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Enthalpy in HVAC economy mode
Engineering and workplace issues. topic
Posted by greenaz on 11 April, 2011 - 3:37 am
I'm programming a DDC for air conditioning control and am required to use enthalpy in the economy mode. The spec is very brief and they are relying on me to come up with the strategy.

I haven't done this before and I'm quite green. Any information would be a great help please!

I have an outside air temperature and relative humidity sensor and I have a function block that calculates the enthalpy in btu/lbm or kj/kg.

My basic understanding is that this should operate much the same as a simple temperature economy mode:
If outside air temp < set point, open outside air damper and close return air damper.
If outside air temp > set point, close outside air damper to minimum and open return air damper.

Is this correct? Any comments or advice on this matter? They have specified modulating dampers (not just open and closed) so I was thinking I would modulate the dampers over a small span over set point. For example modulate from (set point * 0.9) to (set point * 1.1), that is 90% set point to 110% set point. Is this reasonable?

Also what would be a good default set point to program in for the enthalpy?

Many thanks,
GreenAz :-)


Posted by willis5 on 19 May, 2011 - 12:50 pm
You are correct on the If outside air temp < set point, open outside air damper and close return air damper.

If outside air temp > set point, close outside air damper to minimum and open return air damper.

A good starting point with the enthalpy would be 65 degrees which has worked out good for my application for free cooling. You might want to find out about outside air CFM minimum which is a requirement. you can obtain this info from the balance report. Depending on you outside air requirement for air exchange you will be able to get an exact setpoint. Suggestion for setting up your dampers. find out you outside air requirement needs are and you can set your damper to the minimum when you are out of economizer mode and back to 100% open when you are in economizer mode.

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