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Need help explaining use of ASHRAE Design Condition Charts

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AVSteve

AVSteve

Mechanical
Jun 5, 2009
1
US
I am a new HVAC designer. Right now I am helping with the design of a new nursing home facility in Salmon Creek WA. The owner has not dictated any specific design conditions for the job. That being the case it is left up to me to use my professional judgment to design a comfortable facility. My problem arises with selecting appropriate 100% O/A units to use in ventilating the building. Every ASHRAE Design conditions chart includes 4 different sets of ambient conditions: Cooling DB/MCWB, Evaporation WB/MCDB, Dehumidification DP/MCDB and HR, & Enthalpy/MCDB. I need to find a simple and concise explanation of what each of these sets of temps means. I found an explanation in the ASHRAE Fundamentals book in section 14.6 but I am not certain that I understand completely the implications of these temperatures as they describe it therein.

It seems to me that:

- The 'Cooling' values should be used to select a system that has no interaction with outside air, a non-ventilating split system.

- The 'Evaporation' would be used when designing the operating efficiencies of an evaporative system used mainly for cooling, like a cooling tower

- and the 'Dehumidification' values would be used where moisture control from outside air ventilation was the main, or secondary, concern.

I am very concerned that I choose the correct conditions in my equipment selection because these conditions for the Portland area have vastly different implications on the equipment performance.

These O/A units will be used solely for ventilation purposes and will be supplying neutral air at 75 °F / 50% RH.

Any advice on comprehending these ASHRAE charts is GREATLY appreciated!

Thanks!

AVSteve
 
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You are in Portland, and you will be cooling outside air. Even though you are only going to 75 you will probably be seeing some dehumidification.
 
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Correction I did not read well enough and jumped the gun with my response. You want 75/50%RH in Portland you are going to be looking at a unit with dehumidification and reheat. You are in a moist enviroment and will need to get the dew point of your outside air to 55 to be able to reach 50%RH at 75db.
 
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You may need humidification as well to maintain 30% winter condition. As it will be all OA you should also be looking at preheat requirements.

A starting point would be whether you are using the 0.4% or 1% column. Being a nursing home, I would assume 0.4%.

Another item to consider would be total ACH requirements. If running 100% OA and not having a perimeter heating/cooling system, I would assume they will ask for 6 ACH. If the ASHRAE standards are applicable and you are using 100% OA, a perimeter system would cut a third off the DOAS requirement.

If going to 100% OA at 6 ACH you may need to look at State or ASHRAE standards, you may need to include heat recovery. Fortunately, there should not be any more asbestos enthalpy wheels.
 
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Ha! I had no idea they used to make enthalpy wheels from asbestos! That is a fun fact. Gold star for you maurice.
 
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