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Refrigeration Heat Reclaim

In a refrigeration system, heat normally wasted at a condenser can be recovered and serve some other useful purpose.  As a low-to-moderate temperature source, heat reclaim can be used wherever some type of refrigeration system exists.  Properly installed, a heat reclaim system will soon recover the additional costs involved in installing the system.

A well-designed system is more than simply putting another heat reclaim condenser in series or parallel with the normal condenser.

A refrigeration system can supply heat reclaim only when the cooling system is in operation.  Usually, the system is a refrigeration system first, and heat reclaim is a secondary consideration.  The addition of heat reclaim to a system must not handicap the cooling system.

In order to make heat reclaim work properly, control of condensing pressure is a necessity.  Usually, condensing pressure higher than needed for refrigeration only will need to be maintained.  Higher condensing pressure will raise the temperature level to make heat available.  A system without condenser pressure control may operate at condensing temperatures as low as 80°F or less.  This may be adequate for refrigeration, but not high enough for heating anything.  By using regulators to control the condensing pressures to get 110°F, air can now be heated to provide, say, space heating for comfort conditions.

Before adding heat reclaim to an existing refrigeration system or incorporating it in the design of a new system, certain basic requirements must be met.

As mentioned, the system is first and foremost a refrigeration system.  Therefore, heat reclaim will only be available when the cooling system is in operation.  This means there should be a plan to use, or store, the reclaimed heat at the same time the cooling system is in operation.  Adding heat reclaim should not handicap the refrigeration system.  Condensing pressure must not go so high that too much capacity is lost, or the operating cost gets too high.

The system should not operate for heating when cooling is not required.

If the basics can be met, then the details can be addressed.

Discharge temperature, condensing pressure, and compressor compression ratios must be considered, and their affect on operation evaluated.

An example will best illustrate the process:

In a supermarket being considered for heat reclaim is a system with two 10 H.P., R-404a, compressors operating at –20°F (17.1 psig) suction.  Condensing pressure is controlled to maintain a minimum of 203 psig (90°F).  The compressors can do 112,000 BTU/HR cooling capacity, rejecting 176,000 BTU/HR at the condenser.  Electrical power required, 20.2 kW.  The system’s actual maximum cooling requirement is 80,000 BTU/HR.  Of course, the suction temperature must remain at -20°F.

The condensing temperature of 90°F is too low for heat reclaim.  A condensing temperature of 120°F is needed for heating in this case.  The compressor’s capacity at 120°F condensing and -20°F suction is reduced to 82,000 BTU/HR, and the heat of rejection to 150,000 BTU/HR.  Electrical power requirement goes up to 21.4 kW.

The refrigeration performance has not been degraded below the needed 80,000 BTU/HR.  Checking with the compressor manufacturer’s literature shows that the compressors will perform with 120° F condensing temperature provided head-cooling fans are installed.

The increase in electrical use of only 1.2 kW is small, especially in view of the fact that the heat reclaim is 150,000 BTU/HR.  No other heat source could produce that many BTU’s for so little cost in fuel.

Once the system has been analyzed and heat reclaim is shown to be cost-effective and possible, system configuration and components to make it work can be selected.

(Only systems where there can be cold weather will be discussed.)

Figure 1 illustrates a system with cold weather condenser pressure control.

Figure 1.

This system has:

  1.  Field adjustable setpoints for control of the condensing pressure, the setpoints for refrigeration and heat reclaim being the same. 

  2. Excessive discharge pressure will release to the normal condenser. 
  3. The solenoid valve-action (on/off) controls the heating.

When refrigeration only is called for, the solenoid valve is closed, preventing flow into the heat reclaim condenser.  Flow is through the inlet and outlet regulators to maintain discharge pressure as needed, through the normal condenser.  Check valves prevent gas from backing up into the heat reclaim condenser.  (This is the usual “flooded condenser” head pressure system.)

When heat is called for, the solenoid valve is opened, allowing flow through the heat reclaim condenser.  The inlet regulator maintains discharge pressure at or above its setpoint and this pressure is supplied to the heat reclaim condenser.  If the pressure falls below the inlet regulator setpoint, it closes, and all the gas goes to the heat reclaim condenser.  If the heat load is large, the heat reclaim condenser pressure and receiver pressure will fall, and the outlet pressure regulator will begin to open to build receiver pressure back up.  Should the heating load reduce, discharge pressure will rise and the inlet regulator will begin to open, allowing excess pressure to flow to the normal condenser. 

Note:   The outlet regulator setting should be about 10 psig below the inlet regulator setting.

Some systems require a higher condensing pressure for heat reclaim and a lower pressure for refrigeration.

Figure 2 illustrates these systems.  It is the same as figure 1, except another inlet regulator has been added with an electric shutoff.  For refrigeration only, the solenoid valve is closed.  The pilot solenoid on the inlet regulator (Low-Press) is energized, allowing the regulator to control at its setpoint, which is lower than the setpoint of the inlet regulator without electric shutoff.

Figure 2.

The system is performing the same as the system in Figure 1.

When the solenoid valve is energized, allowing flow into the heat reclaim condenser, the inlet regulator with electric shutoff is de-energized, closing it and forcing control to the high-pressure inlet regulator.  This higher pressure is now supplied to the heat reclaim condenser, raising its temperature.

If the receiver pressure drops below the setpoint of the outlet regulator, it will modulate open and build receiver pressure back up.

If the heat load is small, the compressor discharge pressure will rise, opening the high-pressure inlet regulator, causing excess pressure to flow to the normal condenser.  Pressure is maintained to give needed heating but not allowed to get too high.

When no heat reclaim is needed, the heat reclaim solenoid valve is de-energized and the solenoid in the low-pressure inlet regulator is energized.  The condenser pressure will return to the lower, more economical pressure required by refrigeration only.

Systems that have multiple heat reclaim condensers require additional regulators as shown in Figure 3.

Figure 3.

Only in refrigeration mode, everything works as previously described.

When a solenoid valve opens, activating its corresponding heat reclaim condenser, the corresponding inlet regulator on the outlet side of that heat reclaim condenser will maintain the heat reclaim condenser at its set pressure.  The low-pressure electric shutoff inlet regulator will be shutoff, forcing discharge gas to the heat reclaim condensers.  The high-pressure inlet regulator relieves only excess pressure to the normal condenser.  If receiver pressure drops, the outlet regulator will modulate open, building the receiver pressure back to its minimum level.

Any of these systems can be piped with the condensers in parallel instead of in series.  See Figure 4. This reduces the amount of piping required.

Figure 4.

Precautions must be taken, however, to make sure that complete condensation will take place in the heat reclaim condenser.  This may be difficult in an air heating application.  If a water heating application, a flow switch should be used in the water line to insure adequate flow is present before allowing the heat reclaim solenoid valve to be energized.

Since it is cost prohibitive to try and “store” hot air, more and more heat reclaim systems are being used to heat water.  It is easy to store hot water.

If properly evaluated, heat reclaim systems can be a cost effective way to heat air or water.

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