Open loop air system vs closed loop
Open loop air system vs closed loop
(OP)
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
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





RE: Open loop air system vs closed loop
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.
RE: Open loop air system vs closed loop
RE: Open loop air system vs closed loop
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.
RE: Open loop air system vs closed loop
RE: Open loop air system vs closed loop
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.
RE: Open loop air system vs closed loop
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.
RE: Open loop air system vs closed loop
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.