lowering the condensing temperature reduces chiller's capacity?

[capaula]

I have Trane 2000 tons centrifugal duplex chillers, and I am having problems getting them to reach full capacity.
I am resetting the cooling towers temperature. Could this be contributing to the capacity shortage?
I am short on 300 tons. The chillers were designed for 85F condense water, and right now they get around 78F.

 

[ChasBean1]

I'm not sure, but right now you're measuring 85% of capacity and you need to consider the error of the temperature and flow measuring devices to calculate this tonnage (esp. if you have a low delta-T). On the outset, I'd think a lower condenser water temperature wouldn't hurt capacity, maybe help it, but I'd defer that one to the chiller gurus here...

 

[KiwiMace]

I don't think the lower CWS temp is hurting you any, should help. Try looking at all the indicators for capacity and make sure they agree; chiller amps, CHW delta x flow; refrigerant discharge pressure. Calibration or sensor resolution problems - use test grade instruments to verify.

If all these agree;

Operating in accordance with design setpoints? reduced CHWS temp?
Age, cleaning and water quality issues - new?
Maintenance issues - sight glass ok?

 

[capaula]

The chillers are brand new, 2 years old. Of course I am relaying on my instruments, in fact, the chillers TET and my flowmeters.
Delta T is high now, they get to 16F in the middle of the day. I mean, they are operating according to design, except the condenser water temperature.
It is a primary-secondary system, and I noticed that everytime the bypass flow goes down, and the chillers get the real delta T, they stop reaching setpoint (39F) and get maxed out. I am monitoring these on an excel spreadsheet. I also don't see how the condenser reset temperature would be contributing to that...

 

[ChasBean1]

I don't either. Where is the 'bypass' in this primary-secondary loop? - does that refer to the decoupler? There are too many variables - if you could post data and a sketch of system piping it would be helpful...

A lot of times the system configuration gets screwed up.

CB

 

[zuccus]

Capaula,

Typically Trane Centrifugal Duplex chillers will operate better with a lower entering condenser water temperature than design. Therefore if you are not making your specified load with a better ECWT, you might have bigger problems.

You say these chillers are two years old, have you cleaned the tubes, especially on the condenser side in the chiller lately or at all?

Since you said you have a flow meter, I am assuming you have one on the condenser side also, are you getting the required flow rate on that side also?

Zuccus

 

[willard3]

Refrigeration machines are Carnot cycles and the closer the temperature of the tower water and chilled water temperature, the greater the capacity and efficiency.

You have other problems if you can't reach rated tonnage with lower tower water temps than rated.

 

[capaula]

ChasBean1, it is the decoupler bypass.
I agree with everybody, but I had huge arguments with my technicians about this. They go nuts when they see the cooling towers working on 100% of their capacity to provide this cooler condense water.
I am a 5'4" woman and they are 6' tall
We never opened the condenser side...and I am afraid we will have to. I am getting approaches between 6 and 3.0 ...
I thought I had excess flow in the evaporator side. I installed portable ultrasonic Panametrics flowmeters on both sides to get the right balance. It turned out that I did have excess flow, I closed some valves but the problem was still there...
We have automatic condense tube cleaning (

http://www.balltech.co.il),

but they did not work well after start up because they did not have enough differential pressure to operate, then we installed pumps on them.

 

[zuccus]

Excess flow on the evap side will definitely hurt the problem that you are having.

The water could be flowing to quickly through the machine to transfer the heat out.

Zuccus

 

[ChasBean1]

There are too many possibilities to jump to a conclusion. Assuming all is piped right and controls right: could be the secondary load is too high for the plant. If you’re trying to provide 78°F condenser water during hot OA conditions, there’s no question that you’ll use all your tower fan power to try to meet set point.

This issue might not be ‘closable’ here without a good onsite evaluation.

 

[pretendfarmer]

Capaula,
It is not unusual for multiple chillers piped in primary secondary configuration to be unable to fully load. The reason is that the primary loop flow rate and secondary loop flow rate are very seldom if ever the same. This is because the primary pumps are typically constant speed and the secondary pumps are typically variable speed. That means there is almost always flow in one direction or the other in the de-coupler/crossover bridge/bypass line. I would check the Delta T across my primary loop (chillers) and compare that to Delta T across the secondary loop (coils/load). If there is no difference then you have no flow in the de-coupler line. If there is a difference in the Delta T between the two, then there is flow in the de-coupler line in one direction or the other. You indicate that you are unable to reach setpoint when the flow in the de-coupler goes down. (Is your flow meter one directional? Is it installed in the de-coupler? In my humble opinion, I have always been suspect of flow meters.) Typically that would indicate that you are pumping more water in the secondary than in the primary and are blending the nice cold chilled water you just made with return water. When this is happening, compare chiller discharge temperature with secondary supply chilled water (downstream of the de-coupler). If the supply to the secondary is warmer than the chiller discharge temperature, then you are blending return water with your chiller supply water. There are steps you can take to remedy that problem also. A simple sketch would be very helpful. Let us know.

 

[capaula]

Good answer pretendfarmer.
I do have panametric ultrasonic flowmeters on the decoupler. They read in both directions.
Primary pumps are constant speed, secondary on VSDs controlling delta pressure. The chillers sequence on and off based on decoupler flow. Chiiler sequences on when decoupler is 500 GPM, off when they reach 5000 GPM. Each chiller yields 3100 GPM.
I have 15 seconds data of every parameter on each chiller, so I am able to monitor and build graphs using SQL.
I opened the condenser side of one of those chillers, and the tubes seem very clean. Still, 5F approach.
I will make a diagram and attach it here later today.
The last resource will be flying one of those guys from Lacrosse here to explain this. 300 tons shortage on each chiller means I lack 2700 tons in my plant.

 

[capaula]

Here is a schematic of the installation.
There are 3 chillers doing heat recovery not shown, but only one in use now.

 

[capaula]

Opss. file here

http://files.engineering.com/getfil...04-4f20-b90c-b510d2ae43ae&file=CH-diagram.png

 

[ChasBean1]

capaula,

If this control diagram is anatomically correct, this is not a decoupled loop. All primary water is forced through the secondary.

Please see attached for a recommended solution (schematic only).

CB

 

[ChasBean1]

Wait a minute, I didn't see that little thin red line above the HR chillers on the right. That will decouple the loop if it's a full size line...

 

[capaula]

Yes, that is the decoupler line. It is a full size line

 

[ChasBean1]

capaula,

In the snapshot, looks like 5 chillers on, total capacity 15,500 gpm. The 56.3° return temperature is on the secondary side of the decoupler, so mixed with the primary decoupled flow should be a real primary return to the chillers of about 52°.

Crunching numbers, I'd get about 1,650 tons per chiller. Which just means I'd agree with you, which doesn't really help you!

The X factor as was mentioned could be the evap. flow. If that 15,500 gpm primary flow (with the 5 chillers running) is really closer to 19,000 gpm, we're right back at design tonnage. Do you have balancing/triple duty valves at these pumps and are you confident in their flow balance?

What are the secondary pumps' capacities? There are four running at 68% speed, and secondary flow should be 11,402 gpm. That would make each pump (very roughly) ~4,200 gpm capacity? There would obviously be errors based on head differences from design but the secondary flow could be used for rough comparison.

CB

 

[pretendfarmer]

Capaula,
Don't fly those guys in just yet. Your piping diagram indicates a typical primary secondary chilled water system. Is this process cooling? Why are you running such low chilled water supply temps (39F)? Are your coils designed for 39F or do you have to run so low to maintain discharge air? What is the piping arrangement at the coils? Are there tertiary pumps at the individual air handlers/coils? Are there 2 way control valves at the individual buildings to hold back the return water until the it warms to 55-56F? These valves could also be 3 way valves. Can you maintain discharge air set-points at higher than 39F chilled water set-point? Please provide the typical piping connection drawings at the air handler coils. These connection drawings will tell a lot. I know its a bit of a pain but we have to look at the entire system to analyze correctly. Typically, de-coupler lines are an excellent capacity thief.

 

[pretendfarmer]

Capaula,
Also, are there any valves in your de-coupler?