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heat exchanger manufacturers usually present thermal performance data as a function of heat load and incoming air and water flow rates. this works well for applications where the heat exchanger is used to cool water with air, as you can simply plug in your heat load, air temperature, and liquid temperature to determine if it offers sufficient thermal performance.
cabinet cooling applications use the heat exchanger in the opposite configuration - cold water flows in the liquid circuit and the warm air from the cabinet is cooled as it passes across the heat exchanger's fins. in cabinet cooling applications, you usually need to know the temperature of the air as it enters the cabinet, and the maximum temperature that the air in the cabinet will reach. neither of these can be read directly from the heat exchanger performance curves.
the usual way to calculate the temperature change of the air is to use the mass flow rate calculation:
this can be time-consuming and prone to error.
to avoid these calculations, lytron has developed charts to quickly estimate temperature rise in common heat transfer media at various heat loads. graphs are available for air, water, oil, and 50/50 ethylene glycol-water (egw). to calculate the temperature change, simply select the appropriate graph, look up your flow rate and heat load, and read off the temperature change.
air temperature change graph
water temperature change graph
oil temperature change graph
50/50 egw temperature change graph
when used in conjunction with product performance curves, these offer a quick and simple way of calculating the temperature of the cold air entering the cabinet, and the maximum air temperature in the cabinet.
example:
you are evaluating a lytron 6310 heat exchanger with a caravel fan for cooling an electronics cabinet. the water entering the heat exchanger is at 20°c and a flow rate of 1gpm. the heat load, q, is 2400w.
what is the temperature of the cooled air entering the cabinet (i.e. the temperature of the air leaving the heat exchanger) and what is the maximum temperature in the cabinet (i.e. the temperature of the warm air entering the heat exchanger)?
first check the manufacturer's performance curve for the heat exchanger you are considering. you will see that with a 1gpm water flow and the caravel fan which supplies approx 250cfm, its performance is 80w/°c.
since we know that q is 2400w and q/itd is 80°c/w, we can calculate itd.
itd = 2400w ÷ 80°c/w = 30°c
we also know that the incoming water temperature is 20°c. we can therefore calculate the incoming air temperature:
the incoming air temperature = 20°c + 30°c = 50°c.
to determine the outgoing temperature of the air, we use the 'air flow' chart using the parameters 250 cfm and 2400w.
we find that the change in temperature is approximately 17°c. the outgoing air temperature is 50°c - 17°c = 33°c.
we know that this heat exchanger with the caravel fan will cool the air to 33°c, and the hottest temperature the air in the cabinet will reach is 50°c.
to determine the outgoing temperature of the water we use the 'water flow' chart.
at 1gpm and 2400w, this shows that the change in temperature is approximately 9°c. therefore the outgoing water temperature is 20°c + 9°c = 29°c.
graphs for air, water, oil and egw are available in downloadable pdf format on lytron's website. these are helpful for sizing heat exchangers and cold plates and are also useful in a variety of other temperature change calculations.
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