When defrosting an air coil using hot gas, the basic procedure is to interrupt the supply of liquid refrigerant to the evaporator, restrict the outlet of the coil, then supply high-pressure vapor. The interior of the coil then achieves a pressure such that the saturation temperature is high enough to melt the frost on the exterior of the coil. During defrost, then, the evaporator temporarily becomes a condenser.
The next several pages will be organized as follows: (1) standard piping and control sequence for a bottom-feed liquid recirculation coil, (2) defrosting a flooded coil, (3) recommended piping of the defrost gas supplying the coil and of the condensed refrigerant leaving the coil, and (4) recommended sequence for a safe defrost.
The elements15 of a bottom-feed liquid recirculation coil equipped with hotgas defrost are shown in Fig. 6.46. The coil has an odd number of rows of tubes with the liquid entering the header on the near side and the mixture of liquid and vapor leaving through the header on the far side and passing on through
Suction Valve V1 to the liquid/vapor return line. Admission of liquid refrigerant to the coil is controlled by a solenoid valve that may serve several purposes. It may be connected to a thermostat that closes the valve when the air temperature in the space is satisfied. The liquid supply valve V2 also acts during the defrost cycle. The balancing valve that follows the solenoid valve in the line is set in conjunction with other coils in the system to insure adequate liquid refrigerant supply to all coils.
Table 6.9 shows the status of the four valves in Fig. 6.46 during refrigeration operation and during defrost. The two valves that figure into the defrost operation are V3, the solenoid that controls the admission of defrost gas (hot
gas) into the coil, and valve V4, a pressure-regulating valve that opens or closes as necessary in an attempt to maintain the pressure upstream of the valve. Thus, Valve V4 attempts to maintain the coil pressure constant. As Table 6.9 indicates, during refrigeration operation the refrigerant supply solenoid and the suction valve, valves V2 and V1, respectively, are open. The defrost gas valve, V3, is closed as is the pressure regulator, V4. Valve V4 is closed because during refrigeration operation the upstream pressure is low—much lower than the pressure setting of the valve. The regulating valve does what it can to try to increase the upstream pressure, which is to close.
During defrost, the liquid supply valve V2 and the suction valve V1 close, which isolates the coil from the normal segments of the refrigeration system. The defrost gas valve V3 opens, allowing vapor from the high-pressure receiver
or the compressor discharge line to flow first into the tubes of the drain pan and then into the coil. The drain pan must be heated during defrost, otherwise water and frost would slide down from the coil as it defrosts, but then refreeze in the drain pan. At the initiation of defrost the pressure-regulating valve V4 is still closed. Defrost gas flows into the coil, bringing up its pressure. Since the coil is cold, the incoming vapor condenses in the coil, and during the defrost process the evaporator coil temporarily acts as a condenser. The pressure in the coil continues to rise until the pressure setting of V4 is reached, which is typically about 620 kPa (90 psia or 75 psig) for ammonia or about 680 kPa (99 psia or 84 psig) for R-22. These pressure settings correspond to approximately 10°C (50°F) saturation temperatures of the refrigerants, and represent a high enough temperature to warm the coil and melt the frost from its outside surfaces. The disposition of the condensed refrigerant will be discussed in Sec. 6.23.
Several refinements must be made on the basic coil equipped with hotgas defrost shown in Fig. 6.46. Figure 6.47 shows the installation of two check valves. The purpose of Check Valve A is to prevent high-pressure refrigerant from the coil from backing up through Solenoid Valve V2 during the defrost process. The pressure in the coil may reach 620 to 680 kPa (90 to 99 psia) during defrost and the liquid ahead of Valve V2 is only slightly higher than the operating evaporator pressure. The second check valve, Valve B, prevents liquid refrigerant from entering the drain pan during refrigeration operation. If the drain pan became as cold as the coil, it would collect frost which would drop off during the defrost process on product or whatever is beneath the coil.
The defrost process seems in recent years to have been the cause of occasional damage to equipment and in some cases the rupture of refrigerant lines, resulting in excessive cost and in several cases personal injury. The critical periods during a hot-gas defrost are at its initiation and at its termination. In both situations high pressure vapor that may be moving at a high velocity is brought into contact with cold liquid causing pressure shock waves. The conditions that occur at the initiation of defrost will be discussed in Sec. 6.23. The critical nature of the termination of defrost results when the mixture of liquid and vapor at a pressure of 600 to 700 kPa (85 to 100 psia) rushes into a low-pressure liquid/vapor return line, perhaps at a pressure below atmospheric. One or both of two phenomena may occur. The high-pressure vapor may drive the liquid in the return line at high velocity to the end of the suction pipe or to an elbow with such force that the pipe ruptures. Another event may be condensation shock,16 wherein the high-temperature vapor condenses so rapidly on the cold liquid and collapses with such force that the resulting shock wave ruptures a pipe.
To prevent the extreme stresses at the end of defrost, Valve V1 should be equipped with a bypass that slowly drops the pressure in the coil before returning to refrigeration operation. In Fig. 6.48, this bypass consists of a solenoid valve V5 in series with a throttling valve, which opens before V1 opens to slowly relieve the coil pressure. The throttling valve is set to provide the desired rate of pressure decline. There remains the potential hazard that the bypass line, particularly the throttling valve may become plugged, in which case the bypass does not function. However, many hot-gas defrost processes are managed by a microprocessor-based controller which can sense the pressure in the coil and not open V1 until the pressure is low enough to be beyond the danger point.
Because Valve V1 is usually large it is often gas-powered, as shown in Fig. 6.49, drawing its power pressure from the defrost gas.
Operators should be cautioned that if there is excessive noise and shaking of pipes during a defrost, that this is not a normal condition, and the causes should be corrected.
When the liquid overfeed coil is fed from the top, some changes in the piping compared to the bottom-feed coil are required. Figure 6.50 shows the liquid entering the top of a header on the back side of the coil and the mixture of liquid and vapor leaving at the opposite end of the coil. During defrost the liquid valve V2 and the suction valve V1 are closed, but valve V3 admits defrost gas, first to the drain pan and then to the normal entrance of refrigerant to the coil. When the pressure rises sufficiently in the coil, the pressure-regulating valve V4 opens to allow condensed refrigerant to return to a low-pressure suction line.
The elements of the defrost arrangement for a flooded coil are shown in Fig. 6.51. As a preliminary step to defrost, the valve in the liquid supply line to the surge drum should be closed and the operation of the coil continued. This step reduces the amount of liquid in the coil and surge drum. To defrost the coil, isolate it by means of the two valves—one in the liquid leg and the other in the return line from the evaporator to the surge drum. The valve in the defrost gas line is then opened allowing high-pressure gas into the evaporator. Condensation of refrigerant vapor begins immediately, which begins warming the coil.
The pressure inside the coil gradually increases until the setting of the pressure regulator is reached, at which time refrigerant is relieved from the evaporator to a pipe or vessel at low pressure, or to the surge drum as shown in Fig. 6.51. When venting to a suction line, the line should be equipped with a vessel to remove liquid before the refrigerant reaches the compressor, because liquid refrigerant issues from the pressure-regulating valve. Since flooded evaporators are often not associated with liquid-recirculation systems which always incorporate liquid/vapor separation vessels, the discharge from the pressure regulator to the surge drum has advantages. It may be necessary, however, to provide an enlarged surge drum to accommodate the liquid condensed in the coil during defrost.
The main diagram in Fig. 6.51 shows a horizontal line between the liquid leg and the lower tube of the evaporator. A problem sometimes occurs because of vapor reaching the pressure regulator restricting the discharge of liquid from the evaporator. In this case the lower row of tubes may not defrost properly. One approach to prevent this problem is to slope the line from the liquid leg p to the tube, as shown in the inset of Fig. 6.51, which provides a liquid seal at the entrance to the pressure regulator and permits only liquid to flow through the regulator.