Multiple Units on common duct - air flow issues
Multiple Units on common duct - air flow issues
(OP)
I'm trying to explain in technical terms what is happening with the attached system. There are 3 Liebert model VH380AUAAE1A546 upflow units dumping into a common duct system with provisions for a fourth. Each unit is rated for 15,200 CFM @ 0.5" ESP.
With only one unit running, I have no issues. As a second unit is brought on, however, I am seeing liquid slugging back to the compressor on the first unit. I assume that this is caused by a sudden loss of air flow across the first coil.
Just adding up the diffusers (12 @ 3,720 CFM) I don't have enough air outlets for the fourth unit, but that not my concern right now. I need to address the immediate problem of slugging and make a reommendation on how to correct the problem.
It too late to get units with high external static capabilities, so I am almost thinking the the solution is to leave one unit connected to the duct and the others to "free blow". The entire space is only about 50'x40' and it is a server room. I think the duct may be undersized for multiple units with only 0.5" esp, but I haven't done this type of system before.
I didn't create this problem (thank goodness), but I have been asked to write a letter explaining the problem and a recommendation for correction. Any thoughts?
With only one unit running, I have no issues. As a second unit is brought on, however, I am seeing liquid slugging back to the compressor on the first unit. I assume that this is caused by a sudden loss of air flow across the first coil.
Just adding up the diffusers (12 @ 3,720 CFM) I don't have enough air outlets for the fourth unit, but that not my concern right now. I need to address the immediate problem of slugging and make a reommendation on how to correct the problem.
It too late to get units with high external static capabilities, so I am almost thinking the the solution is to leave one unit connected to the duct and the others to "free blow". The entire space is only about 50'x40' and it is a server room. I think the duct may be undersized for multiple units with only 0.5" esp, but I haven't done this type of system before.
I didn't create this problem (thank goodness), but I have been asked to write a letter explaining the problem and a recommendation for correction. Any thoughts?





RE: Multiple Units on common duct - air flow issues
RE: Multiple Units on common duct - air flow issues
RE: Multiple Units on common duct - air flow issues
RE: Multiple Units on common duct - air flow issues
I think your main problem is the diffuser locations, particularly on CRAC-1 and 3 will dump cold air straight back into the RA intake on the front of the unit. Poor balancing would make it all the worse. This duct configuration would be v difficult to balance with the 2/3 duty cycle. Can't think of an easy way to fix that off hand. Maybe headers across 1,2 and 3,4, and H the other way between across the room, then branch all diffusers L-R from the H crossbar?
I can barely make out the rack layouts, but it looks like a hot/cold aisle layout, so you might be able to effect a reasonable solution by containing the ends of the cold aisles to prevent this short circuiting back to the CRACs.
RE: Multiple Units on common duct - air flow issues
RE: Multiple Units on common duct - air flow issues
RE: Multiple Units on common duct - air flow issues
RE: Multiple Units on common duct - air flow issues
I would look into your back draft damper selection. I prefer a non-powered high pressure BDD with a viewing port window to make sure the damper is not stuck open. We have seen air leaking in the standby unit and fan trip on start due to fans spinning backwards.
I believe your operational issues are caused by low airflow. I have posted my design of a switch room with Four VH380 units in an N+1 design (2 units running and 2 standby). I listed this Lieberts at 15,200 CFM each. This is an all overhead, non hot isle design and front return. We did operate all 4 units w/o slugging issues. We did not use alternate unloading. The duct to the side is for smoke purge.
RE: Multiple Units on common duct - air flow issues
I didn't even mention the backdraft damper issue. They don't have any BDDs, but they do have isolation dampers with Belimo motors. They are slow to open which isn't doing the compressors any favors either. I'm going to take your recommendation on the HP BDD's with viewing ports.
Aside from being grossly shortly on diffusers, how does the ductwork look? Is is suitable for 3 to 4 units assuming adequate diffusers are added? I'm not a duct expert, but it seems to me that it is sized for no more than 2 units with 0.5" esp fans. Any thoughts on what I should recommend? Perhaps leaving the two on the same wall connected to the duct, and disconnecting the third and future units. I don't know ceiling height, but it's probably 12' to the deck. There is no suspended ceiling.
RE: Multiple Units on common duct - air flow issues
RE: Multiple Units on common duct - air flow issues
RE: Multiple Units on common duct - air flow issues
You could ride up the curves toward the 'do not select' region with the added static of fans fighting each other, but enough air outlets (reduced backpressure) would help with that.
RE: Multiple Units on common duct - air flow issues
I believe you are referring to the duct loops used on VAV systems in office blocks.In such instances the duct splits into halves at the AHU outlet and take a big loop around the space and join at the farthest end from the AHU so the velcities are likely to be low at the point of joining.
RE: Multiple Units on common duct - air flow issues
Forward curved fans can run at two pressure points for the same volume and do not operate well in parallel. Are the ducts drumming when more than one unit is on?
Control of forward curved fans in parallel is also a nasty control problem and you don't say how they are controlled.
RE: Multiple Units on common duct - air flow issues
Disculpeme, disculpeme......
RE: Multiple Units on common duct - air flow issues
I think if he has enough outlets, the heads of air that collide should have less ill effect - fans are lower on their curves and further from surge with less backpressure.
RE: Multiple Units on common duct - air flow issues
Any fan manufacturer will vouch that by adding Plenum you get into problem area.