[Equest-users] Humidity control in PSZ Heat Pump system in Base design (CRAC unit modeling)

[Peter Worley]

Derek,

I'm not sure if this is exactly what you're running in to, but I thought I'd share what I learned over the weekend re: modeling CRAC units using PSZ systems, with humidity control.

I have set up my CRAC unit as a PSZ with DX cooling and electric heat + humidification. Required space conditions are 70 degF +/-5 degF and 50% RH +/- 5% RH; supply air is 55 degF. Initially, in an hourly report, I saw that space temperature and high humidity level were being met. However, the low humidity limit was not being met. In fact, the unit's return air humidity ratio was tracking the OA humidity ratio (as low as 15% RH at 70 degF), until it reached the high limit (dehum worked fine). This tracking was particularly strange as the unit is 100% recirculation (0 CFM outside air). Further checking showed that no heat was being used by the humidification coil, even though it was installed and set properly.

After some back and forth, I tried setting the outside air quantity at 0.0001% (~1 CFM OA, for this particular unit). A re-run and re-check of the hourly data showed that all required conditions (including the low humidity level) were being met. Strange, no? In my case, I don't think it would impact the energy consumption at all. Theoretically, I shouldn't need any humidification if I don't have any outside air coming in (I haven't modeled any moisture losses, and my 55 degF off-coil condition (55 degF DP) won't remove moisture from the 70 degF, 50% RH (50.5degF DP) room air).

If you're seeing something similar, some questions to think about are: what are the conditions that you're trying to achieve? What is your supply air condition? How much outside air are you bringing in? Do you have any moisture losses (infiltration)? I hope that's of some help. If not to you, than to someone else!

Regards,
Pete


Peter Worley
Mechanical Engineer

155 Avenue of the Americas  New York  NY  10013  USA
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From: Derek Huang [mailto:Derek.Huang at atce.com]
Sent: Saturday, July 10, 2010 9:10 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Humidity control in PSZ Heat Pump system in Base design

Hi all,

Per ASHRAE 90.1, I have to specify PSZ heat pump for each zone. Each unit serves one zone of a data center. From the DD Wizard, I have modified to add humidity controls in Detailed Data Edit mode but it seems the Base model didn't take this into account (can tell from the energy summary). Any ideas ?

Thanks

Derek Huang, LEED AP, CPD
Mechanical Engineer

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97 E. Brokaw Road, No 300
San Jose, CA 95112
Direct: 408 487-1305
Fax:      408 436-1511
www.atce.com<http://www.atce.com/>
derek.huang at atce.com


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