap disc device located on the muffler plate between the high and low pressure sides of the compressor. It is designed to open and route excessively hot discharge gas back to the motor protector. During a situation such as loss of charge, the compressor will be protected for some time while it trips the protector. However, as refrigerant leaks out, the mass flow and the amperage draw are reduced and the scrolls will start to overheat.
A low pressure control is recommended for loss of charge protection in heat pumps for the highest level of system protection. A cut out setting no lower than 2.5 bar for ZW**KA* models and 0.5 bar for ZW**KS* models is recommended. The low pressure cut-out, if installed in the suction line to the compressor, can provide additional protection against an expansion device failed in the closed position, a closed liquid line or suction line service valve, or a blocked liquid line screen, filter, orifice, or TXV. All of these can starve the compressor for refrigerant and result in compressor failure. The lo
w pressure cut-out should have a manual reset feature for the highest level of system protection. If a compressor is allowed to cycle after a fault is detected, there is a high probability that the compressor will be damaged and the system contaminated with debris from the failed compressor and decomposed oil.
If current monitoring to the compressor is available, the system controller can take advantage of the compressor TOD and internal protector operation. The controller can lock out the compressor if current draw is not coincident with the contactor energizing, implying that the compressor has shut off on its internal protector. This will prevent unnecessary compressor cycling on a fault condition until corrective action can be taken.
Quiet Shut down
All scrolls in this size range have a fast acting valve in the center of the fixed scroll which provides a very quiet shutdown solution. Pressure will equalize internally very rapidly and a time delay is not required for any of the ZW compressors to restart. Also refer to the section on “Brief Power Interruption”. Discharge Check Valve
A low mass, disc-type check valve in the discharge fitting of the compressor prevents the high side,
high pressure discharge gas from flowing rapidly back through the compressor. This check valve was not designed to be used with recycling pump down because it is not entirely leak-proof.
Motor Protector
Conventional internal line break motor protection is provided. The protector opens the common connection of a single-phase motor and the center of the Y connection on three-phase motors. The three-phase protector provides primary single-phase protection. Both types of protectors react to current and motor winding temperature.
Accumulators
The use of accumulators is very dependent on the application. The scroll’s inherent ability to handle liquid refrigerant during occasional operating flood back situations often makes the use of an accumulator unnecessary in many designs. If flood back is excessive, it can dilute the oil to such an extent that
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