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Padmanabhan Iyer
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Join Date: Jul 2008
Posts: 63

2 Way Modulating Valve in AHU

07/05/2015 6:42 AM

A 100 % fresh air hospital project AHU with constant speed chilled water circulating pump has been fixed with 2 way modulating valve in the out let of the chilled water coil controlled with thermostat instead of 3 way bypass valve by a Butcher of HVAC. In case any of 2 or more of these valves of AHUs out of 5 closes on reaching inside conditions during lean hours or evening what will be the effects on the pump head?
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Crabtree
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#1

Re: 2 way modulating valve in AHU

07/05/2015 8:02 AM

Your pump head will go up as your flow rate goes down. The gradient you will find by using the pump manufacturer's performance curve, which you used to select the pump in the first place. Make sure you have a copy on file.

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Padmanabhan Iyer
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Join Date: Jul 2008
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#2
In reply to #1

Re: 2 way modulating valve in AHU

07/05/2015 8:26 AM

Which will be economical and time saving? Replacing the constant speed pump with a VFD pump or replacing the 2 way modulating valve with 3 way by pass valve?
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Wal
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#7
In reply to #2

Re: 2 way modulating valve in AHU

07/06/2015 12:20 AM

Fastest and cheapest is a VFD installed to control the existing pump motor.

Don't go slower than say 50% or you may have motor heating problems if it isn't VFD rated.

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spades
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#3

Re: 2 way modulating valve in AHU

07/05/2015 9:37 AM

Assuming the pump is a centrifugal type, the pump will accelerate slightly, and head pressure will increase but this will not cause any damage to the pump, and it should not be capable of reaching such pressure as to be a concern for the piping.

You can safely throttle a centrifugal pump severely with no danger to the pump.

You can even dead head (totally close the discharge) the pump for short periods, but don't do it for long times as the fluid in the pump can over heat and vaporise thus causing pump damage.

Again I assume that the pump is controlled by at least one valve being open (demand switch or similar on the valve or thermostat) so that the pump is not dead headed, but if not then you should have some provision for bypass should all valves close.

Changing just one valve to a 3 way bypass would achieve this, as would having a smaller diameter bypass from the discharge to the suction, but don't put the connections too close to the pump as the recirculated fluid may still overheat.
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Padmanabhan Iyer
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Posts: 63
#4
In reply to #3

Re: 2 way modulating valve in AHU

07/05/2015 9:46 AM

Pump is end suction end delivery split case centrifugal over sized to the demand of 31 KW to 44 KW. There is a DRV of full size of the pipe with a globe valve also in the delivery of the pump installed by the HVAC Butcher for balancing.
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Tornado
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#5
In reply to #4

Re: 2 way modulating valve in AHU

07/05/2015 12:29 PM

The trouble-shooting diagnostic of "HVAC Butcher" seems to capture matters quite well.

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lyn
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Join Date: Oct 2008
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#6

Re: 2 Way Modulating Valve in AHU

07/05/2015 2:35 PM

Padmanabhan Iyer,

Is this the same installation you botched in 2013 or a different one?

Butcher Distributors Inc. is a Trane distributor.

Trane has a good reputation as far as I know.

It sounds like you are not competent to be in charge of a large hospital HVAC installation.

I'd ask Trane directly.

Past history:

"Could some one advise what PN rated valves and fittings such as strainers, DRV & elbows fittings to be selected for a chilled water HVAC sysstem circulating"
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Padmanabhan Iyer
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Join Date: Jul 2008
Posts: 63
#8
In reply to #6

Re: 2 Way Modulating Valve in AHU

07/06/2015 12:49 AM

Yes it is the same installation we discussed in 2013. Infact i rectified the Butcher HVAC design and made the system functional without much expenses. It is not Trne but Carrier. Trane dont do such unprofessional jobs.

Regardnig ur query about PN for fittings it depends upon the head of pump. Normally PN 16 should be OK upto 8 floor hight. Incase if it is district cooling system, you should go for PN 20 as there are chances of pressure reducing valve in main line failure.
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lyn
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#9
In reply to #8

Re: 2 Way Modulating Valve in AHU

07/06/2015 3:14 AM

OK, Thanks.
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Users who posted comments:

Crabtree (1); lyn (2); Padmanabhan Iyer (3); spades (1); Tornado (1); Wal (1)

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