Variable primary chilled h2o loop

[york83]

Hey guys, I am new to the site and I am NOT a mechanical engineer (hopefully I am not violating any rules) but I would like to become one eventually. I currently work for a large HVAC mfgr. on the service end. Let me start by saying that hydronics are not my strong suit. I came upon a chilled water system today that was having some flow issues. Upon inspection I found that the make up water and expansion tank were on the discharge side of the pump. The PRV has a tag on it that says set at 30# and the relief valve is set to open at 50#. The discharge pressure at the pump was 36# @60 Hz. The suction pressure was 0 or in a negative (gauge does not read neg. pressure). Shouldnt the make up be on the suction/low pressure side of the system to ensure the loop is always full? If anyone has any links to online information or training classes/videos regarding hydronics, I would love to check it out. Im sure im leaving a lot of information out but hopefully I can get a few responses. Thanks in advance..
-Dan

 

[EnergyProfessional]

This is the problem right there. Expansion tank needs to be at location of no pressure change
Apparently hydronics also wasn't a strong suit for the guy who designed that system.

Get this book if you ant to become a mechanical engineer. Expensive but worth it.

 

[cdxx139]

Take a look online, plenty of free stuff out there, especially from vendors.

To answer your specific question, read:

http://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/2013/02/TEH-1196B.pdf

Hydronic heaven!

knowledge is power

 

[CarolinaPE]

Here is another schematic, hope this helps.

http://www.jmpco.com/page.cfm?sp=schematic&app=5

 

[york83]

Thank you guys for the responses and links.. Be well..

 

[IBechir]

I really hope this thread is still active and someone replies to this. I know that the expansion tank should be located at the pump suction but I read something confusing. I quote and reference the author below.

Expansion tanks should be located at the point where the system is to
be stabilized, not necessarily at the suction of the pumping system.
With the advent of variable-speed pumps and their digital control, the
operation of a pump is not dependent on its suction pressure being
constant. In the past, the expansion tank was always located at the
pump suction and took its air from the air separator. This expansion
tank was of the plain type without an internal bladder that separates
the air from the water. The result was absorption of air by the water
and waterlogging of the tank.
This nuisance was eliminated by the bladder-type expansion tank.
This expansion tank is located away from the air separator, and this
enables the designer to locate the expansion tank at the desired point
of pressure regulation in the water system, not just at the point of
installation of the air removal equipment.
On low-rise buildings, the expansion tank for both hot and chilled
water systems can be located at the suction of the pumps. On high-
rise buildings, the expansion tank and water makeup equipment
should be located at the top of the building to ensure continuous pres-
sure at that point in the system.

from Rishel, J. B., Durkin, T. H., & Kincaid, B. L. (2006). HVAC pump handbook. New York: McGraw-HiIl.

So there's no need to keep the expansion tank at the pump suction and it should be kept at the top? I don't understand. Can someone clarify please?

 

[york83]

I cannot clarify for you, but in my situation the building is only about 20' tall and I believe the expansion tank DOES have a bladder. The expansion tank is situated at ground level.

 

[EnergyProfessional]

first, read the resources we provided. the reason you want a certain pressur at all point is, that the air won't go out of the system if you have points of negative pressure, it will suck air in. So the advise to have expanion tank on top of system has some validity. Negative pressure (compared to atmosphere) also cna cause boiling.

VFD didn't change anything for location of expansion tank, at least i don't see how. whne a VFD throttles the pump, all it does is make the large pump be a small pump from a pump performance point.

the fact that we no use bladder tanks also doesn't have anything to do with the pressures. At least I don't see why this woudl matter.

 

[317069]

Can you explain your system in more details and what is the flow issue do you have.

 

[IBechir]

HerrKaLeun, I still do not know the answer but I do know that the type of tank matters because in the HVAC Design Guide for Tall Commercial Buildings (ASHRAE, Donald Ross, 2004) it is stated:

Examples of closed water systems in a building would be both the chilled water and hot
water systems that deliver water to the various heat transfer equipment that functions to
provide conditioned air and heat to the building. These systems always contain an
expansion tank, which can be either an open expansion tank or a closed expansion tank.
The open expansion tank is always at the highest point of the particular system and is
open to atmosphere, but the area of the water in the tank is insignificant to the point that
it does not alter the definition of a closed water system as one that is not exposed to
atmosphere.

So the type of the expansion tank matters.

Also the reference you kindly provided advises to install the system pump or pumps to pump away from the point of no pressure change so that the pressure differential produced by the pump will at every point in the system always increase the operating pressure above that of the non-operating pressure.

When I compare this advise to the advise from the ASHRAE guide above and the one from the handbook in my last point I find it confusing.

 

[317069]

don't be confused, the article explain about open tank location, the article provided by herrkaleun talke about connection point not about location.
take it this way, it is better to connect tank on the pump inlet side but if you connect it on the pump outlet side, it will not be a disaster, it still work fine.
you can read ASHRAE handbook, it advise to install it on inlet side but did not prevent installation on the outlet side.

 

[IBechir]

Thank you. I found this article it clarified everything.

http://www.taylor-engineering.com/d... - Understanding Expansion Tanks - Taylor.pdf