On occasion we are asked about condenser piping. There are several misconceptions by some engineers and contractors passing about in our industry.
Condensers have more “mystique” about them than any other portion of the refrigeration system. Understanding condensers necessitates understanding not only refrigeration thermodynamics, but also the psychometric relationships of water, water vapor, and water vapor and air. In a very technical sense, this gets into vapor pressures and relationships of wet bulb and relative humidities.
I will try, in layman’s terms, to explain some of the “phenomena” that may be observed. As a preface, you can be assured that the following are true in the operation of evaporative condensers:
1. “Open drain” piping systems for evaporative condensers equalized to the receiver through their condensate drain lines, is the best equalizing method. It uses the entire pipe size for equalizing. Trapped drains with small equalizing lines will sometimes have trouble equalizing pressures. This can be observed by feeling these lines on occasion and observing that they become warm in their efforts to equalize.
2. Purging of air is much simpler on condensers equalized through open drains. This is because the air can be purged directly from the receiver or drain line header going to the receiver (individually trapped condensers need to be purged at every outlet connection).
Exceptions to the “normal” operation include the following:
1. Temperature inversion – On occasion, condensers and condenser platforms (the same as the automobile sitting in the open) will experience lower surface temperatures because of radiation to a clear sky. The surface temperature of the metal will be lower than the air adjacent to the metal. When the surface temperature becomes lower than the dew point (the inversion on a clear night will normally reach a temperature difference of ~7° F), (water) condensation will form on these metal surfaces.
2. When there is an abundance of condensers on a refrigeration system, such as on cool days, and the relative humidity is low, the wet bulb temperature of the air can be 10° to 15° lower than the ambient air. There are occasions when this occurs that “natural convection” will produce air temperatures (and consequently, refrigerant temperatures) that are lower than the temperature of the air (if the fans were blowing). When this occurs, some unusual phenomena can be observed in the evaporative condensers. If enough convection (or slight breeze) is available, it would be possible to operate condensers without the fans running (when the refrigeration load is low) and provide discharge pressures lower than if the fans were operated on a few of these condensers. This, of course, is not recommended as standard procedure, since you never know when the humidity will be high. Humidity is normally high in the early morning, but can be high for extended periods, depending on the “frontal” systems active at the time.