McQuay MicroTech III RAH Operation And Maintenance Manual page 89

McQuay OM 920-1
• A waterside economizer is installed AND
• Economizer operation is enabled.
In Mechanical Cooling states, the Bypass Valve is open under all other circumstances.
In the Economizer state, the Bypass Valve is normally closed. It is opened in the Economizer
state only if:
• The unit has an Airside economizer AND
• ClgStatus= Enabled or Off Ambient AND
• Economizer Position is greater than or equal to 95.0% AND
• Discharge Air Temperature is greater than the DAT Clg Spt by more than ½ the DAT Clg
deadband
Water Regulating Valve Control
The water flow to the condenser can be modulated in response to refrigerant pressure signals
when the unit is equipped with the water regulating valve option. This option is only available
when a unit is not equipped with a waterside economizer. When mechanical cooling is active
(unit is in the Cooling operating state), the water regulating valve is modulated to maintain the
refrigerant system head pressure within the Water Regulating Valve Deadband of the Head
Pressure Setpoint. There are two refrigerant pressure sensors included with this option. The
water regulating valve is modulated to maintain the higher of the two pressure readings at the
Head Pressure Setpoint. The editable parameters used for Head Pressure Control are located in
the Extended Menus/Head Press Setup menus.
The following Startup Sequence is followed when a stage up from stage zero is required and
the Water Regulating Valve is closed:
• The Water Regulating Valve is set to an editable minimum water regulating valve position
(MinWRVPos). (Default = 10%)
• The Entering Water Temperature (EWT) is measured after the WRV has been at
MinWRVPos for a MinWRVTime (Default = 60 seconds)
An Initial Position (InitPos) is calculated based on the measured EWT using the following
formula:
• InitPos = MinWRVPos + (EWT - MinWRV@Tmp) x (100% - MinWRVPos) / (100%@ -
MinWRV@Tmp)
The MinWRV@Tmp is the edited temperature where MinWRVPos is OK
The 100%WRV@Tmp is the edited temperature where WRV at 100% is OK
A Time to Initial Position (ToInitPosTime) is calculated using the following formula:
• ToInitPosTime = (InitPos - MinWRVPos) x WRVRampTime/100
The WRVRampTime is the time required for the WRV to be driven from 0 to 100%
The Unit transitions to Cooling and the first compressor is started when the ToInitPosTime has
passed since the command was given to drive the actuator to InitPos and ClgStatus=Enabled
or Off Ambient.
The first compressor runs for an editable Initial Operation Time (InitOpTime) with the WRV
at InitPosControl reverts to normal PI control when the first compressor has been on for the
InitOpTime.
In the Cooling state, the Water Regulating Valve stays at its last commanded position when the
last compressor is turned off. When all compressors are off and a compressor needs to be
turned on, the Water Regulating Valve must be driven open long enough to prevent the
compressor from being locked out due to high pressure, but not so long that it is locked out
due to low pressure. This is required when transitioning to the Cooling state from the Fan
Only or Economizer state.
Operator's Guide
89