100% Outside air application

[ClydeMule]

Dear Experts,

I would like some input on an HVAC/Chiller problem (oppurtunity).

Given:
10000 ft^3 semi mobile building. (probably will never move)
12-15 volume changes/hour (2000-2500 CFM)
Desired air supply 68 DB 50% RH
100% Outside Air.
Outside air conditions: worst case 90 DB 90 RH.

THe trick is that this customer will be making many of these mobile facilites and they want the same one no matter where they go in the US. I am trying to convince them that this is not practical. That issue aside, my questions are:

Since this is 100% outside air, I have been calculating the coil load by CFM*dH(plus the conversions required). Do I also have to account for the sensible and latent loads in the room even though they will not "touch" the cooling coil?

When I am mostly only needing dehumidification, what would be the best soure for reheat? I have condenser exhaust from an adjacent process chiller and the superheated compressor discharge. Or do I need to look at hot water or duct heaters?

Right now I don't have any controls selected, so I can select and program those to fit my application.

Yes, we have to have 100% ventilation due to air quality issues.

Thanks in advance for any input.


Clyde

 

[VEEKRISH]

Hi ClydeMule,

Interesting, what you are trying to do.
To answer your first question - If you have already decided that 2,500 CFM is what you will pass through the coil, then the internal load is not relevant.
Internal load will become relevant in case you are trying to maintain the internal space temperature. In such an eventuality, you will require the internal load to establish the actual air quantity required to maintain the space temperature, given the off coil conditions.

In case you want re-heat - again the amount of re-heat will be decided again by what internal loads are present. If waste heat is available, it would be a good idea to use that.

You could also use an energy wheel to pre-cool the fresh air
before passing it through the cooling coil. This will reduce refrgigeration load (also may be reduce coil size?) - you could do this provided contamination is not an issue.

 

[ChasBean1]

Clyde, quite a complex problem to build one system to satisfy all regions! Interesting...

I assume you mean 68/50 to be maintained in the space? Are these parameters critical, and within what tolerance?

If your client isn't too energy conscious (and to keep things simple), a system that discharges a constant 49°F and reheats as necessary to maintain room temperature would do the trick. Humidification would be needed in the winter.

Installing a cooling system to satisfy your design summer condition at the flow you specify would be about a 24 ton unit based on enthalpy differences at 68/50 versus 90/90. It would be capable of one ton of cooling for every 50 ft2 (picture a large A/C unit in a 7 ft x 7 ft bathroom window). Keep in mind your space is about the size of a small house that would be fine with a 2-3 ton unit.

General thoughts:
• AC unit size would be excessive for normal situations (multi-compressor staging, hot gas bypass would be required).
• Consider heat recovery via glycol water loop. Recommend against heat wheel if air quality is of high concern.
• If you have to produce steam for humidification, you might use that same source for reheat.
• If this is one open space with no interior walls, reheat could probably be done at the central AC unit, otherwise it should be done at each zone.
• Discharge 49°F above any outside air conditions greater than 50°F. Discharge temperature could probably be scalable below that (e.g., 0°F outside = 65°F discharge up to 50°F outside = 49°F discharge).
• Start with communication of needs with good AC manufacturer: Munters, Pace, Haakon, Temptrol. Stay away from Trane, Carrier, York, Mammoth (only my opinion).

Just some thoughts for starters... good luck. -CB

 

[quark]

Clyde!

As CB pointed out the tonnage comes out to be 24 (by enthalpy difference)plus heat load in the area. I couldn't even plot the process on a psychrometric chart. This process certainly needs reheat after cooling down to ADP. Even if I consider total sensible heat (Sensible heat load factor as 1) I couldn't plot it.

The design load far exceeds the average consideration of 1ton per 200 sq.ft. (minimum value of a range)

My suggestion incase of strict adherence to above conditions is to go for a desiccant dehumidifier to reduce the moisture and then sensible cooling.

Regards,

Repetition is the foundation of technology

 

[kck115]

ClydeMule
A company called DryKor makes a unique ‘dry conditioning’ unit. The units are typically for applications such as swimming pools, ice rinks, locker rooms, etc. but this may be good application.
Good Luck

http://www.drykor.com

 

[superheat]

Your client will never be happy. The system that does a great job in Detriot will not make a dent in load in Houston. The system that does great in Houston will be short cycle and not last long in Detriot. Not to mention it will cost lots more money.

 

[ClydeMule]

Experts:

Thanks for all of your responses. They were all very helpful and have gotten me pointed in the right direction.

It is nice to be able to discuss ideas and be free of hidden agendas, office politics, and the like.


Thanks again.

Clyde

 

[mdslmn]

1. The designer should cross check the space cooling load (ie. the sensible and the latent heat gain) and compare with that of the 100% fresh air unit in order to ensure that the quantity of supply air and its temperature are able to remove the heat from the space.
2. For dehumidification, I would recommend the use of a heat pipe coils wrapped round the main cooling coil or run around cycling coils(ie.preheat-cooling-reheat coils). this will ensure propre dehumidification of the outdoor air under various conditions.