Water Cooled Chiller Efficiency Monitoring

Water Cooled Chiller Efficiency Monitoring
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Chiller Check Software
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CONDENSER
Inlet Temp (Entering Condenser Water, originated as Cooling Tower Leaving Water)
Most chillers are designed for a max condenser inlet water temperature of 85F. If the entering condenser water
temperature exceeds 85 degrees the efficiency of the chiller is reduced by as much as 2% per degree above 85F.
Outlet Temp (Leaving Condenser Water, becomes cooling tower entering water)
Refrigerant Temp
 Liquid Line Temp?  Subcooling ? Excess Approach
Condenser approach is the difference between liquid refrigerant temperature as measured on the liquid line, and
leaving condenser water temperature. Normal condenser approach is 0 to 3 degrees.
If condenser approach is 4° or more, it's an indication your water cooled chiller has fouled tubes.
Evaporator approach can be used to evaluate the refrigerant charge. Take all readings with the water cooled
chiller at full load.
In a 1 pass evaporator, approach should be 10° to 14°.
In a 2 pass evaporator, approach should be 7° to 10°.
In a 3 pass evaporator, approach should be 3° to 6°.
A higher than normal evaporator approach can indicate an undercharge. A lower than normal evaporator
approach can indicate an overcharge.
This is the calculated full load approach minus the target approach. Approach is the difference between the
leaving condenser water temperature and the saturated condenser refrigerant temperature. It is a measure
of the heat transfer efficiency between the condenser water and the condenser refrigerant.
As condenser tubes foul or division plate gaskets leak or tubes get blocked etc, the approach temperatures will
increase. The program takes the actual approach temperature, factors it up to a full load approach by taking into
account the % load, and then subtracts the target approach or what the approach should be with clean tubes at
full load. The result is the Excess Approach. Each 1F of excess approach reduces efficiency by approximately
2%.
Problem: High Condenser Approach
What To Check
What To Correct
1.
Check instrumentation for accuracy and calibration.
Recalibrate or replace instruments.
2.
Review water treatment logs to insure proper
operation, treatment and blowdown.
Contact water treatment company if
necessary.
3.
Check refrigerant level control or refrigerant metering
device.
Adjust level control. Adjust refrigerant
float. Clear orifice.
4.
Inspect condenser tubes for fouling, scale, dirt etc.
Clean tubes if necessary.
5.
Check for division plate bypassing due to gasket
problems or erosion.
Replace division plate gasket.
6.
Check for non condensable in high pressure
refrigerant chillers.
Purge system of non condensable.
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Pressure
 Entering condenser water and leaving condenser water pressure?
Non Condensable
In low pressure refrigerant chillers (R-11, R-123) air can leak into the chiller and if the purge does not remove the
air or non-condensable it will accumulate in the condenser adding to the condenser pressure. Non-Cond is a
measure of the amount of air in PSI added to the condenser. 1 PSI of air reduces the efficiency by approximately
5%.
If condenser approach is 4° or more and the condensing water delta t is low, it's an indication of noncondensables in the condenser, insulating the tubes and reducing heat rejection to the water.
With non-condensables, condensing pressure will be higher than normal.
Whether your water cooled chiller has a reciprocating, screw, or centrifugal compressor, If the head pressure is
rising and your pumps, tower, and piping all seem to be ok, get a tube cleaning machine and gasket material
ready, and don't hesitate to pull an end bell off to inspect the tubes and clean them if they need it.
Pressure drop
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EVAPORATOR‐SIDE
Inlet Temp (Entering into the Evaporator from load Chilled Water)
Outlet Temp (Leaving from the Evaporator Chilled Water)
Sat Refrig Temp (where is this measured?)
Excess Approach
Evaporator approach is the difference between evaporating temperature measured at the well(?) in the
evaporator, and leaving chilled water temperature.
This is the calculated full load approach minus the target approach. Approach is the difference between the leaving
evaporator water temperature and the saturated evaporator refrigerant temperature (suction line temp?). It is a measure
of the heat transfer efficiency between the evaporator water and the evaporator refrigerant. If refrigerant is leaking from the
chiller or evaporator tubes foul or division plate gaskets leak or tubes get blocked etc the approach temperatures will increase.
The program takes the actual approach temperature factors it up to a full load approach by taking into account the % load and
then subtracts the target approach or what the approach should be with clean tubes at full load. The result is the Excess
Approach. Each 1F of excess approach reduces efficiency by approximately 2%.
Problem: High Evaporator Approach
What To Check
What To Correct
1.
Check instrumentation for accuracy and calibration. Recalibrate or replace instruments.
2.
Review maintenance logs and determine if excess The refrigerant should be reclaimed or an oil
oil has been added and if so, how much. If
recovery system added if the oil content is
indications are that excess oil has been added
greater than 1.5 to 2%.
take a refrigerant sample and determine the
percent oil in the charge.
At this point there are two likely causes of the problem; low on charge or tube fouling. Some considerations
in determining the direction to take are:
 Has the chiller had a history of leaks?
 Is the purge indicating excessive run time?
 Is the chiller used in an open evaporator system such as a textile plant using an air washer?
 Has there been a history of evaporator tube fouling?
If the answers to the above questions do not lead to an obvious diagnosis a recommended course of action
is as follows:
1.
Trim charge using a new drum of refrigerant. If
approach starts to come together as refrigerant is
added, continue to add charge until the approach
temperature is within specs. This indicates a loss
of charge and a full leak test is warranted.
2.
If adding refrigerant does not improve the
Clean evaporator tubes.
evaporator approach, the next step is to drop the
evaporator heads and inspect the tubes for fouling,
as well as inspecting the division plate gasket for a
possible bypass problem. Clean tubes if
necessary. Replace division plate gasket if
necessary.
Trim refrigerant charge.
Pressure (where is this measured
Pressure Drop
"Water cooled chiller" means that the refrigerant in the condenser releases heat to water instead of air.
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Do you know what 'condenser approach' and 'evaporator approach' are?
Another condition you might be encountering with your centrifugal water cooled chiller is 'surging'.
This would be when the chiller makes a noise like a squealing elephant, and it indicates that there's a problem.
It happens when the difference between the evaporating temperature and condensing temperature exceeds
design conditions, which is about 70 degrees. (Temperature of the refrigerant or water?)
When this difference, which is called lift, exceeds design conditions, refrigerant flow will reverse through the
impellers, which causes the squeal, and can lead to other problems.
Surging can be caused by non-condensables, fouled condenser tubes, low condenser water flow, high entering
condensing water temperature, and low refrigerant charge.
Surging can damage the thrust bearings and impeller, so if you hear your centrifugal water cooled chiller surging,
find and correct the problem immediately.
CoolingTowers
Look for the temperature of the water entering the cooling tower to drop about 10F as it passes through
the cooling tower.
Cooling tower approach: is the difference between the web bulb temp of the air entering the cooling
tower, and the temperature of the cooled water leaving the cooling tower. Cooling tower approach is a
significant indication of the efficiency of the water to air heat transfer taking place in the cooling tower.
In geographical locations where 78F web bulb temp would be considered an average outdoor ambient,
look for the cooling tower approach to be 5 to 7 F. (7 would be considered optimum).
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