The defrost process impacts the system beyond the confines of the coil itself and its associated defrost piping. For example, many plant operators maintain an artificially high condensing pressure, even when ambient conditions would permit the condensing temperature to drop, to provide sufficiently high defrost gas pressure. Since operating with low condensing pressure saves energy, the maintenance of artificially high condensing temperatures is challenged.18 Some tests conducted with R- 22 in the laboratory and with ammonia in the field indicated that a hot-gas supply pressure of 100 kPa (15 psi) above the setting of the pressure-relief valve could still achieve a defrost. Operators would want to be safe and not push this limit, but the values do suggest that some experimentation be conducted in the plant to determine how low the condensing temperatures can be safely operated.
Another issue is where to direct the discharge from the pressure-regulating valve during a defrost. It may be most convenient from the standpoint of simplicity of piping to discharge into the low-pressure liquid/vapor return line. This arrangement functions reliably, but results in some loss of efficiency. During the early stages of hot-gas defrost, the refrigerant passing through the pressure regulating valve is likely to be in liquid form. In later stages, when the rate of refrigerant condensation has dropped, vapor is also likely to pass through the pressure-regulating valve. Figure 6.52 is a pressure enthalpy diagram showing throttling processes of both liquid and vapor.
The saturated liquid at u drops in pressure at constant enthalpy to point u, which is mostly liquid but also contains some flash gas. When the vapor at x throttles down to y, however, the process is wasteful because no refrigeration can be performed with vapor at y, and the compression from y to the condensing pressure incurs a cost of energy.
In two-stage compression systems, the intermediate pressure is likely to be lower than the setting of the pressure-regulating valve. When those relative pressures prevail, many designers provide a separate pipe to return the discharged refrigerant from defrosting coils to the intermediate-pressure intercooler/flash tank, as in Fig. 6.53, rather than to the low-pressure suction line. This arrangement improves the efficiency in several ways. Figure 6.52 shows the vapor at x only throttling down to the intermediate pressure, so the recompression from point y is avoided. The liquid at point u is much warmer than the liquid in the separating vessel of Fig. 6.53, and would cancel some potential refrigeration. Instead, the liquid is dumped into the intercooler/flash tank where the liquid temperature is the saturation temperature corresponding to the intermediate pressure, so less refrigeration is lost.
If the refrigerant leaving the pressure-regulating valve is discharged to the intermediate pressure, it is recommended that a check valve be placed in series with the regulating valve, as shown in Fig. 6.54. This check valve prevents refrigerant vapor from leaking back through the pressure- regulating valve when the evaporator is on the refrigeration cycle and operating at a lower pressure than the intermediate pressure.
A traditional question associated with the hot-gas defrost process is whether to take the defrost gas from the compressor discharge where it is superheated (as in Fig. 6.55a), or from the top of the liquid receiver where the gas is saturated (Fig. 6.55b).
The superheated vapor has a higher temperature than saturated vapor, but this benefit may be canceled by the lower heat-transfer coefficient of the gas in comparison to the condensation coefficient. In a plant with screw compressors, the discharge gas already has been cooled. When the defrost gas is taken off the top of the receiver, gas is drawn through the condenser or through its equalizer line uncondensed and supplemented with vapor flashed from the liquid in the receiver. When the liquid flashes into vapor, the liquid temperature drops. But a potential benefit of extracting gas at this location is that the warm liquid provides some thermal storage to aid in supplying defrost gas if the amount being compressed is in short supply.
In developing the defrost plan, provision must be made for an adequate supply of defrost gas. An adequate supply is assured if several evaporators are refrigerating while one is being defrosted. A frequently used rule-of-thumb is that two evaporators should be in refrigeration service while one is defrosted. Some measurements of the flow of hot gas during defrost19 indicate that this rule is safe, and even conservative when the condensing temperature is maintained low, as discussed above. Another estimate of the heat required for defrosting is in the range of 63 to 126 watts per m2 of heat-transfer area (20 to 40 Btu/hr per ft2). The defrost of a large industrial coil may require 10 to 15 minutes, but the defrost must continue longer because the frost that has melted and slipped down to the drain pan must be disposed of. This latter process may require another 10 to 15 minutes.
The pressure-regulating valve may introduce inefficiencies, as suggested by process x-y in Fig. 6.52. The sequence of events occurring in the pressure regulating valve is likely to be that only liquid refrigerant arrives at this valve
in the initial minutes of defrost. As the coil becomes warm, the condensation rate decreases and vapor along with liquid enters the regulating valve. Because passing vapor through the valve is inefficient, another approach to controlling the flow of condensed refrigerant is to use a liquid drainer, as in Fig. 6.56. The liquid drainer is a float valve that opens to allow liquid to pass, but closes when the liquid level drops. Also shown in Fig. 6.56 is a small vapor bypass to prevent vapor binding of the valve. The liquid drainer controls a different behavior of the pressure in the coil during defrost in comparison to the pressure-regulating valve. The regulating valve allows the pressure in the coil to rise to its pressure setting, where it remains. With the drainer, the pressure in the coil will continue to rise until it nearly reaches the pressure of the incoming defrost gas. Equipping the coil with the bypass valve of Fig. 6.48 is particularly critical so that high coil pressure is not suddenly discharged into the suction line when the defrost terminates.