Industrial make-up air lowest temperature? 2

[m2e]

I'm doing a high level review for an industrial HVAC system where the indoor temperature is too high in both summer and winter. There is a lot of heat and moisture produced by the process in the plant and the ventilation is not enough. I understand that air change is not a good criteria for judging and designing an industrial HVAC system, but currently the plant has about 3 ACH of ventilation in the winter. It is mostly limited by the make-up air unit because in the summer they can turn on several extra exhaust fans and open the roll-up doors for ventilation.

I need to recommend several high-level recommendations for the retrofitting of their system. One option of course is the addition of MUA units. However, there should also be options where MUA units isn't added (for cost or real estate reasons).

What is the lowest temperature where untreated air can be introduced into the building before problem can occur? This is only for the additional ventilation on top of the existing MUA units. The building is large enough where the cold air won't freeze anything before it's mixed with the warm air inside. The outdoor air can get down to -40F.

 

[317069]

(What is the lowest temperature where untreated air can be introduced into the building)
(The outdoor air can get down to -40F.)
you want to intreduce untreated outside air to a building, the temperature would be tha same outside, I couldn't find a reason to ask about lowest tepertature for non heated air.
correct me if I am wrong please

 

[m2e]

Obviously I wouldn't ask the question it's that simple. Maybe I didn't word it clear enough, but if you could think outside the box you should be able to think of a reason why this question was asked.

The question is whether there is a recommended minimum temperature before the outside air cannot be introduced into the room untreated.

Let's say the minimum temperature is X degree.

If the outside air can get down to -40F, then there are two possibilities:
1. Bring in the outside air when the O/A temperature is above X degree, and stop the fans and close the damper when the temperature drops below X degree.
2. Bring in the outside air all winter long because X degree is below -40F.

In either case, we need to know what X degree is, if it exist at all.

 

[317069]

What about the case that you need outside air, the outside temperature is lower than X degree,and your existing MUA's are not enough, then what would your high level review or rcomondations be.
you have posted a question and someone asked you about a point, you do not have to be Impolite, high level of review needs high level of polite person

 

[DRWeig]

First, I'm a controls guy who has had to deal with ventilation in cold areas -- but mostly for occupied areas or areas storing hazardous materials. Wait for the actual experienced mechanical engineers to respond as well.

If you are concerned about human comfort, there may be guidance in ASHRAE 55 or (heaven forbid) ASHRAE 62.1.

If the air is blowing on water pipes or people, you're going to have to temper the air or shut the dampers at a pretty warm temperature.

If you are concerned about freezing stuff only, temper your air to 35°F at a minimum. Heating loops with glycol are common. I don't know of a lower limit for makeup air, but a lower limit of discharge air is a comfort or freeze-up issue.

Best to you,

Goober Dave

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[tys90]

If the space is warm even when it's -40F outside just find a way to mix indoor air with outdoor air to a decent temperature (40-50F) and supply it to the space. I would also design the mixing unit so it can switch to 100% exhaust in the summer. Doesn't get much cheaper than that, all you need is a fan, filter, dampers and a casing with some basic controls. Don't count on air mixing once it gets into the space, you may get a cold spot and freeze something. Pre-mix it and deliver it to the space at a high velocity if possible.

 

[Drazen]

it's hard to figure out everything from limited data, but it looks to me that your whole existing concept does not work properly - summer air changes are insufficient for adequate heat rejection, while you don't have enough preheating capacity for extremely cold outdoor temps.

as tys said, your main concern is how to mix introduced air to reach acceptable indoor supply scheme.

the only specific help you can get is the one from air distribution manufacturers. you need to find adequate diffusers, supply grilles or what you want, and you need to calculate air mixing by their recommendations, and, on the other hand, minimum air temperature is directly dependent on adopted air speed in occupied area.

ashrae handbook do have it somewhere - charts with air speed - air temperature relations, for different comfort criteria, including industrial. i don't know where it is, but i'm sure it exists as, for that specific issue i use european norms.

again, the lowest limit to start with is the one allowed in air distribution devices calculations, and you need to obey it as there is no other way to find out how to rectify cold draft issues. cold drafts can cause serious health troubles even at temperatures higher than interior design temperature, so that matter should have number one priority.

 

[DRWeig]

You may find some guidance this way as well --

Stick the following phrase in Google:

handbook of industrial ventilation

Best to you,

Goober Dave

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[m2e]

Thanks for the responses. No this isn't an easy problem to solve. One of the buildings has a volume of over 5000000 cubic feet and an internal heat rejection of over 12000 MBH. The structures and equipment inside the building are also highly complex. I was looking for creative ways of solving the problem.

I have already read through the ASHRAE handbooks, journals, and several industrial ventilation handbooks. However, so far I haven't found a guideline or a method of determining the lower limit of MUA temperature. I was just hoping to get some extra ideas from experienced people. Thanks to those who provided ideas.

 

[KiwiMace]

This seems like a fairly simple problem for winter conditions. Your space is too warm and it is under ventilated. You need to bring in more OA and mix it with the MUA unit either in a box or at a high level such that you get good mixing before it reaches the occupied zone. Your design effort will be to determine how much additional air you need to bring in to satisfy your heat load and remain at the temperature setpoint. If you do not plan to control the temperature of this air, you should plan to control the airflow. With a lower supply air temp and humidity, you need less air changes to get where you want.

In Summer when the OA temp is higher than the setpoint, you can do little to reduce the high temperature without greatly increasing your air change rate or adding some kind of cooling. As the OA is warmer and also more moist, you need to increase the air change rate to ensure the moisture coming from within is adequately purged. Your temp is going to be limited to OA + a delta of 5 or 10 degrees.

In unconditioned machine spaces, the OA demand for cooling usually far exceeds that required for adequate ventilation for the occupants, but please check your particular situation.