Closed Loop Chilled Water System

Mechanical Engineering general discussion HVAC-R
#1

QUESTION

  1. I have a closed loop chilled water system and the static head from the centerline of the pump to the top of highest point in the piping system is only 20 ft.
  2. I have no expansion tank in the system because my rationale is that my piping can take the pressures changes in pressure due to expansion and contraction of the water. We are only talking about 10 oF water temperature movement. The volume in the piping system is relatively very small
  3. The chiller stops when the chilled water is 45 oF and it stops when it is 55 oF
  4. The computed friction loss in the system is 15 psi
  5. We have pressurized the system to 20 psi.
  6. At no flow, the pump suction and discharge pressure is 20 psi…which it should be
  7. BUT, when the pump runs AND the chiller cools the water down I notice that the pump suction pressure drops and the discharge pressure drops the same amount. The difference between the suction gauge and the discharge gage is still 15 psi though. Why is it dropping?

REPLIES

MTES93
Expansion tank…

A ‘duty’ of the expansion tank is to act as the hydraulic ground for your system. Think of it as the ground rod in an electrical system. It ‘grounds’ your system and more importantly the pump and allows it to operate correctly. An expansion tank should ALWAYS be installed on the suction side of pump so that the pump adds its pressure to the system. Putting it on the discharge side would cause it to subtract from the system pressure and possibly cause an air entrainment issue. Right now your system could be floating all over heck and a hand basket.

Typically we design chillers and pump systems to pump into the chiller. Thus the chiller pump, expansion tank and the air elimination device is on the chilled water return line going into the chiller. If your system has a direct feed makeup water line from a domestic service one can safely assume that entrained air is coming in w/ the cold domestic water. An air separator will eliminate the entrained air and makes sure the system is fluid and not solution. Physics (I think its Dalton’s law…) tells us that air comes out of a water solution at the WARMEST temperature and the LOWEST pressure. Which on your system is the chilled water return and the suction side of the pump. The direct feed make up water connection should be made at the expansion tank tie in or in some cases to the air elimination device directly… varies and really up to you (they are essentially in the same spot).

Now you did mention that the system has relatively low volume. I strongly encourage you to consult your chiller manufacture’s representative or installation guide and determine either the recommended minimum system volume (given in gallons per ton) or (more accurately) the run time you/they want to see on the compressors. Essentially a low system volume coupled w/ a limited stage compressor(s) (say a single or two stage scroll jobber) you end up beating the crap out of your compressor via rapid starts and stops. Think of driving a car using only the pedal to the metal and the brake. The chiller mfgr may recommend a buffer tank. I typically install one when the system flow versus system volume is in the three to five minute range. (minutes = system volume/pump gpm). Essentially the buffer tank will give your system additional volume to allow the chiller to operate smoothly. In addition, the buffer tank, coupled with an automatic air vent will then be your AIR SEPARATOR.

Peruse Cemline’s website (one good mfgr, many, many others). To save costs closely look at your local code authorities for complying w/ the ASME tank requirements. If you don’t need it, don’t do it. Spendy!

SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=323431
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