Economy cycle - free cooling

[remp]

I have an air handlig unit that has a fixed outside air damper and a fixed return air damper.
Outsdie temps vary from -5 to 25 dgeC so I want to add economy cycle control to the unit.
I will install an entalphy sensor outside and one on the return air duct. If the return entalphy is greater then the outside air during cooling mode I will use use O/A and modulate the return damper closed and the outside damper open as my first stage in cooling....there after I will use the cooling coil valve. Is this OK?
Is there anything else I need to be careful off??

I have seen some control descriptions mention disabling economy cycle when the outsdie air is too humid or too cold and dry...why I do not know...heard mention of humidity problems... do I need to worry about this with my proposal??

Your help would be appreciated... I wanna keep it simple but good.

REMP

 

[ProcessHVAC]

You didn't mention if you are in humid or dry zone. You might want to read this before you design:

http://www.taylor-engineering.com/d...y Economizers Don't Work - Taylor & Cheng.pdf

 

[Drazen]

you need to make psychrometric calculation, or to employ fortune teller, which does not belong within engineering practice.

some contractors tend to act as fortune tellers, but only if they are sure that they will not be rendered accountable for lousy performance.

in general, it is said that such free cooling investment is feasible if you need cooling at outdoor temperature of 0 deg. C or lower.

 

[urgross]

You probably also want to look at ASHRAE 90.1, Chapter 6, to see which economizer configuration is allowable for your climate zone. I work in a 4a climate zone, so I prefer differential enthalpy control so that high humidity won't be mistaken for cooling.

 

[DRWeig]

You have the right idea. The enthalpy sensors you mentioned will prevent issues with too-humid outdoor air. You'll want to cut off at some temperature below 40°F to deal with the dryness problem. Do the psychrometrics to choose your low limit.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[willard3]

Enthalpy or web-bulb controllers go out of calibration quickly and fail even more quickly. Avoid these problems and use a dry-bulb controller; the repeatability and longevity are much better.

 

[urgross]

Again, I would recommend first consulting with ASHRAE 90.1-2010. In my climate zone, a dry bulb controller is not allowed. First step is knowing what climate zone the unit is in according to the manual.

 

[MTES93]

The only thing I could add to these comments is make sure that the 'extra' outside air does not cause any pressure issues w/in the building. Don't know what the building function is and if there is a path for relief air.