CORRECT CHARGE LEVEL IN CHILLER WITH FLOODED EVAPORATOR

DETERMINING CORRECT CHARGE IN A CHILLER WITH FLOODED EVAPORATOR

Determining when a Turbocor compressor based chiller is very different as compared to a DX type system.

All units should have a nameplate which should include the design charge of the system.  On smaller systems, if there is doubt about the charge, system charge can be removed and the correct charge weighed back in.

However on larger systems, this may not be practical.  The typical Turbocor compressor based chiller will have something on the order of 1 to 1.5 kilos of charge per TR.

With a DX system, if it is not practical to remove the charge and weigh in the correct charge, charge can be added until suction superheat stabilizes  and until the discharge pressure starts to increase (indicating liquid is backing up in the condenser).

However, in a Turbocor compressor based chiller with flooded evaporator, determining correct charge is completely different.

On a water cooled chiller with flooded type evaporator and condenser you must determine the correct charge level in the condenser and the evaporator. 

Normally, the water cooled condenser will have a subcooling circuit at the bottom of the condenser tube bundle that designed to be immersed in liquid during normal operation.  In most cases, there will be a sightglass mounted low on the condenser such that, when condenser charge is correct, a liquid level will be visible in this sightglass.  If there is no sightglass, depending on the size of the chiller, you will need to estimate the height of the sub-cooling circuit.  The sub-cooling circuit will be the bottom 6 to 12 rows of tubes (6 on smaller capacity chillers, 12 on larger capacity chillers) with tube spacing about 2/3 of an inch.

In the case of the air cooled condenser, correct level of liquid in the condenser is somewhat more difficult to determine.  Normally, the air-cooled coils will have a sub-cooling circuit near the bottom of the coil.  This can only be determined through visual inspection of the header end of the coil.  If the air cooled coil has a sub-cooling circuit, correct charge of the condenser can be determined by the fact that the liquid header of the air cooled coil will have a lower temperature in the area of the sub-cooling circuit as compared to the area of the liquid header in the area where only hot gas is present

Normally, the evaporator will have a sightglass mounted such that the top row of evaporator tubes is visible through this sightglass.  Evaporator charge is correct when during full load operation, the liquid refrigerant in the evaporator is visible just barely covering the top row of evaporator tubes.

To determine correct charge in a chiller with flooded water cooled condenser, adjust the setpoint on the liquid level sensor (which will be measuring the liquid level in the condenser) until the liquid level in the condenser is at the desired level.

Once the liquid level in the condenser is correctly established, with the chiller fully loaded, the liquid level in the evaporator should be just covering the evaporator tubes.  If the liquid level in the evaporator is not visible, add charge until the liquid becomes visible.

HINT FROM THE AUTHOR -  When I have the correct level in the condenser established, I reduce the condenser liquid level setpoint by 5%.  Then when I see the evaporator liquid level come up over the top evaporator tubes, I can make a fine adjustment to the evaporator level by increasing the condenser setpoint slightly.

With the flooded evaporator in an air cooled chiller, the liquid level sensor will be directly in the evaporator. To determine the correct charge in a chiller with air cooled condenser coils, you will need to adjust liquid level setpoint until the liquid level in the evaporator just covers the top tubes in the evaporator.  Then, sufficient charge must be added to back liquid refrigerant up into the liquid line and up into the very bottom part of the air cooled coils.

On the Turbocor compressor based air cooled chiller, there is a motor cooling connection on the condenser liquid line and a sightglass on the cooling line going to the compressor.  As a minimum, when the chiller is correctly charged, there will be no bubbles in the motor cooling lone sightglass during compressor operation.

Absolute correct charge on the air cooled unit will be when liquid is backed up into the sub-cooling circuit of the air cooled coil during full load operation.

As with air-cooled DX systems, excessive charge in the unit will result in an increase in the discharge pressure of the air cooled chiller.