Dehumidifying with a sensible coil

[McCormick93]

I have a cooling coil with the following specs:
Entering 76.5F DB / 63F WB
Leaving 57F DB / 56F WB
Which gives it a horizontal condition line on the psych. chart. Under normal conditions, air is circulated within the building with very little latent load, therefore very little condensation going on.

Now if I want to open up the outside air damper in mild weather in order to take load off the chiller, normally I would only do this when outside DB and enthalpy are below the specified Entering conditions above.

But say for instance I meet these criteria, and I'm bringing in 62F saturated air? I'm still below the maximum enthalpy, but can my sensible coil get to the design Leaving temperature and remove all this moisture?

Thanks for any input!

 

[lilliput1]

Is this a chilled water coil? What is the entering chilled water temperature?

 

[imok2]

You have a cooling coil with the following specs:
Entering 76.5F DB / 63F WB = RH 47.4%, DP 59*F, 64.5 gr/lb,enthalpy= 28.5 btu/lb
Leaving 57F DB / 56F WB = RH 94.2%, DP 55.3, 65.4 gr/lb,enthalpy= 23,8 btu/lb
If you use OSA 62*F, say 61 WB this = RH 94.6, DP 60.4, 78.8 gr/lb, enthalpy= 27.14 btu/lb
So it looks to me that it will not be able to meet the required cooling effect.

 

[McCormick93]

lilliput1,
It's a chilled water coil with entering water temp of 45F.

imok2,
How did you get 59F for the entering dew point? My chart shows it as 55F.

Building automation systems I have worked with have canned routines to enable outside air based on enthalpy. Because this is a sensible cooling application, I'm working on a program that limits outside air based on dewpoint. In this case, if OSA dewpoint is < 55F and DB < 76.5, OSA is OK.

Just wondering if this is really necessary, given that the "experts" who market the software have settled on enthalpy control. Am I missing something?

 

[quark]

DP corresponding to entry coil conditions is 55F. You should not have any problems with total load. The increase in moisture load of 14.3gr/lb will be offset by the reduction in sensible load. You need not worry as long as new enthalpy is less than that of coil entry condition and DP is morethan that of chilled water temperature by 5F(not strict but this is my experience).

But one problem may be that the maximum allowable BPF is less than 5% in your case[(28.5-27.14)*100/28.5] = 4.7%. If the design coil conditions already include coil BPF then you may not have any problems.

Enthalpy control is the better way to go.

Good luck,

 

[imok2]

I actually got 58.752*F for dew point on elite software. I will verify what the correct DP should be. Thanks for pointing this out

 

[imok2]

Software is OK... My mistake...OK, that's one in 30 years...NOT!

 

[McCormick93]

Thanks quark & imok2,
After thinking some more about it (dangerous), I notice I have no humidity sensor for return air. Therefore, I can't compare inside enthalpy to outside. So using strictly enthalpy control may cause me to bring in outside air which is more difficult to cool than return air which is not necessarily as warm as the design point.

If I compare inside DB to outside, AND limit outside dewpoint to 55, this should gaurantee that outside air will provide relief to the cooling coil.

Thanks all for the commentary.

 

[lilliput1]

Just make sure the coil is piped counterflow to the direction of airflow through the coil.