Heat Reclaim in Series and Parallel

March 26, 2007
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Figure 1. (Click on the image for an enlarged view.)

Editor’s note: This is the second in a three-part series that began in the March 12 issue with a discussion of heat reclaim in three-way valve operation.

Heat reclaim can be accomplished with either a series or parallel method. In the series approach, (Figure 1) when a finned tube heat exchanger is used it is properly termed a heat reclaim coil. It is not a heat reclaim condenser.

Its purpose is to desuperheat the discharge vapor only and therefore its Btu capacity is much less than that of the outdoor condenser averaging somewhere between 30-50 percent of the total heat of rejection.

In the series method, when the three-way valve shifts into the heat reclaim mode, the refrigerant discharge vapor flows from the compressor, through the tubes in the heat reclaim coil, and finally to the inlet of the normal condenser. Heat from the refrigerant discharge vapor is transferred to the ventilating air and is subsequently distributed through the ducting system providing heat for the building.

Because the heat reclaim coil only removes some portion of the superheat content of the refrigerant discharge vapor, the normal condenser must remove the remainder of heat content necessary to convert the vapor into liquid refrigerant.

A hot water reclaim tank (also shown in Figure 1) can be substituted for the heat reclaim coil and will serve to preheat the water supply before entering the hot water heater. Up to 60 percent of the heat normally rejected at the condenser can be transferred to the water. This results in an 80°F temperature increase, totaling approximately 65 gallons per hour per 10 tons of refrigeration capacity.

Note: Some applications use plate heat exchangers in place of hot water reclaim tanks.

Since the series method of heat reclaim utilizes a heat exchanger sized to desuperheat the discharge vapor only, there should never be any liquid refrigerant in the heat reclaim coil. While some amount of vapor might be expected to condense during the pump-out period, it should never be present at the point of pump out (heat reclaim coil inlet). As mentioned above, a restriction in the pump-out line should not be required, however, it is always a good practice to install one.

Figure 2. (Click on the image for an enlarged view.)

PARALLEL HEAT RECLAIM

There are some applications where a parallel piped heat reclaim system may offer some benefit. This is an either/or method that employs two condensers of equal capacity - one outdoors and the other in the ventilation system (see Figure 2).

They are both sized for the entire heat of rejection load. During the normal condensing mode, the outdoor condenser provides the heat transfer necessary to handle the entire heat of rejection. The smaller heat reclaim coil in the series application only desuperheats the refrigerant.

The full-sized reclaim condenser in the parallel application, however, completely condenses the refrigerant. The entire heat of rejection load is transferred to circulating ventilation air. This will yield a larger heating capacity for the building, possibly eliminating the need for a second stage of heat.

The very thing that gives the parallel method a greater heating capacity is also what offers the method its greatest challenge - being an either/or application.

During the outdoor condensing mode, the heat reclaim condenser is idle. This means that it receives no refrigerant flow and it contains only a minimal amount of refrigerant vapor and liquid. When the requirement for heat requires the three-way valve to shift, simultaneously the refrigerant flow ceases to the normal condenser and starts to the reclaim condenser. There will be some time lag before the relatively empty reclaim condenser contains enough refrigerant to resume the steady supply of liquid refrigerant to the receiver.

Because of this lag, the ability to quickly pump out the idle condenser is desirable. To accomplish this, it is recommended that the Type C three-way valve be used along with a dedicated pump-out solenoid valve for each condenser. A restriction should be used in the pump out line to prevent liquid floodback.

Even with this arrangement, the compressor(s) will fill the now active reclaim condenser much faster than the pump-out solenoid can drain the idle normal condenser. During the transition from one condenser to the other, the level of refrigerant in the receiver will be drained faster than it can be replenished by the functioning condenser.

Without a sufficient refrigerant charge, the receiver may lose its liquid seal before system equilibrium is restored, temporarily compromising the ability to provide vapor-free liquid to the TXVs.

There is yet another potential problem with the parallel method and that is the accumulation of cold saturated liquid in the idle roof condenser during the heat reclaim mode. Upon resumption of the normal condensing mode, superheated discharge vapor will be re-introduced into the outdoor condenser.

If the quantity of low temperature saturated liquid is excessive, the resulting rapid expansion from the sudden temperature increase may cause liquid hammer. There will be the possibility of piping and/or component failure.

The potential is more severe than in the series reclaim method as there will be areas of the country that will experience extremely low ambient temperatures during the winter months. The amount of liquid hammer will be magnified as the difference between the discharge vapor temperature and ambient temperature increases.

In the parallel method there are two possibilities for liquid refrigerant to be present prior to resuming normal condensing:

1. Any time the ambient (outdoor condenser location) is colder than the suction vapor temperature, the suction vapor will want to migrate to the colder condenser. The colder ambient will cause some of the migrated vapor to condense, resulting in an idle condenser that has a substantial amount of cold liquid inside. A tight seating check valve in the pump-out line is required to reduce the potential for migration.

2. Pump out of the idle condenser is complete when its pressure is reduced to compressor rack suction pressure. When pump out is complete, some refrigerant vapor will remain in the idle roof condenser. If the ambient temperature is lower than the compressor rack’s saturated suction temperature, some of the remaining vapor will condense as its temperature is reduced in the colder ambient location. Since it will be at the pressure corresponding to the ambient, the pressure will be lower than the common suction pressure causing it to remain in the condenser.

Figure 3. (Click on the image for an enlarged view.)

ELIMINATING LIQUID HAMMER

One method of eliminating the potential for any liquid hammer is to slowly repressurize the roof condenser with an auxiliary solenoid valve before shifting back to the normal condensing mode. This can be accomplished with a time delay relay (TD), a standard relay (R1), and a small, normally closed solenoid valve (SV).

The store thermostat would close on a rise of temperature, energizing the TD and R1 when the store temperature reaches its set point (see Figure 3).

A normally open switch in R1 will close energizing the pressurizing solenoid valve. After the TD times out (a minute or two), it will open the two normally closed TD switches, de-energizing the three-way valve and R1.

This will bring the system back into the normal condensing mode and de-energize the pressurizing solenoid valve. The slow introduction of discharge vapor to the condenser will eliminate rapid temperature shock and liquid hammer as the system reverts to the normal condenser mode.

Note: This series is scheduled to conclude in a future issue with service aspects of heat reclaim.

Publication date: 03/26/2007

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