Chiller Flow control 1

[krb]

What is the concensus on how to control flow (chilled water and condenser water) through a chiller? The system I am referring to is a multiple chiller system, primary secondary pumping, with a common supply header for the chill water and condenser water, in other words, the pumps are not dedicated to a single chiller. Also, what other instrumentation is good to have (ie differential pressure guage)? Thanks for any input

KRB

 

[remp]

The best way I know off to set up your CHW system for primary/secondary is to have a dedicated pump for each chiller on the primary side and have a number of pumps in parallel on the secondary side. Primary and secondary are seperated by a by pass line which is sized with very low pressure, just a few ft of H20 i think.

Stage the secondary pumps on/off based on a pressure differential senso acorss the suppy/return lines, maintain a constsant pressure here. Use a VSD (freq. inverter) and/or by pass line and valve on the secondary side.

The difficult bit is staging the chillers on/off. First you must establish if there is demand for chilled water on the secondary side or not. This can be done by putting a flow metre on the by pass line between the sec/prim circuit. The flow direction and water quantity will indicate if there is too much water or too little water in the primary side. This can also be done with temperature sensors only, but is much more tricky...Dont fall into the trap of installing a check valve on the by pass, this will cause problems. Water should be able to flow in both directions in this by pass line.

I'll have to check if you need flow maters on the primary or secondary pumps. Its ages since I did one but i got the feling you may.

Primary pumps shuld be constant volume pumping only.
The condenser water can be set up on constant volume, primary pumping only. If you want consider variable pumping here to conserve energy but talk to chiller manufacture first.

 

[redgdon]

Used a set up similar to the one described by billyq. System didn't have flow meters on the primary or secondary pumps and it worked fine.

 

[quark]

This is a general arrangement when the pumping station is far away from chilers. Common header will save you investment cost.

You can have a modulating flow control valve controlled by a differential sensor installed across supply and return lines of each chiller. Alternatively you can have mechanical flow balancing valves. Even with common header, I would suggest one pump with one chiller to reduce power consumption incase if you are keeping any of the chillers idle.

I operated systems without any automatic flow control valves and never had big problems because primary chilled water and cooling water flow rates are fairly constant.

As for controls, I suggest individual pressure gauges in supply and return branchoffs of each chiller. If you cannot afford flow transmitter, go for a dp transmitter or atleast a paddle type flow switch.

Regards,

 

[remp]

Yea, just checked, you dont need flow metres on the primary or secondary circuit, only the by pass loop.

 

[greenaire]

Hi All

I am curious about the location of Buffer Tank(s) if the system really needs it. What I mean is primary loop or secondary loop.

GA

 

[krb]

When you have a common header arragement as we do, without flow meters or dp guages how can you be sure you have correct flow through the chiller? Our contractor balanced the flow at the pump discharge valve, and I have had several occaisions where condenser water flow through chiller has been low, but I had no direct indication unitl having problems witht the chiller. I have purchased dp guages so I can quickly check flow. It seems to be standard practice to balance the flow at the pump discharge valve, but it makes more sense to me to add a dp guage and balance at the chiller.

 

[Cooky]

To greenaire:

With regard to buffer tank location, I would suggest the following. If your chilled water system is serving a process application, and there is a possibility of sudden surges of warm/hot water returning from the secondary circuit. Then I suggest the buffer vessel is located on the return side of the secondary circuit, this will act to smooth the temperature variations of the water entering the chiller.

If the reason you need the buffer vessel is to increase the water volume of the entire system, then I suggest the buffer vessel acts as the common header. With the primary circuit flowing in at the bottom and out at the top (both at the same side). The secondary circuit flowing out at the bottom and back in at the top. The above description is based on a vertical vessel.

Cooky

 

[greenaire]

Dear Cooky

Thank you very much for your input and it is valuable to me.

GA

 

[remp]

KRB...If your primary chilled water hydraulic system is some what complex, like 4 chillers, there will be a difference in water flow through Chiller No 1 if all chillers are working compared if only Chiller No.1 is working, pressure drops in common pipes go up because flow goes up when more chillers and associated pumps are switched on. This will even happen when there are 2 chillers only
Make sure the system is balanced with all chillers working. This will ensure more than design water flow throgh the chillers when only 1 is working....ok energy wasteage, unless you have plenty money to spend, in which case the best way to design a system is to have flow metres on each chiller and a VFD on the primary pumps to give the exact water fow through the chiller no matter what the situation is, regarding staging of chillers. You can also get automatic balancing valves which will ensure correct water fow throug the chiller with out the VFD.
Keep a closer eye on your contractor too, be there when he is balancing the system and unseure no body messes with the valve settings after the exercise.

Balancing using pump curves and throtting the pump is OK, thats how Id do it, but check DP across the chiller vessel to confirm the flow rates. Get Chiller vessel flow curves from manufacturer

 

[SAK9]

Krb,

I have used chiller sequencing based on instantaneous cooling demand of the building.This can be done by measuring the return flow from the building before the bypass connection and the chilled water supply /return temperaure from the building.The product of flow and the temperature difference gives you the instantaneous cooling load of the building.This input is used to determine the chiller loading sequence.An additional feature you can use is to put in a directional flow switch in the bypass line to give an indication if there is a shortfall in system flow requirement.