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Open loop air system vs closed loop

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desertdingo

Aerospace
Nov 30, 2007
1
Hi all,

I am not very knowledgeable in HVAC and have a basic questions.

If I have an open loop air system where the unit pulls air from the ambient, goes through filters, cooling coils, air blower, then through some ductwork. The cfm needs to be about 2000 cfm at the end of the ductwork and the air blower is sized accordingly.

My questions is that with the same system, if I connect the end of the ductwork back to the intake of the unit to form a closed loop system. Still requiring 2000 cfm at the same point of measurement in the ductwork. Would this lead to less requirement for the air blower?

If so, what is the math/thorem behind it?

Appreciate any feedback
 
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You would not have to accelerate air again, so the velocity pressure would not be wasted in a closed loop. Windtunnels are circular for that reason.

i nopen system air starts out still, then fan accelerates it, and after the diffuser in space it goes back to still. In closed loop you avoid that deceleration.

I have a hard time imagining what the application in HVAC is, though.
 
If the pressure drop is be the same the work required to move the air will be the same.
 
What you describe is the same as a dedicated outside air system. Air is sucked in from ambient and no return is provided.

Whatever the most difficult duct route is sets the static pressure requirements, which sets the energy requirements. Can't think of a system I've seen that has return air having highest static.
 
If you were to compare the duct runs in both systems,open loop has less run as the air can be exhausted through a wall louvre in the room.So closed loop may require a fan which can deliver air at slightly higher pressure
 
Let me get this right. You need the unit to supply 2,000 cfm. You want to cycle some of the air back to the return of the unit. So lets say the unit needs to supply 3,000 cfm, so 1,000 cfm can be returned to the unit.

Your entering air temperature will be higher than ambient in the winter and lower than ambient in the summer. Some kind of pressure regulator will be required prior to the loop back to the return.

This is an okay system, but you will not have rapid heating or cooling in the building if you size the coils in this manner.
 
I think the actual question is being missed. The replies are trying to understand how this will be a functioning HVAC system --- and I agree with them --- but I think the question is strictly about the fan.

If you are asking if you can essentially double the amount of ductwork connected to the fan and still get the same performance, i would almost certainly say NO. Apparently the fan only has supply ductwork with return being drawn from "ambient". In the HVAC world, ambient means outside air, but you may just mean that there is one central return in the space. Now you are asking if you can put a ducted return on the same fan system and get the same CFM. right?

It all depends on the fan curve. CFM versus static pressure. perhaps the fan is large enough to still deliver the same CFM with all the static of return ductwork. A test and balance contractor will have to take readings to see what additional capacity (if any) the fan has. Curious that you say the return will be directly connected to the supply. I'm guessing some kind of process/testing application as opposed to room conditioning.
 
Refer to SMACNA Duct Design or ASHRAE Fundamentals Handbook. Compare pressure loss from ambient air intake system up to the supply fan inlet with the alternate closed loop system to the supply fan inlet. Consider inlet air entrance loss, duct and ductwork fitting and equipment (dampers etc) pressure drop.
In addition to fan energy loss consider heating and cooling energy requirement of each alternate. Consider also need for outdoor air ventilation (get rid of carbon dioxide, offensive odors, etc), pressurization with outdoor air to offset infiltration.
 
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