Chiller Surge

If you have worked in chiller industry, you might have come across this terminology. In this article we will discuss the significance of Chiller surge & how to avoid it.

General refrigeration circuit & flow

First take a look how refrigerant moves along the component of refrigeration circuit.

In normal operation low pressure refrigerant vapors from evaporator enters compressor at suction side.
this refrigerant then compressed in compressor which is driven by electric motor.
rotating impeller sucks the refrigerant in & push the refrigerant towards the end of impeller blades generating high angular velocity.
then it gets collected in the volute (spiral shape channel surrounding the compressor) & passes through the diffuser where its velocity decreases & kinetic energy of fluid gets converts in to pressure.
this pressure builds up in the volute & force refrigerant out through discharge line in to the condenser.
Now the pressure builds up in the discharge line & in the condenser.
refrigerant than gives away its heat (which was gain in the evaporator from building heat load) to the cooling tower water in the condenser & gets converted into low temperature liquid.
the liquid refrigerant than passes through expansion valve & finally at low pressure to evaporator. Then cycle gets repeat.

Chiller Lift

Low Pressure Side: Also called as suction side is on the evaporator side from where the compressor sucks the low pressure refrigerant.

High Pressure Side: Or the discharge side which is downside of the compressor where high pressure refrigerant gets discharged to the condenser.

The pressure difference caused by the compressor between High & low side is called Chiller Lift.

If we put it in the equation:

Chiller Lift= High side pressure – Low side pressure

Each compressor is able to provide certain amount of Lifting from low to high pressure, which totally depends on different sizes & manufacturer.

When the discharge side pressure reaches too high beyond the lifting range of compressor.
Due to high pressure, refrigerant starts moving in opposite direction backwards from condenser towards compressor.
The compressor keeps rotating to move refrigerant towards discharge line but due to very high pressure at condenser side the refrigerant will flow back to impeller & through the suction line into the evaporator.
The refrigerant direction will continue to cycle back and forth as the compressor provides insufficient lift.
This condition of chiller is called as Chiller surge.
In simple terms, When the pressure in the condenser is higher than the pressure that can be applied by the compressor, then chiller surge will occur.

Example:
Suppose the maximum lift of chiller is from 300kpa to 900kpa.That means it takes the refrigerant in from evaporator at pressure of 300Kpa & compress it to maximum pressure of 900kpa to deliver it to condenser.
So the Chiller lift is 900-300=600Kpa
Now if the pressure at condenser side reaches to 1000Kpa, which is way high beyond the max lifting pressure of compressor (which is 900kpa). At this instance Chiller Surge will Occur.

Surge Effects:

It can cause some serious damage to chiller components (compressor, evaporator etc.)
When this happens chiller will make loud, screeching sound made by the compressor or a banging sound on the compressor casing.
Surge conditions create excessive radial vibration
You might also experience extreme changes in motor current and violent fluctuations in the discharge pressure.
In short, chiller surge is not a good scenario & it will affect chiller performance & life.

Causes of Chiller Surge

As we discussed Chiller surge occurs due to increase in pressure in the condenser. So what are the reasons behind this pressure surge?

Poor Chiller Selection/Application: Chiller forced to operate at lower load with high lift. High lift chiller selected too close to the surge line.
Part Load Operation of Chiller: When the chiller is operating at lower than it’s maximum design. Once this load drops to a certain point, this will cause surge because the temperatures will not be sustainable to function the machine.
Blockage in condenser line: If a partial blockage occurs in this condenser return line, then the flow rate is going to change. This might cause the condenser water to drop below the minimum required flow rate. Which will cause the condenser not able to dissipate enough heat to the cooling media, & that’s going to start raise the pressure inside condenser.
Cooling tower unable to reject enough heat: If Cooling tower can’t fully reject the unwanted heat and it can’t generate a large enough temperature difference on the condenser water line, again there will be not enough heat transfer by condenser which will raise pressure. This might because of water distribution within the cooling tower has become disturbed due to factors like build up of leafs in the cooling tower, blockage of distribution tray at the top due to debris & scale, drive belt breaking or the motor might have a fault and break.
The pump strainer might become blocked and that will reduce the flow of water through the system.
Fouling : The tubes inside the condenser may become covered in scale and this would reduce the amount of heat transfer that can occur because it is going to change the heat transfer surface area.
Refrigerant: Low refrigerant Charge. Non condensable gas in refrigerant.

How to Solve Surge problem

Below are some tips to avoid & reduce the possibilities of Chiller Surge.

1.Equipment selection: Requesting minimum chiller load at the design entering condenser water temperature allows the design team to verify possible surge conditions.

Select the cooling tower for the highest ambient wetbulb condition expected and provide adequate head in
the condenser water pump selection to ensure the condenser flow at least meets the chiller selection requirement.

Installing cooling towers in a manifold offers the operational flexibility to reduce chiller lift at part load by running more cells.

2. Hot gas bypass: Can be used to maintain a minimum refrigerant gas volume flow rate through the compressor to avoid surging during low load conditions.

Hot gas bypass recirculates hot discharge refrigerant from the compressor back to the evaporator. The refrigerant passes through a hot gas bypass valve to reduce the pressure.

It is also used in air-cooled centrifugal chiller applications and sometimes referred to as a load balance valve where it can be used to false load the evaporator in low load situations or decrease the pressure ratio between discharge and suction pressures during high head conditions caused by high ambient temperatures.

3. Chiller with Variable speed drive : Some ways to get around this are by fitting a variable speed drive, or variable frequency drive, onto the induction motor which drives the compressor. This will change the rotational speed of the compressor to modulate its capacity.

4. Condenser Entering temp: Another option is selecting a chiller at entering condenser water temperature a few degrees above the design conditions. While this reduces design efficiency a little, it may allow the chiller to stay out of surge and on-line even in unusually hot weather, which can occur given the fact that weather design conditions are seldom at the annual extremes, and may become even more common due to climate change.

5. Proper Maintenance is critical for chiller plants. Following the manufacturer’s recommended maintenance will avoid many problems.

Actively trending and monitoring evaporator and condenser approach temperatures using the chiller plant control system can help avoid surge conditions. Proper water treatment and annual condenser cleaning will keep condenser approach temperatures near design conditions. When condenser approach goes beyond what is originally designed (0°F to 3°F [0°C to 2°C]), condenser pressure will increase.

Timely maintenance of chiller, cooling tower & condenser pumps will eliminate possible causes of Fouling, scale formation, blockage which will eliminate condenser & cooling tower heat rejection inefficiency. which are the possible cause of chiller surge.