The low-temperature liquid recirculating system illustrated in Fig. 10-22 uses several controls. During the cooling cycle, Number 1 pilot valve is opened and Number 2 pilot valve is closed, holding the gas-powered solenoid valve wide open. This allows flow of liquid through the energized liquid-solenoid valve from the recirculator and then through the circuit of the unit. The in-line check valve installed between the drain-pan coil header and suction line prevents drainage of liquid into the drain-pan coil.
For defrost, the liquid solenoid valve is closed. The Number 1 pilot solenoid is de-energized. The Number 2 solenoid is opened, closing the gas-powered solenoid valve tightly. The hot-gas solenoid is energized. This allows distribution of the hot gas through the drain-pan coils, the in-line check valve, the top of the suction header, and the coil. The gas comes out the bottom of the liquid header.
Check valve A prevents the flow of the high-pressure gas in the liquid line. Therefore, the gas is relieved through the safety-valve relief regulator (B). This is set to maintain pressure in the evaporator to promote rapid, efficient defrost.
Bottom hot-gas feed
The multiple system shown in Fig. 10-20 shows a check valve mounted in each of the liquid-refrigerant branch lines. A single solenoid valve is used in the main refrigerant line. The defrost gas is bottom fed.
The system illustrated in Fig. 10-21 shows a check valve mounted directly at the outlet of each of the liquid-solenoid valves. The defrost gas is top fed. This system permits selective defrosting of each evaporator. A single accumulator is used to protect the compressor during defrost, as well as to accumulate both liquid refrigerant and defrost condensate. This protection is accomplished by using a differential pressure–regulator valve in an evaporator bypass circuit.
The differential pressure–regulator valve will open sufficiently to relieve excess pressure across the compressor inlet. The pressure will discharge as excess pressure differential occurs. When the pressure differential is less than the regulator-valve setting, the regulator will be tightly closed.
Liquid refrigerant recirculating systems are frequently fed by upward liquid flow through their evaporators. These systems are called bottomfed. This is accomplished by either mechanical or gas-displacement recirculators during the refrigerant cycle (see Fig. 10-20).
In some systems, more than a single evaporator is fed from the same recirculator, as shown in Fig. 10-20. Then, a proper distribution of liquid between evaporators must be maintained to achieve efficient operation of each evaporator. This balance is usually accomplished by the insertion of adjustable glove valves or orifices into the liquid-feeder line. Similarly, adjustment of the glove valves or insertion of orifices is also often used properly to distribute hot gas during the defrost cycle.
Equalizing orifices or glove valves are not used if the hot gas used for defrosting is fed to the bottom of the evaporators as shown in Fig. 10-20. In such cases, most of the hot gas could flow through the circuits nearest the hot-gas supply line. The same would also happen in circuits where both vertical and horizontal headers are used, as in Fig. 10-21. The more remote circuits could remain full of cold liquid. Consequently, they would not defrost.
Supplying hot gas to the top of the evaporator forces liquid refrigerant down through the evaporator and out through a reseating safety valve relief regulator into the suction-line return to the accumulator (see Fig. 10-21). Reseating safety-valve relief regulators are usually set to relieve at 60 to 80 psig to provide rapid defrost.
The use of check valves is important in flooded liquid-recirculating systems fed by mechanical gas-displacement liquid recirculators. The check valves are used where the pressure of the hot gas used for defrost is higher than the system pressure. The reseating safety-check valve must be used to stop this gas at high pressure from flowing back into the liquid supply-line.
Gas and liquid leg shutoff
Figure 10-18 illustrates a flooded floor unit suitable for operation down to ?70°F (?57°C). The gas-pressure-powered valve used in this circuit has a solenoid pilot operator. This provides positive action with gas or liquid loads at high or low temperatures and pressures.
To defrost a group of evaporators without affecting the temperatures of the common surge drum, the gas-powered valve is used at each end of the evaporator. A reseating safety valve is a relief regulator. It controls the defrost pressure to the relief-line accumulator. A check valve prevents backflow into the relief line. The in-line check valve prevents crossover between adjacent evaporators.
At high temperatures [above ?25°F (?31°C)], use of the gas-powered check valve in place of the gas-powered solenoid valve is recommended.
Gas leg shutoff
The system shown in Fig. 10-19 is similar to that shown in Fig. 10-18. However, a single gas-type, pressure-powered valve is used. Overpressure at the surge drum is relieved by valve B, a defrost-relief regulator. This is normally wide open. It becomes a regulating valve when its solenoid is de-energized during defrost.
Defrost gas flows through the hot-gas solenoid when energized. It then flows through the glove valve and the in-line check valve to force the evaporator fluid into the surge drum.
An optional hot-gas thermostat bulb may be used to sense heating of the bottom of the evaporator. Thus, it can act as a backup for the timed defrost cycle.
Top hot-gas feed
Figure 10-16 shows a modification of the system shown in Fig. 10-15. In the system shown in Fig. 10-16, top-fed hot defrost gas forces the evaporator fluid directly to the bottom of the large surge drum. The defrost regulator (valve A), which is normally open, is de-energized during the defrost to act as a relief regulator.
To minimize heating of the ammonia that accumulates in the surge drum during defrost, a thermostat bulb should be used to sense the temperature rise in the bottom header. This thermostat can be used to terminate the defrost cycle. Once again, the gas-powered check valve isolates the evaporator from the surge drum until the gas pressure is shut off.
Hot-gas feed through surge drum
Figure 10-17 shows a simple defrost. It is a setup for the refrigeration system shown in Fig. 10-16. However, both the evaporator and surge drum are emptied during the defrost, necessitating the use of an ample suction accumulator to protect the compressor. In this system the pilot solenoid valve in conjunction with the reverse-acting pressure regulator limits the system pressure. This permits the use of a simple solenoid valve and globe valve for rate control in the relief line.
Bottom hot-gas feed
Figure 10-14 illustrates a flooded-ceiling evaporator. Upon initiation of the defrost sequence, the hot gas solenoid (Number 1) is opened. Gas flows to gas-powered check valve, isolating the bottom of the surge tank from the evaporator. The hot gas flows through the pan coil and the in-line check valve into the evaporator. Excess gas pressure is dumped into the surge tank. It will bleed through valve A. During defrost, this valve has been de-energized to perform as a relief regulator set at approximately 70 psig.
Top hot-gas feed
Figure 10-15 shows a multiple flooded-evaporator system using input and output headers to connect the various evaporators and the surge drum. Note that, upon defrost, the fluid and condensate, are purged from the evaporator and surge drum into the remote accumulator through the regulator, which is a reseating safety valve. This is usually set at about 70 psig. The accumulator must be sized to accept the refrigerant, plus hot-gas condensate.
The system shown in Fig. 10-13 is similar to that shown in Fig. 10-12. However, the liquid leg of the evaporator dumps directly into the surge drum without a relief valve. In this system, valve C is a defrost regulator. It is placed in the suction line, where it is normally open. During defrost, valve C is de-energized, converting to a defrost regulator. In such a system, it is recommended that a large-capacity surge drum or valve A be used as a bypass valve. This will bleed defrost pressure gradually around valve C into the suction line. Note how the in-line check valve is used to prevent cross flow.
Figure 10-12 shows a flood-gas and liquid leg shutoff (top hot-gas feed) system. Here, the gas-powered valve is used on both ends of the evaporator. It is a gas-powered check valve. At defrost, the normally closed type-A pilot solenoid is energized. Hot-gas pressure closes the gas-powered check valves. Hot gas flows through the solenoid, globe valves, pan coil, and in-line check valve into the top of the evaporator. Here, it purges the evaporator of fluids. The evaporator is discharged at the metered rate through valve B. that has been de-energized and acts as a regulator during defrost.
At the end of the defrost cycle, excess pressure will bleed from the relief line at a safe rate through the energized valve B. The gas-powered valves will not open the evaporator to the surge drum until the gas pressure is nearly down to the system pressure.
In the evaporator shown in Fig. 10-10, when the defrost cycle is initiated, the hot gas is introduced through the hot-gas solenoid valve to the manifold. It then passes through the balancing glove valve and the pan coil to a check valve that prevents liquid crossover. From the check valve, hot gas is directed to the top of the evaporator. Here, it forces the refrigerant and accumulated oil from the relief regulator (valve A). This regulator has been de-energized to convert it to a relief regulator set at about 70 psig. It meters defrost condensate to the suction line and accumulator.
When the three-position selector switch is turned to defrost, solenoid valve A and pilot-solenoid valve D close as hot-gas valve C and evaporator-pilot valve B open. This allows hot gas to enter the evaporator. Valve D now acts as a back pressure regulator, maintaining a predetermined pressure above the freezing point. After a regulated delay, preferably toward the end of the defrost cycle, the time delay relay allows the water solenoid to open. This causes water to spray over the evaporator, melting ice that may be lodged between coils and flushing the drain pan.
When the evaporator is defrosted, the system is returned to the cooling cycle by turning the three-position selector switch. The hot-gas solenoid (valve C) and built-in pilot (valve E) close as the liquid solenoid (valve A) opens.
This system can be made completely automatic by replacing the manual selector with an electric time clock. Table 10-2 shows some of the valve sizing for the low-temperature system.