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ACH based air system design [closed]

asked 2016-04-14 08:01:34 -0600

FSilenzi's avatar

updated 2017-05-06 08:36:53 -0600

I need to model a hospital, which has some strict requirements for the air changes per hour. I want the air system (for the moment the "ideal loads" one, in future a full mechanical ventilation system) to supply air in order to respect thermostats temperatures, air changes per hour and if possible, also the humidity comfort levels. Is this possible? I have read a couple of answered question but I could not find exactly what I need, I have also tryed to discover it by myself, but I cannot find anything useful.

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Closed for the following reason the question is answered, right answer was accepted by FSilenzi
close date 2016-05-02 04:43:47.358388

2 Answers

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answered 2016-04-14 09:07:18 -0600

updated 2016-04-14 09:08:59 -0600

Hospital ventilation requirements typically have a minimum outdoor air ACH requirement as well as a minimum total ACH requirement.

For the outdoor air component, you can indeed specify an ACH in the Design Specification Outdoor Air Object as mentioned in Julien's answer.

To set the minimum total ACH you will need an air terminal attached to your zone (you can't use an ideal loads system). Fixing the minimum total ACH supplied to a zone by an air system will require you to set inputs in both the air terminal object as well as the zone sizing object. In the air terminal object (SingleDuct:VAV:Reheat in this case), I typically set the 'Zone Minimum Air Flow Input Method' to 'Fixed Flow Rate' and then enter my total ACH converted to CFM in the 'Fixed Minimum Air Flow Rate' field.

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In the zone sizing object, I also set the 'Cooling Design Air Flow Method' to 'DesignDayWithLimit' and enter the total ACH converted to CFM in the 'Cooling Minimum Air Flow' field.

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Setting the parameters as shown above will ensure the minimum total ACH (converted to CFM) will be supplied to your zone during the normal system operation in addition to the outdoor air ACH defined in the Design Specification Outdoor Air Object.

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I've never done a hospital so I didn't know about the min ACH requirement, that's really interesting.

I'm voting for this answer to be accepted! It even answer questions that weren't asked :)

Julien Marrec's avatar Julien Marrec  ( 2016-04-14 09:10:57 -0600 )edit

Do this prcedure need to be implemented also for the "Heating desing day"?

FSilenzi's avatar FSilenzi  ( 2016-04-18 03:38:57 -0600 )edit

answered 2016-04-14 08:44:50 -0600

It is indeed possible. You can specify ACH numbers in the Design Specification Outdoor Air (DSOA) object, which you usually apply at the space type level (but you could apply it at the space level).


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Crystal clear. Thank you again @Julien Marrec

ps: in case I can also use each zone total volume, knowing the ACH, to calculate the flow rate for each AHU right?

FSilenzi's avatar FSilenzi  ( 2016-04-14 08:56:54 -0600 )edit


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Asked: 2016-04-14 08:01:34 -0600

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Last updated: Apr 14 '16