CVHE-SB-32A 99
Hot Gas Bypass
Customer Benefit:
Hot Gas Bypass eliminates or reduces cycling on process or other installations having load
variations that occasionally include brief periods of no or very light load. Eliminating cycling
shortens the time required to respond to load increases, and prevents chiller starts during
periods when chiller starts are prohibited.
HGBP is also used as a way to correct applications where the chiller has been oversized.
Explanatory Comments:
Hot Gas Bypass is a chiller option that is intended to reduce the capacity of the chiller at
minimum load conditions by bypassing condenser gas to the evaporator. On standard chillers,
minimum load is determined by the amount of gas that leaks through the inlet guide vanes when
they are fully closed. By adding HGBP a portion of this gas comes directly from the condenser,
thus reducing the amount of cooling done by the chiller.
For CVHE/F/G/B Chillers:
See also High Compressor Discharge Temperature Protection.
In the Machine Configuration menu, UCP2 has an entry to indicate that hot gas bypass is
installed. The installation of HGBP will enable all of the HGBP valve diagnostics, HGBP
screens in the CLD, and communications diagnostics with the I/O module. The HGBP valve
can be independently controlled in the Service Tests menu. Tracer is also be able to Enable/
Disable HGBP.
When HGBP is Enabled, HGBP control will check to see if the IGV position is less than or
equal to the larger of either Fully Closed or the “Guide Vane Closed Travel Stop”. When this
condition is satisfied and the ELWT is 0.5°F below the chilled water setpoint an integral is
permitted to ramp up. When the integral reaches a predetermined value, the HGBP mode is
Entered.
Once the HGBP mode is entered, the output of the evaporator leaving water temperature
control algorithm will be scaled for tics (versus steps) and sent to the I/O module to modulate
the HGBP valve. The inlet guide vanes are given a loading pulse until the “HGBP Mode, IGV
Target” is attained.
Exiting Hot Gas Bypass:
While in the HGBP mode, the same integral that allowed HGBP to be entered is ramped if the
ELWT is 0.5°F above the chilled water setpoint. This allows the HGBP mode to be terminated
if the load increases. When this integral is greater than or equal to some preset value, HGBP
control is terminated, and LWT control is directed to the inlet guide vanes.
If the user Enables the “Maximum HGBP Time” timer, the algorithm shuts the CenTraVac
down after the timer has expired. The “Maximum HGBP Time” will be user adjustable. When
the timer is Enabled, the mode display is modified to show the Hot Gas Bypass time
remaining.
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