Freeze Avoidance and Pump Control Requirements

Issue:

Pump Control Requirements

Resolution:

Please reference the Freeze Avoidance Bulletin (RF-PRB002*-EN) applicable to CGAM, RTAC, RTAE and RTAF chillers to determine what is required to protect the evaporators from freezing.

The bulletin is attached below. Please reference e-library for the most current version.

Notes:

The following information was provided when we first introduced the falling film evaporator to the market. It is provided here as a historical reference. Detailed information as it applies to the new evaporator designs can be found in the Freeze Avoidance bulletin referenced above.

Chilled Water Pump Control is required for the RTAC. 
This decision was not arrived at lightly, and Trane did not make it a requirement to make anyone's life miserable.  One of the most challenging issues our design team faced in the RTAC development effort to date, is to address the chiller's susceptibility to water side freeze.

RTAC utilizes what is known as a Falling Film evaporator.  Similar to a flooded evaporator, this type of evaporator has water inside the copper tubes, and refrigerant residing inside the shell surrounding those tubes. There exists the potential for a substantially higher mass of liquid refrigerant present than water, once the chiller is stopped and the chilled water flow through the evaporator ceases.  Just after a circuits compressor is stopped (if the outdoor ambient is colder than the water temperature) the liquid refrigerant left in the evaporator will migrate to the condenser as it rapidly cools.  The saturation temperature of this refrigerant will rapidly fall and approach the ambient temperature at the condenser.  All this time, the stagnant water in the tubes will give up its heat causing the refrigerant liquid to boil and move its heat energy to the condenser.  As there is more liquid mass than water mass, and a huge condensing surface exposed to the cold ambient, it is quite easy for the refrigerant to carry away enough heat to freeze.  The most susceptible tubes are those of the bottom row with the freeze starting in the middle of those tubes.  To give an idea of how fast this can happen, a model has been run that suggests that at an ambient temperature of -10F and an initial water temperature of 36, freezing in the tubes will begin in less than 4 minutes after shutdown. This even considers that there is 800 watts of electric heat being added to the water of the water box.  Freezing can eventually occur with ambient temperatures as high as 28F.

Some customers choose to control the pump from another source and put the chiller pump control contacts in parallel with the input from the chiller.  This is acceptable, but the bottom line is this:  If the chiller detects a need for a pump to start, then the correct pump must start and valves must shift so that design flow is immediately established.  The factory takes the stand that freezing an evaporator is not a warranty issue, and is no different than running a new car into a brick wall.  It constitutes a misuse of the chiller.  Again, it is ultimately up to the customer whether they comply with the factory requirements.  If the requirement is ignored, the risk is transferred to the customer's shoulders.

Changes:

Other Question/Problem/Cause Statements:

• Pump Control Requirements
• Do we require pump control for the CGAM?
• Is pump control required for the CGAM?
• Do we require pump control for the RTAC
• Is pump control required for the RTAC?
• Do we require pump control for the RTAE?
• Is pump control required for the RTAE?
• Do we require pump control for the RTAF?
• Is pump control required for the RTAF?

Facts:

• Air-Cooled Chiller (Pueblo) (CGAM)
• Air-Cooled Chiller (Pueblo) (RTAC)
• Air-Cooled Chiller (Pueblo) (RTAE)
• Air-Cooled Chiller (Pueblo) (RTAF)