Some disastrous incidents appear to have as their root cause the sudden expansion of high-pressure vapor into a low-pressure space. If liquid refrigerant is in the path of the high-velocity vapor, this liquid can be driven at high velocity against the end of the pipe, possibly causing a rupture. Two typical situations where there could be a rapid release of pressure are both associated with the hot-gas defrost process:
1. Opening of the valve admitting high-pressure defrost gas into the coil that is to be defrosted
2. Opening of the suction solenoid valve between the coil and the suction line when defrost is complete
In both of these situations, vapor rushes into the low-pressure region and high vapor velocities exist during the pressure equalization. In the first situation, a coil may be damaged at the initiation of defrost. During refrigeration service the coil, particularly a liquid-overfeed coil, contains considerable liquid, as illustrated in Fig. 13.4a. Furthermore, additional liquid may enter the coil because of condensed refrigerant in the hot-gas line. When the defrost process starts, high-pressure gas rushes into the coil that is at a low pressure and drives some of the liquid with high velocity against U-bends or headers of the coil, as shown in Fig. 13.4b. The force may be enough to rupture a tube in the coil or the drain pan.
Another possible cause of hammer-like blows and the knocking sound within the coil at the start of defrost may be attributable to the sudden condensation of bubbles of vapor in the subcooled liquid. When the liquid is subcooled by more than about 35°C (63°F), the rapid condensation of vapor causes the liquid to close on itself with such velocity that the impact generates high localized pressures.
Some steps that may be taken to avoid liquid hammer at the initiation of defrost are the following:
• Limit the pressure of the entering defrost gas by means of a pressure regulating valve, as shown in Fig. 13.4b. In summer operation when the condensing pressure and thus the pressure of the defrost gas is high, the entering pressure permitted is reduced. Dropping the defrost. gas pressure from, say, 1275 kPa gauge (185 psig) to 950 kPa gauge (140 psig), does not cause a noticeable lengthening of the defrost time.
• Employ a pumpout process. In this operation, the first step on initiation of defrost is to close the solenoid in the liquid supply line to the coil. The fan continues to operate giving the liquid in the coil time to vaporize. Only after this pumpout operation (perhaps 5 to 10 minutes) does the fan stop, the valve in the suction line closes, and the defrost gas valve opens. The pumpout operation rids the coil of most of its liquid so the entering defrost gas finds little or no liquid to drive against the ends of tubes.
• Keep the defrost gas main as free of condensed refrigerant as possible. The defrost gas line often passes through low-temperature spaces which condense the refrigerant; but some means of preventing liquid accumulation are to: install a main solenoid valve in the machine room that opens only when one of the solenoids controlling flow to a coil opens, and/or install a small highside float valve near the coils and pitch the defrost gas pipe to this float valve. The float valve drains liquid to a wet suction line.
• Choose slow-opening solenoid valves for the defrost gas valve.