On many occasions, I find myself in meetings with energy companies, and they tell me they think the chiller is recirculating hot discharge air back into the condensers. I ask them, “Well, what percentage of air is recirculating, and I get a blank stare 100% of the time.

With that lingering image in mind, I am now going to explain a simple way to determine the amount of recirculation, so we can take the guess work out of this.

Dan Mizesko

But, before I explain how to calculate the recirculation percentage, it is important to understand the ill-effects of condenser air recirculation. First and foremost, the way an air-cooled chiller is installed can cause hot discharge air to circulate from the exit of the condenser fans back to the intake of the condenser coil. The hot recirculated air has little to any cooling capacity, so the recirculation reduces the capacity (tonnage) of the chiller. Second, it increases the condensing temperature and pressures as well as suction pressures, causing compressors to work harder, thus decreasing their reliability. The chiller’s ability to sub-cool liquid refrigerant will also be affected, causing excessive superheating of the compressor suction line as well as lack of refrigerant liquid cooling at the compressor; so, all in all, hot air recirculation should be avoided at any cost.

Now, let’s see how we calculate an air-cooled chiller’s hot air recirculation rate. The air temperature at the inlet of the condenser section should be the same as the ambient air temperature away from the chiller. If the condenser air intake is warmer than the ambient air away from the chiller, there is some amount of hot air recirculation occurring. The formula, below, will give you the percentage of hot air recirculated back into the condenser.

To estimate the amount of recirculation, all you need is an accurate thermometer – F or C degrees is fine – and use the following formula:

Intake Temperature – Ambient Temperature

Percent recirculation = ———————————————————- X 100

Discharge Temperature – Ambient Temperature

After you have determined the percentage of hot discharge recirculation, you are going to have to determine the cause, which could be many, and get it corrected, as any hot air recirculation is wasting energy and causing chiller damage.