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CHILLED WATER SYSTEM-DIFFERENTIAL PRESSURE SENSOR

PUMPMANPLEE (Mechanical)
23 Jun 05 2:56
GREETINGS HVAC GURUS !

I NEED SOME URGENT ASSISTANCE ON FOLLOWING:

-I HAVE A CLOSED LOOP CHILLED WATER SYSTEM.
-ESSENTIALLY IT CONSISTS OF 1 OFF AIR COOLED CHILLER
& 1 OFF CHILLED WATER PUMP.
-THE CHILLED WATER IS CIRCULATED TO INDOOR FAN COIL
UNITS LOCATED IN THE BUILDING.

-NOW, MY QUERY IS REGARDING THE DIFFERENTIAL PRESSURE
SENSOR WHICH SENSES THE WATER PRESSURE ON THE INLET AND OUTLET PIPES FROM THE CHILLER, THE DIFFERENCE BETWEEN
THE TWO (INLET & OUTLET PRESSURES) BEING THE DIFFERENTIAL PRESSURE.

-WHAT HAPPENS TO THE DIFFERENTIAL PRESSURE AS THE
FAN COIL UNITS LOCATED IN THE BUILDING ARE NOT USED,
IE; CHILLED WATER MAY NOT BE REQUIRED TO FLOW THROUGH
ALL THE FAN COIL UNITS OR ONLY SOME FAN COIL UNITS
ARE USED... WOULD THE DIFFERENTIAL PRESSURE START TO GET SMALLER ???

-ALTERNATIVELY, WHAT IS THE PURPOSE OF INSTALLING THE
DIFFERENTIAL PRESSURE SENSOR IN CHILLED WATER SYSTEMS.

...ANY ASSISTANCE APPRECIATED AS I'M CURRENTLY ABOUT
TO IMPLEMENT A NEW DIFFERENTIAL PRESSURE SENSOR FOR
A NEW CLOSED LOOP CHILLED WATER SYSTEM.

PUMPMANPLEE





quark (Mechanical)
23 Jun 05 3:19
My first advice - Don't write in all uppercase letters and try using sentence case. In internet etiquette, all uppercase letters mean shouting.

Answer to your question depends upon what type of control valves you use for the FCUs. A three way valve bypasses the redundant flow and thus flow is maintained constant in the circuit. In this case you don't have any change, hypothetically, in the pressure difference. If you are using a two way control valve, the flow across each FCU reduces with reduction in load and thus the pressure difference across the chiller inlet and outlet gets reduced(low flow means low pressure drop).

Having said this, you don't have any advantage of providing a differential pressure sensor across chiller inlet and outlet unless you have to take care about lessthan recommended flowrates across chiller and subsequent freezing of circulating fluid which results in damaged evaporator tubes.

We generally use differential pressure sensors across supply and return headers of FCUs to take care of extra pumping energy during low loads. When you have two way valves in the system and when you supplement the pump motor with variable speed system, the dp sensors try to maintain constant dp across the headers by speeding the pump up and down as per the load requirement.

What is your intention?

Regards,

KenRad (Mechanical)
23 Jun 05 7:27
quark,

You stated:

"If you are using a two way control valve, the flow across each FCU reduces with reduction in load and thus the pressure difference across the chiller inlet and outlet gets reduced(low flow means low pressure drop)."

If this is a constant speed pump and there is no differential pressure regulating means (like a d/p control vavle), then as the FCU 2-way valves shut, the supply/return differential pressure will increase as the operating point on the pump curve is shifted to the left.  The increase in system pressure drop occurs at the control valves as they stroke toward full shut.

---KenRad
krb (Mechanical)
23 Jun 05 7:41
KenRad is right, that scenario is called riding the pump curve.  As flow decreases, delta P goes up.  If all valves are closed, the pump will be deadheaded.
As quark stated, the dp sensor is generally used in conjunction with a vfd to control pump speed and maintain constant dp.

KB
McCormick93 (Mechanical)
23 Jun 05 13:38
PUMPMANPLEE indicated that the dp sensor is across the inlet and outlet of the chiller.  If the pump is upstream of the chiller, then the dp across the chiller will decrease as flow decreases.

If PUMPMANLEE meant to say that the dp sensor is aross the supply and return systems, with the supply being downstream of the chiller & pump, and the return upstream of the chiller & pump, then the dp will increase as flow decreases, as the pump curve would suggest.
lilliput1 (Mechanical)
23 Jun 05 19:39
The DP sensor may also be used to control a bypass valve to make sure the flow to the chiller do not get below the allowable minimum when two way chilled water valves are mostly closed.

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