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- EXTRA EDITION
For example, if the design discharge air temperature for a particular application is 25°F, and the evaporator is applied at a 10° temperature difference, then the saturation temperature of the refrigerant in the evaporator should be 15°. The goal of the vapor-compression cycle for this application would be to deliver the necessary mass flow of saturated liquid refrigerant at 15° to the evaporator inlet. The amount would be sufficient to transfer the design heat load from the refrigerated space to the refrigerant.
So, if the goal is to convert the refrigerant to the state necessary to be a useful heat transfer medium (the evaporation stage), the other three stages can be looked upon as a means to an end. They are all essential and each can be altered to improve efficiency. However, they perform no useful function other than preparing the refrigerant in its journey of transformation into something useful. Or do they?
Consider the compression stage. Every experienced technician knows how hot the discharge line can be. This heat is the waste byproduct from the vapor compression process and is normally rejected at the condenser. Heat is a form of energy. Rather than “throw it away” at the condenser, couldn’t it be used somewhere? Yes it can. If the proper controls and accessories are added to the system, this heat can be reclaimed and put to use.
The modern supermarket consists of several hundred feet of refrigerated display fixtures in the sales area, along with several medium- and low-temperature walk-in boxes for storage. It is not uncommon to employ large multiplexed compressor racks to provide the necessary Btu capacity to refrigerate the various fixtures in the store. These may be in the range of 30 to 40 tons each. Each one of these large capacity racks rejects a substantial amount of heat at the condenser as part of the vapor compression cycle.
Heat reclaim takes a portion of the condenser heat of rejection and uses it to heat water and/or the supply air in the building’s HVAC system. Instead of refrigerant flow from the compressor discharge to the condenser inlet, a three-way heat reclaim valve (Figure 1) diverts the flow of refrigerant from the compressor discharge to the heat reclaim coil inlet.
THREE-WAY VALVE OPERATION
The three-way valve is in the normal condensing mode when the three-way pilot valve’s solenoid coil is de-energized. This allows the main piston chamber to vent to the suction header, and the resulting pressure differential (discharge pressure acting on the bottom of the lower main piston, and suction pressure acting on top of the upper main piston) causes the main piston to shift upwards. The three-way valve’s upper outlet port (heat reclaim port) closes and allows full refrigerant flow to the bottom outlet port (normal condenser port).
Energizing the three-way pilot solenoid coil will shift the three-way valve into the heat reclaim mode. This simultaneously closes the pilot’s suction port and opens the pilot’s discharge port. The piston chamber is pressurized with discharge refrigerant vapor. The resulting pressure differential (from the discharge pressure, the opening spring acting on top of the larger diameter upper main piston, and discharge pressure acting on the bottom of the smaller diameter lower main piston) causes the main piston to shift downwards.
The three-way valve’s lower outlet port closes (normal condenser port) and allows full refrigerant flow to the upper outlet port (heat reclaim port).
If heat reclaim is used for heating the building’s supply air, the three-way valve will be controlled by the building’s HVAC controls. When the building temperature falls below its set point, the HVAC controls energize the three-way valve solenoid coil, shifting the main piston and allowing discharge vapor to flow into the heat reclaim coil. Heat would then be transferred from the discharge vapor to the ventilating supply air, passing through the heat reclaim coil’s fin-tube bundle.
In milder winter climates this may provide enough heating capacity to meet the comfort needs of the building; no additional heating equipment need be purchased, installed, and maintained. If not, a second stage of gas or electric heat can be employed.
RECLAIM COIL PUMPOUTWhen the building temperature is satisfied, the HVAC controls de-energize the three-way valve solenoid coil, bringing the system out of the heat reclaim mode.
Proper refrigerant management requires transferring the residual refrigerant remaining in the idle reclaim coil back to the active part of the system. More importantly, if it’s not removed, the remaining mixture of saturated liquid and vapor in the heat reclaim coil will be at the ventilating supply air temperature. When heat reclaim is required again, the superheated discharge vapor entering the reclaim coil will mix with the lower-temperature liquid refrigerant remaining in the reclaim coil tubes, causing it to boil quickly.
If the temperature difference between the entering discharge vapor and the idle saturated liquid is great enough, the resulting agitation may cause liquid hammer.
There are two ways in which the heat reclaim coil can be pumped out. Both transfer the high-pressure refrigerant from the heat reclaim coil to the suction header on the compressor rack. Pumpout is complete when the pressure in the heat reclaim coil is equal to the suction header pressure.
Some refrigerant always remains in the idle heat reclaim coil. Pumpout only allows the higher-pressure refrigerant in the coil to vent to the suction manifold, resulting in a drop in heat reclaim coil pressure. A substantial amount of the refrigerant is removed in the process; however, some vapor will remain.
Method 1: Pumping out the refrigerant from the idle heat reclaim coil can be accomplished with the Type B three-way valve, which has a bleed orifice in the main piston (Figure 1). De-energizing the Type B pilot solenoid coil causes the main piston to shift upwards, stopping the refrigerant flow to the heat reclaim port. The refrigerant in the heat reclaim coil will flow through the bleed orifice, into the upper pilot port, and proceed to the suction header. The bleed orifice provides the necessary flow path for the refrigerant to re-enter the system, eliminating the need for a dedicated pumpout solenoid valve.
Method 2: The Type C three-way valve, which does not have a bleed orifice in the main piston, requires the use of a dedicated pumpout solenoid. This typically would be a normally open solenoid valve, wired in parallel with the three-way valve, and piped from the outlet of the reclaim coil to the suction header. With the three-way valve de-energized (no heat), the normally open pumpout solenoid would be de-energized as well, allowing refrigerant to flow from the heat reclaim coil to the suction header.
LOCATION CHARACTERISTICSIn most instances the inlet to the heat reclaim coil is located at the top, with the outlet located at the bottom. This allows free-draining flow of refrigerant and reduces the possibility of any oil logging. If any liquid refrigerant is present in the heat reclaim coil during the pumpout mode, it will drain to the bottom or outlet fitting.
With the Type B three-way valve, pumpout allows refrigerant to flow from the valve’s outlet fitting (inlet of the heat reclaim coil) to the suction header. Because of the free-draining nature of the heat reclaim coil, liquid refrigerant and/or lubricant should never be present at this point. Therefore, it would be unusual to experience any liquid return to the suction header with this method of pumpout.
Nevertheless, it’s always good practice to install a restriction to the pumpout line upstream of the suction header. The resulting pressure drop will expand any liquid refrigerant that might be present into a vapor before entering the compressors.
With the Type C three-way valve, pumpout allows refrigerant to flow from the reclaim coil’s outlet fitting (bottom of the heat reclaim coil) to the suction header. If any liquid is present in the heat reclaim coil, it will accumulate at this location during the pumpout. Therefore, a restriction in the pump out line is a necessity with this method of pumpout.
REFRIGERANT TRAVELIf there is no refrigerant flow between two points in the system, the refrigerant will migrate to the colder of those two points.
For example, once the idle heat reclaim coil has been pumped out, there is a connection between the suction header and the heat reclaim coil through the pumpout line. If there is a possibility that the temperature of the idle heat reclaim coil (or hot water reclaim tank) would be lower than the saturated suction temperature of the compressor rack, the refrigerant would migrate to the colder heat reclaim coil. A tight-seating check valve should be added to the pumpout line to reduce refrigerant migration.
Note: You may think that a check valve would completely eliminate the possibility of refrigerant migration, and in theory it should. In reality, however, all check valves will experience some amount of leakage, albeit minor.
The type of seating material used also affects the check valve’s ability to provide as close to a leak-free seal as possible. A metal-to-metal seat may have an accepted leak rate of 750 cc/min, while a synthetic to metal seat may be as low as 10 cc/min.
This series will continue in the March 26 issue and will cover heat reclaim in series and parallel. For more information, visit www.parker.com.
Publication date: 03/12/2007