Free-cooling on packaged DX AHUs

asked 2018-05-15 12:23:20 -0500

NasimMirian gravatar image

updated 2018-05-18 06:18:09 -0500


I'm trying to make a water-cooled DX, VAV AHU with the option of a “free-cooling” hydronic coil. In these case the free-cooling coil is condenser water from the cooling tower which is controlled to provide entering water close to the ambient wet bulb + the cooling tower approach. If the free cooling can’t meet the cooling load the downstream DX coil will energize.

Waterside Economizer - Effectively uses low cooling tower water temperatures to offload compressor operation The waterside economizer is a condenser water pre-cooling coil located before the direct expansion cooling coils, allowing the condenser water to provide free cooling. When the condenser water temperature is 5 F or more below the return-air temperature, condenser water is directed to the economizer coil to obtain free cooling. When free cooling is available the economizer coil functions as the first stage of cooling. The economizer coil valve can be modulated to control discharge-air temperature when the economizer can meet or exceed the cooling needs. If the economizer coil cannot control the discharge-air temperature, compressor stages are brought on to control the discharge-air temperature. The economizer and valves can be controlled to operate either constant or variable flow.

I used the same approach mentioned here: the problem is the Free cooling coil does not show any cooling effect on the passing air while the water temperature after the coil is increased and there is some cooling rate. should there be any set point temperature after the FCC? any suggestion?

you can download the E+ file here:


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