Economizer and Humidity Control

[torpor]

I am facing an excessive humidity issue for some packaged RTUs introducing OA and with economizer options for a commercial space. The trouble with this is, the amount of OA will then be governed by the eco. options. If the economizer control is dry-bulb type it would allow OA with all the outside RH into the space (Is that so?). I am assuming this operation leads to 100% OA at some point. Space humidity would then hit the roof? Right? If its an enthalpy type control it would have control over the humidity introduced to the space. With no reheat in either case.
I am not sure what controls are on the RTUs economizer now - but the space almost starts drenching. And this is in NJ. From what the installers tell me its an enthalpy control.
We are constantly monitoring indoor&outdoor humidity to figure out the problem. No success yet. Any ideas?

 

[NCDesign]

First, to directly answer your economizer question. You have three types dry bulb, enthalpy, and comparative enthalpy. As you stated dry bulb just looks at the temperature of the air, so if your economizer set point is 60 F and it is 59 F and raining you will dump moisture into the space like crazy. An enthalpy economizer takes the moisture into account but just checks against a single set-point. It would be possible to dump excess moisture into the space if your set-point is above the enthalpy of your space. A comparative enthalpy economizer has sensors in the return and outside air. It compares the two and if the enthalpy in the outside air is lower than the enthalpy of the return air by some adjustable factor it will allow the economizer to operate. Every economizer set-up I have ever seen, or spec'd, on a standard packaged rooftop is an all or nothing thing. It's either closed (or at its minimum) or cooling with 100% outside air.

There are some things that could be wrong with the set-up that would make it run in economizer mode when it shouldn't. Check the location and calibration of the enthalpy sensors. Make sure you don't have any ponding issues underneath your outside air intake. Check the operation of the damper itself, is it closing and opening as it should?

As far as the problem you are describing, I would bet that during occupied mode the fan is in the on position, the outside air damper is at its minimum, and the economizer is not even in play. I have seen this many times when there isn't enough sensible load in the space, the outside air, or some combination of both to actually get the compressor(s) to run. So the unit sits with the fan on, dumps humid outside air into the space, and the compressor doesn't run because space temp is met. When the compressor does run it is only for a very brief time which doesn't get any of the moisture out of the space. This is very common when the unit is over-sized.

 

[AbbyNormal]

try to avoid pumping in air with a dew point higher than 57F. a cheap fix on the drybulb one is then do not set it higher than 57

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Golestan]

NCDESIGN,
thanks for your insight. I am also podering this issue on my designs. Based on your explanation, we should always spec the comparative enthalpy option. The Lennox rep and the trane rep have both shared this with me, but I thought is was salesmanship, (I am in Dallas, TX ) and it gets really humid here. So I will spec the comparative enthalpy from now on. I am sure the contractors will complain about the cost. Is this option expensiv?

What did engineers do before the comarative method?

thanks-good weekend...

 

[marcoh]

Before engineers used the comparative methods they experienced the same issues you are facing!!

 

[NCDesign]

Contractors would not know what to do if they couldn't complain about those 'stupid' engineers adding too much cost to a job. I should know I worked for a contractor before moving strictly into consulting engineering.

Once you've paid for an economizer, the comparative enthalpy is a relatively small increment in price. It's also much cheaper than dealing with the problems that led to your original post, assuming it is the economizer causing them. Keep in mind that getting it spec'd and the contractor supplying it is just the first part of the equation. It still needs to be installed and set-up properly.

I am not sure which code Dallas is under but the International Energy Code gives you some flexibility on the use of economizers. Check section 803.2.6

 

[torpor]

Thank you all.

There are a few options we are planning to put to test. I think the system has the simple enthalpy type control. If at all anything - dumping in const. vol OA will definitely work. Only thing is - the system loses its energy efficiency.

Thanks once again - Ncdesign

 

[lilliput1]

If humidity concern is a problem with economizer it can only be because you are using enthalphy economizer which is actually always the comparative type. It compares the enthalphy of the outdoor air with the return air. If outdoor air enthaphy is less, it dumps all the return air and bring in 100% OA. You can get in trouble if the humidity outdoors is higher than the humidity level corresponding to your design cooling coil leaving humidity which would be at about the same humidity at 55°F db. You should just use straight outdoor air economizer which mixes OA with return air to get 55°F mix. Look at the pschrometric chart and you will see that at or below 55°F outdoors the outdoor air humidity will be below the maximum allowable. Do not oversize DX systems. Consider using demand control ventilation using CO2 detectors. Provide vestibules at major entrances. If units are oversized consider making the highest loaded unit be the ventilation air supply of the other units by closing off the OA damper of the other units and ducting supply air from the ventilation units to the return duct of the others.

 

[NCDesign]

liliput1 all enthalpy economizers are not comparative. See my first post. Also, the nature of an economizer means that you are using 100% outside air to cool the space and not some mixture of outside air and return air. Finally, if the enthalpy of the outside air is less than the return air it is impossible to dump additional moisture into the space. Take another look at a pysch chart.

With that being said demand control ventilation is a very good idea because during normal times (not economizer mode) it can usually allow you to use less outside air. You just have to make sure your CO2 sensors are kept calibrated.

 

[AbbyNormal]

75F @ 50%RH return air would have a dewpoint of just over 55F, a wet bulb of about 62.6 and an enthalpy a little over 28 Btu/lb da.

If you had outdoor air with a dewpoint higher than 55F and less than 62.6F it would have an enthalpy less than or equal to the return air condition stated, yet would be pumping moisture into the space as this outdoor air has a higher dewpoint than the return air.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[NCDesign]

Insert foot in mouth...

Finally, if the enthalpy of the outside air is less than the return air it is impossible to dump additional moisture into the space. Take another look at a pysch chart.

AbbyNormal your post made me pull my pysch chart back out and take a look at economizer operation again. In fact I'm going to have to go back and talk to a couple of my manufacturer's reps about exactly how their economizers control on standard packaged equipment. Thank you and my apologies to liliput1.

 

[AbbyNormal]

My nit pick was not well worded. You could have outside air at or near saturation from 55 to 62F and be less than equal to the enthalpy of the return, is a better way of putting it I suppose.



Take the "V" out of HVAC and you are left with a HAC(k) job.