lowering the condensing temperature reduces chiller's capacity?
lowering the condensing temperature reduces chiller's capacity?
(OP)
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.
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.





RE: lowering the condensing temperature reduces chiller's capacity?
RE: lowering the condensing temperature reduces chiller's capacity?
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?
RE: lowering the condensing temperature reduces chiller's capacity?
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...
RE: lowering the condensing temperature reduces chiller's capacity?
A lot of times the system configuration gets screwed up.
CB
RE: lowering the condensing temperature reduces chiller's capacity?
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
RE: lowering the condensing temperature reduces chiller's capacity?
You have other problems if you can't reach rated tonnage with lower tower water temps than rated.
RE: lowering the condensing temperature reduces chiller's capacity?
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.
RE: lowering the condensing temperature reduces chiller's capacity?
The water could be flowing to quickly through the machine to transfer the heat out.
Zuccus
RE: lowering the condensing temperature reduces chiller's capacity?
This issue might not be 'closable' here without a good onsite evaluation.
RE: lowering the condensing temperature reduces chiller's capacity?
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.
RE: lowering the condensing temperature reduces chiller's capacity?
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.
RE: lowering the condensing temperature reduces chiller's capacity?
There are 3 chillers doing heat recovery not shown, but only one in use now.
RE: lowering the condensing temperature reduces chiller's capacity?
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RE: lowering the condensing temperature reduces chiller's capacity?
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
RE: lowering the condensing temperature reduces chiller's capacity?
RE: lowering the condensing temperature reduces chiller's capacity?
RE: lowering the condensing temperature reduces chiller's capacity?
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
RE: lowering the condensing temperature reduces chiller's capacity?
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.
RE: lowering the condensing temperature reduces chiller's capacity?
Also, are there any valves in your de-coupler?
RE: lowering the condensing temperature reduces chiller's capacity?
As PretendFarmer mentions, is demand being satisfied?
RE: lowering the condensing temperature reduces chiller's capacity?
I run 39F in summer because it dehumidifies a lot.
I need a dew point of 44F in the end of the dehumidification coils.
We are talking about 850,000 of make up air, (100% OSA units).
The dehumidification coils were designed for 40F. In the rest of the year I reset the SP, it can go up to 42.5F.
Tertiary pumps are in part of this system, 6,500,000 CFM on recirculation units, doing 2F delta T (air side) but most of the load is on the secondary system. There are NO 2 way control valves at the individual buildings to hold back the return water until the it warms to 55-56F.
There are no 3 way valves, each coil has its control valve (proportional Baumann butterflies).
I have no real balancing valves on the condenser water and chiller water pipe before the chillers. We managed to balance it with 18" butterfly valves before each chiller, which was a nightmare.
There are actually hundreds of coils on this system. I have to check the coil arrangements.
In fact I am dying to change this to variable primary flow, but have no way to justify this financially yet...
RE: lowering the condensing temperature reduces chiller's capacity?
I will check those guys one by one.
RE: lowering the condensing temperature reduces chiller's capacity?
If you can believe your flow meters and temperature sensors, you are bypassing almost 4100 GPM of nice cold chilled water you just made and are dumping it into the return line, warming it up.
In order to maintain the setpoint you need, you need to chill that mixed chilled water return temperature of 39ºF and 56.3ºF flow down to 39ºF. What is the mixed chilled water return temperature at the point #1 on the attached PDF?
RE: lowering the condensing temperature reduces chiller's capacity?
I think the rule was 3% capacity lost per degF reduced sp? This works out to 300 tons, which is what you say you are missing.
RE: lowering the condensing temperature reduces chiller's capacity?
The 4,100 gpm being "wasted" is also cooling the return which means less chiller work. The temperature at Point 1 should be 51.7°F (I called it 52° earlier for errors' sake). The data is all there – 3,100 gpm primary pumps x 5 running is 15,500 gpm; 4,100 gpm in the decoupler means 11,400 gpm in the secondary.
4 secondary pumps at 68% speed delivering 11,400 gpm implies 2,850 gpm per pump and about a 4,200 gpm capacity per pump.
Some of this isn't directly metered. To me the data begged the question of flow measurement accuracy. Temperature measurement is usually pretty good...
RE: lowering the condensing temperature reduces chiller's capacity?
To me, running additional chiller(s) to accommodate the additional 4100 GPM being bypassed qualifies as wasteful and inefficient. You not only have to to run the additional chiller(s), but you have to run the additional chiller pump(s), additional condenser water pump(s), and probably additional tower fan(s).
Just because you have pumps and chillers rated by nametag information at 3100 GPM does not mean that is where the pumps are actually operating on their curve. That is why I believe knowing what the actual versus calculated temperature at point 1 is important. Trust but verify.
I also believe eliminating uncontrolled bypass flow will allow your chillers to fully load as they are designed to do. That way you will be able to satisfy your chilled water requirements with fewer chillers. The calculated payback of meeting your load requirements with fewer chillers will quickly convince you to change over to a vari-prime type piping system. All you need to do is to install a valve in the bypass, shut it, and let the system run as is currently designed.
You mentioned that you have some tertiary pumps piped in your system at the coils (hopefully not as much as 4100 GPM). These tertiary pumped loads should be checked to make sure their flow rates will provide minimum chiller flow protection.
This simple system modification will allow you to operate in both vari-prime and primary secondary and then you can convince yourself (and others) which mode works best for you.
RE: lowering the condensing temperature reduces chiller's capacity?
Why do you want to change to a variable primary system? Primary/secondary, especially for a large system, should be easier to control. If you install variable speed drives on the primary pumps you should be able to realise the same energy savings.