Pressure drop in hot gas line

Many people believe that pressure drop in hot gas line at lower condensing pressure is a reason of poor hot gas defrosting. My experience has shown that this pressure drop is not significant. Typically, in the middle of defrosting, pressure drop in hot gas line is less than 5 psig. It is easy to check my statement. Put the gauges on the hot gas line and compare the readings.

Example

A cold storage facility has 24 evaporator coils. The main hot gas line was designed for simultaneous defrosting of 8 coils (one third of all coils). But during winter operation, only one evaporator coil per hour will be defrosted. The pressure drop in the hot gas main will not be significant because actual mass flow will be one eighth of design mass flow.

According to the Darcy-Weisbach equation, the pressure drop in a pipe varies according to the square of the velocity of flow in that pipe. This velocity is directly proportional to the vapor flow. Reduction of the vapor flow in a hot gas line by 8 times will lead to a reduction of the pressure drop in this line by 64 times. This is the reason that a real life pressure drop in a hot gas line is not significant.    

My experience has shown that hot gas oversupply or poor ammonia condensate draining are often the real reasons of poor hot gas defrosting at low condensing pressure.