HVAC CHILLER help

[T77911S]

we have a small chiller system here at work. I have been to HVAC school but not chiller. its a YORK condenser system.
it has not been keeping up, oh its only a few years old.

my question is about water flow.
the controls are set to run the pumps at 40% "minimum" and that is where they stay no matter the heat load.
Q: should the water flow increase with heat load?

I had upped the one pump from 40% to 50% about 2 weeks ago and there did not seem to be an issue. then yesterday the tower cab (I work for the FAA) got up to 87.
this morning I noticed the pumps switched and the pump it was on was at 40% so I just upped it to 58%. water flow went from about 45gpm to 65gpm.

there is a "set point" for gpm of 75gpm but not sure if that is an alarm or just the max it can be.

I am letting the water temps stabilize as the water supply temp out of the condenser was going up from 41deg to 44 last time I checked.
BUT the return from the AHU was coming down, it was at 60deg.

my thinking is if there is more water flow then the water will stay colder making the air supply colder. (it was about 58deg.) the room temp was already up to 74

[Ziggythecat]

The slower the flow, the more efficient the transfer of cold to the liquid you are trying to cool.
More contact /longer dwell time is better.
It also reduces the friction of water through the pipes and friction adds heat.
In many situations, the best thing to do is run the liquid just fast enough to prevent freezing.

[billybek]

Could you get the Following info?
Chilled water temp entering and leaving evaporator
Chilled water pressure in/out evap.
Refrigerant r134a?
Evap ref press
Ref temp leaving evap
Cond press. Ref temp leaving cond

Keep in mind this could be an air handler problem.
Could you check air temp drop across the cooling coil?

[biosurfer1]

You say the pumps are running at 40% so I would assume everything is on VFD's? If so, I dont understand why they are set to a specific speed...usually they modulate based on an input (inlet temp, static pressure, etc), otherwise just get a single speed pump that runs at whatever GPM you're looking for?!

[T77911S]

its a 410 unit, 2 comps
water temp WAS set at 45
pumps at 40%=45gmp

cond unit
supp-42
ret-52

evap
supp-43.3
ret-58.5
air temp- 59
ret air 74
flow-8.8gpm
energy-5.6btu


when I upped the pump to 58%=66gmp
it raised the supp temp out of cond a little. (only 1 comp running)

I dropped the water at the cond to 40 degrees.
temp in cab was already up to 76 and now its coming down,.

cond unit
supp- 36
ret- 47
70gpm

evap
supp-37
ret-55
air supp-56
ret air-74.7

evap says 8.6gpm
energy -6.4btu

when would the water supply rate need to be changed

[T77911S]

water pumps have a minimum setting of 40%, I have never seen them any higher.

that's my question, why or when would the flow rate need to go up and would that help with cooling.
the cond unit was not maxed out, was just thinking if more water in through the evap faster that would drop the return temp which would drop the air temp. no?

[red-beard]

Originally Posted by T77911S

water pumps have a minimum setting of 40%, I have never seen them any higher.

that's my question, why or when would the flow rate need to go up and would that help with cooling.
the cond unit was not maxed out, was just thinking if more water in through the evap faster that would drop the return temp which would drop the air temp. no?

That is a motor/VFD limitation. VFDs have a range generally of 40% speed to 110% speed, for turndown.

The flow should be varying with the heat load. Temperature should be constant. Control can be set based on many conditions, room temp, outside temp, return temp and and combination. The entire system, HVAC and the chilled coolant flow, should ramp together.

We use chillers on our gas dehydration skids.

[BK911]

Chillers have a minimum flow requirement.
Chillers are also just one part of a system.
What is the other part?
Air handling units with 2 way valves or 3 way valves?
Any bypass in the system?
Primary / secondary system?
Variable volume system?
Constant volume system? yes you can have a CV system with VFDs.

[billybek]

Someone may have changed some settings recently.
One sure fallback is to find the IOM (installation operation and maintenance ) books for the unit and ahu.
Maybe the pumps have been overridden to manual?
Seems like you could get a bit more performance on the temp drop of air with more flow.
Too much flow can make for problematic operation and poor heat transfer on the water side.
On the off chance that it could be the ahu check damper positioning/operation.

[MysticLlama]

Far from understanding what a bunch of the numbers posted were, but we had a restaurant that we just could not keep cool no matter what we did. Turns out a big part of the problem was that the float that added water to the supply when it dipped too low was stuck and we were constantly adding new water to the system and dumping other water out the overflow. Didn't work real well like that.

[T77911S]

room temp is down to 74 and its hotter out so maybe I wont get bothered over the weekend
supp air temp is down a little more to 54.7
I dropped the water supply down to 50%

the colder water temp setting did it. before I don't even think the second compressor was coming on due to the load on the outside unit was not hi enough.


no one else is going to mess with it. its scary but I am the most knowledgeable one here on it.

still would like to know if the water supply should be increasing with heat load like a variable air handler might.

[BK911]

Chiller should modulate to maintain a constant supply water temperature.
Water flow modulates to maintain AHU discharge air temperature setpoint, down to the chiller minimum.

edit:
Water flow modulates to AHU to maintain discharge air temperature setpoint.
If 2 way valves, this also increases/decreases water flow in the system as the AHU valve modulates.
If 3 way valves, the overall system flow does not change. or should not change, if the 3 way bypass was balanced properly.

[billybek]

Another important point.
If it truly is chilled water and not glycol, make sure you don’t turn it into a slushy machine!😉