EVAPORATIVE CONDENSERS: FUNDAMENTALS
Evaporative condenser basic design and performance can be numerically examined using Newton’s equation.
Q = U * A * LMTD
Q = heat reject, btu/hr
U = transfer coefficient
A = surface area, e.g. st.ft.
LMTD = mean logarithmic temperature
In our experience, this formula has proved successful enough in terms of interpretation of unit specifications, as supplied by each particular manufacturer. Unfortunately, not all industrial equipment suppliers choose to openly publish their specifications and actual performance data. Vendor catalogs and many engineering proposals often contain “inaccuracies” only to be noticed once the under-performing equipment becomes operative. While the model represents a significant simplification (basically assumes an air behavior following the saturation line, starting from a specific wet-bulb temperature and condensing temperature) it closely matches most available trade literature.
Standard evaporative condensers, unlike cooling towers, are almost always selected/ designed/built with liquid-to-gas (L/G) ratios typically hovering 1.1 – 1.2, which obviously considerably limits its optimization. Any cooling tower designer can work with ratios L / G ranging between 0.6 and 2.2, which greatly allows job optimization and size to meet or accommodate a variety of criteria, e.g. lower fan HP, lower required area, direct drive fan restrictions. Selection by way of catalog further handicaps the final decision.
Blurred bid specifications only go against the future equipment owner. Calculation formulas and algorithms cannot be technically more honest than what participants are bent to install in reality no matter what.
An incorrect wet bulb selection results in am extremely, non-linear underperformance of the unit. An EC selected to dissipate a certain amount of heat in an area with a wet bulb temperature of say 75 ° F, will thermally collapse in locations with significantly higher prevailing wet bulb, 78-80 ° F.
The selection of the heat transfer coefficient U must take into account the “de-rating” caused by unfavorable conditions in the field. The consequences of ignoring this item, unintentionally or maliciously, will gravely short-change the trusting end user.
MATERIALS OF CONSTRUCTION
Most common EC enclosure is hot-dip galvanized steel. Tube material can be galvanized steel or copper or other material depending on the application/refrigerant. Most common water distribution materials include plastic pipe and nozzles. PVC drift eliminators are frequently the standard.