Dehumdification Software Recommendation 1

[DrRTU]

I normally use HAP for my loads but it will not work in this case. I have a space that has a heating load, minimum ventilation load and substantial internal moisture production. I would like to heat the space with direct fired MAU with a minimum 40% outside air. I need to size the MAU fan cfm as a function of heating load AND indoor space RH\DP. Space will only be used when OA is less than 50F. Building is located in Midwest. Thus this is a dehumidification process w/o a cooling or desiccant wheel. I have completed the heating loads / MAU sizing based upon IAT&OAT + minimum OA flow but have the issue of using OA to lower space gr/ft3 in a direct fired application.

 

[Zesti]

"with a minimum 40% outside air" : you mean 40% of the air needed to heat the space?

The 60% that is recirculated will of course not remove the moisture created from the space.
Based on required inside Absolute Humidity and worst case OA Absolute Humidity you can calculate what volume of outside air is needed. If this is less than the 40%, then you are in luck. If it's more, the control system has to vary the OA % based on humidity.

I guess al you need is a Carrier/Mollier diagram and the appropriate climate data for the location.

 

[Zesti]

In addition: If by "direct fired" you mean burning gas directly in the airstream that goes into the space, then you have to take into account that burning gas creates water vapour...

 

[EnergyProfessional]

reconsider the direct fired option. Most manufacturers only allow 20-40% re circulation for good reason. And if a damper gets stuck, you create Auschwitz. Engineers are supposed to protect life...

IMHO direct-fired is only appropriate where you ventilate a dirty space, like a garage, shop or welding area etc. As mentioned above, combustion also produces water.... so it is questionable how you dehumidify with direct-fired units.

 

[DrRTU]

All, thank you for your input. Space data is as follows, IAT=73F, IRH = 50%, OAT=-4F, OARH%@-4F=80%, internal moisture release to space = 325#/hr., maximum OA temp = 30F and at 80%RH. Total conduction @-4F = 650,000 BTU/hr with 20% safety factor, Area = 54,000ft2, Volume = 1,863K ft3, Infiltration = minimal due to positive pressure. Basis of design is to utilize direct fired heating / vent. My design was an iterative approach - I did a mass balance on moisture load, arrive at a minimum outside air flow, add conduction loads, add for moisture addition of burner, recheck OA flow and size machine at various OA loads. Resulting design ends up with 16,000 of OA. VFDs run exhaust fans based upon space RH/DP up max 16K exhaust flow. 60/40 MAU unit runs to maintains positive space pressure. MAU has the burner ability to do 100% OA @-4. Based upon my plots, worst case is when OA is 30F@80%RH, I can only remove 336#/hr and this is without burner load. Thus is was looking for software to help with sizing of OA flow. A small change in internal or external moisture loads adds a large increase in flows.

 

[EnergyProfessional]

Not sure there is a canned software. but what you did per hand you could program in EES. that way you can change your parameters and it automatically "looks up" the values.

Overall your approach sounds correct. I still don't think recirculating in a direct-fired unit is good. Especially in your case you want to remove moisture with OA, so reducing OA seems counter intuitive. If you want to make up some additional building load, you could install a separate recirculating indirect-fired unit.

I once reviewed a design where the "designer" proposed to recirculate 100% of air in a direct-fired unit. They were a bit bitter when i made them change that... but I figured the unit will flame out after some run time for lack of oxygen, long after it had killed all humans.

 

[Zesti]

ROBATHERM have AHU's that fire gas or oil, but NOT in the airstream.

And it might be energetically more efficient to make heating/ventilation and moisture removal two separate processes.

 

[Leon75]

I would also look at a munters heat recovery wheel, they can handle both latent and sensible parts of the cycle.

I have seen them used in a similar application where there was a large % of OA air and a requirement for latent cooling (de-hum) and not much sensible cooling. It was in a bar / club where they were using 100% OA to improve comfort level and allow patrons to smoke in certain areas of the bar. It worked a treat.

As a bonus it was very energy efficient.