[Equest-users] Simultaneous humidification and desiccant dehumidification with enthalpy wheel

Brian Fountain Brian.Fountain at sa-footprint.com
Tue Feb 25 08:08:24 PST 2020

I have a project aiming for careful temperature and humidity control of an interior zone - looking for cool, dry space conditions at 50F and 40% rh - which results in an absolute humidity of 0.003 lb//lb-da or 21 grains.

The mechanical system has:
                Enthalpy heat recovery wheel
                Solid desiccant dehumidification - only on the outdoor air stream
                Cooling coil served by low temperature chilled water (glycol)

What I am seeing is excessive dehumidification by the desiccant system - and corresponding humidification in the same timestep - especially in the summer months.  In the extreme, there are some timesteps where the absolute enthalpy leaving the desiccant unit is negative (a neat trick).  The problem occurs when I specify an enthalpy heat recovery wheel.  It does not occur when I specify a sensible heat recovery wheel.  I have tried broadening the deadband between max and min humidity without resolving the issue.

My hypothesis is that while the desiccant dehumidification sees the heat recovery wheel output air conditions, it is "controlling" to conditions upstream of the wheel.

I have attached a sample project that displays the same behavior  - significant humidification in the summer months when an enthalpy wheel is included but none when only a sensible wheel is used.  The sample project hourly report is also attached as a spreadsheet.  I ran it on Toronto CWEC2016 weather but I see the same phenomena on other weather data sets.

I am running the model DOE-2.2 in build 7175 but it also occurs using 7173.  DOE-2.3 does not have the desiccant keywords implemented so there is no help there.

I would appreciate any suggestions and or observations.  Have others also encountered this situation?




Brian Fountain P.Eng., LEED AP BD+C, BEMP
d 416 572 8501 m 416 562 6831
Brian.Fountain at sa-footprint.com

1100 - 100 Sheppard Ave. East, Toronto ON M2N 6N5
t 416 218 7025   f 416 487 9104   sa-footprint.com


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