b5 Low Evaporator Refrigerant Pressure RTHA

Issue:

b5...Low Evaporator Refrigerant Pressure

Resolution:

RTHA Chillers:  Low Pressure Cutout

• Low Pressure Cutout (1TB3-24, -25)
  • These terminals monitor the pressure in the evaporator.  If the pressure falls below 45 +/- 10 psig for design Sequence A thru E closure of 4S6 between 1TB3-24 and 1TB3-25 will shut down the unit giving a latching diagnostic (b b5).  For design sequence F and on, the low pressure cutout is 26 +/- 10 psig.
  • The LPC input is a Normally Open Input (module logic).
• When the chiller has refrigerant in it, the LPC switch is OPEN; when there is loss of charge the switch CLOSES (close on Low Pressure) generating a latching Low Pressure Cutout (b b5).
• This could be caused from catastrophic loss of charge or a faulty low pressure switch.
• Use gauge to verify pressure in the evaporator if this diagnostic cannot be cleared.

Notes:

Note: Observe the gender ( Normally Closed ) of this switch; it is different than other switches used in this capacity (Normally Open, which open on a fault rather than close).

Changes:

Other Question/Problem/Cause Statements:

• b5...Low Evaporator Refrigerant Pressure?
• Low Pressure Cutout?
• Low Pressure diagnostic?
• Low Pressure?
• LPC?

Facts:

• Water-Cooled Chiller (Pueblo) (RTHA)