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Low Delta T

Low Delta T

(OP)
Dear All,

Can anyone explain to me what physically happens within a chiller when it receives water at a temperature lower than it is sized for. I have read many articles which explains the drop in efficiency with formulas. the chiller doesn't know formulas..Can anyone explain how the chiller responds in such a situation?...
As I understand, when the saturated suction pressure provided by the compressor is fixed, the refrigerant keeps evaporating at the same temperature in the evaporator. Hence the supply chilled water temperature remains same (pls correct me if I am wrong here). Hence when the return chilled water temperature drops, the LMTD of the evaporator drops and hence the heat transfer is reduced.
Is there a simpler language to explain this? I'm done reading articles..some one with hands on experience or who has figured out this stuff in equipment performance level detail - please help me out..

Regards,

SDTH

RE: Low Delta T

I'm not sure what you're really after. Newton's law of cooling basically says that cooling is proportional to temperature difference, so cooling will be less. but the cold side temperature is a function of the heat being removed, so it's not impossible that the cooler will attempt to hot side further down in temperature, until that side drops below the freezing point, which could then potentially result in no heat transferred because the hot side is frozen solid. If the hot side is expected to go much colder that the design point, the designer needs to account for this by possibly shutting down the compressor to prevent that from happening. There are a lot of variables, so there are lots of possibilities.

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RE: Low Delta T


SDTH,
I'm not sure what you're after either and you appear to have answered your own question "Hence when the return chilled water temperature drops, the LMTD of the evaporator drops and hence the heat transfer is reduced. "

What is physically happening is that if the cold side stays the same temperature, then, if all else stays the same then the temperature difference between hot and cold is reduced and hence heat transfer reduces.

In practice what will happen is that the cold side, because it has less heat transfer, at the "hot" end it will be colder than it was designed for and hence temperature of the hot side will drop more than it was designed for when it finally gets to the end point, assuming you have the classic counter flow system to try and get equal heat.

Hence as IRstuff says, the reality is that unless you control and start cycling or reducing flow in the cold side, you will freeze the hot side..

Does that help?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Low Delta T

Chiller doesn’t know the formula, but operation configured as per the formulas. while chiller receive low water temp that below its design, the same time mass flow rate of refrigerant feed to heat exchangers need to be controlled/reduced. For this, compressor comes to unloading condition.

RE: Low Delta T

(OP)
Thanks Little inch and IRstuff,
I think I got it. Ive seen chillers receiving water at 8 degree celcius and still delivering at the set 6 degree celcius. the compressor must have ramped down (VFD operated chillers) or the pre rotation vain on compressor suction must have closed as a part of part load operation of chiller.
All what operator can do is to over pump the chiller (variable primary system) and switch on another chiller and pump (primary secondary system) to increase the flow to make up the decrease of delta T.
Ok..
thanks again...

SDTH

RE: Low Delta T

For a water system 6C is about the lowest you can go without getting ice formation.

At 2 C difference I'm surprised you're running any chillers....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Low Delta T

(OP)
Thanks moideen..
Little inch,
I remember 2 C was a result of low delta T syndrome. When the air handler's and FCU's coil and 2 way valve doesn't extract heat as intended and 2 way valves are still open due to demand on air side being not met, the option is to keep running chiller and pump.
This happened during commissioning stage. Eventually the system was tuned I believe.
Regards,

SDTH

RE: Low Delta T

To better understand the Low Delta T issue and possible solutionS, additional information is needed on your application.

What is the piping configuration?

Primary/Secondary or Primary

What type of pumping is used?

Constant Primary Flow Pumping (CPF)
Variable Primary Flow Pumping (VPF)
Constant Primary Flow - Variable Secondary Flow Pumping (CPF-VSF
Variable Primary Flow - Variable Secondary Flow Pumping (VPF-VSF)

Low Delta T can occur in a primary/secondary piping arrangement when more chilled water is produced in the primary piping loop than is used in the secondary piping loop.

RE: Low Delta T

(OP)
Hello LowdeltaTsolver,

Thanks for your response. I wasn't discussing about a running scenario. Just curious about how the Chiller responds when it receives chilled water at lower than the tuned temperature.
Lets take the example of variable secondary, constant flow primary. The secondary keeps working based on DPT setting. The primary based on chiller on/off.
Say low delta T occurs due to any of the common reasons - CV oversizing, clogged coils etc. Load is not met at the building side. So the demand is ON.
Chillers achieved the set point and as per the time delay sequence, one chiller stops and one pump stops. Now the flows doesn't match in the primary and secondary loop. Return water starts moving to return header of secondary through the decoupler. System control is lost.
From this point I hear that the operator switches on additional chiller and primary pump. Why should the operator switch on additional chiller?.. He can simply run a primary pump to balance the flows.
1. Please correct my understanding on the above.
2. In addition, can you tell me what other measures are commonly adopted in such situations - on building side as well on plant side.
3. What would be the measures adopted in such situations - on building side as well as on plant side if the system is not constant primary-variable secondary but a district cooling plant - Variable primary.

SDTH

RE: Low Delta T

SDTH

SDTH Question:
Why should the operator switch on additional chiller?.. He can simply run a primary pump to balance the flows.

Response:
The primary objective of a chilled water system should be to maintain the distribution supply temperature at or below the design temperature at all times during operation.
Energizing a primary pump with without energizing the associated chiller will cause the distribution supply temperature to rise above the design temperature assuming you have more than one chiller and the additional flow circulates through the chiller is not energized.
When a coil entering temperature rises above design, the coil leaving air dew point temperature is affected, which decreases the dehumidification capability of the coil, which will cause an increase in space humidity.


See ASHRAE October 2014 technical article "Simplified Chiller Sequencing for a Primary/Secondary Variable Chilled Water Flow System" for a more in depth discussion of the flow and temperature relationships in a district chilled water system.

The article discuss the conversion of a Constant Primary Flow - Variable Secondary Flow Pumping (CPF-VSF) to a Variable Primary Flow - Variable Secondary Flow Pumping (VPF-VSF).

Only four temperature sensors are used to sequence chillers on/off and control speed of primary pumps such that primary flow equals secondary flow while maintaining the distribution chilled water temperature at or below the chilled water design temperature.


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