Microtek MHP1-HD Installation Manual download pdf (Page 30)

30 OMRCPMICRO

WHR units with remote condenser

For WHR split systems, fan staging for remote condensing units is controlled in the same manner as ALR

units as explained above. The unit must be declared as air cooled on the Misc Setup Menu 23. Also on Menu

23, the number of fan stages should be set to a value equaling the number of fan contactors per circuit minus

one. The MicroTech controller works in conjunction with the required field wiring from the controller's relay

board to the fan contactors. The first bank of fans will start whenever one of the compressors is running.

Each additional fan bank will run as required to control head pressure. Refer to the remote condenser option

on the field wiring drawings towards the back of this manual.

SpeedTrol settings for air cooled units (ALRs and WHRs with remote condensers)

When a chiller's first fan stage is controlled by a variable speed head pressure control device, the head

pressure control setpoints should be adjusted slightly so that MicroTech and SpeedTrol do not conflict with

one another.

For each circuit, the SpeedTrol fans are wired in parallel with the first compressor stage so they are energized

with the compressors. The SpeedTrol pressure modulation device is not hooked into the MicroTech control

and therefore operates independently of MicroTech.

The desired goal is for the SpeedTrol device to modulate the first fan up to full speed before MicroTech head

pressure control logic decides that a second condenser fan should be turned on. To meet this objective, the

"MinLift-35%" (item A on Menu 17 "Head Pres Spt") plus the Head Pressure Dead Band at 35% circuit

capacity plus the evaporator pressure should equal the high end of the throttling range for the SpeedTrol

device. The "MinLift-35%" is the minimum lift pressure that should be maintained at 35% circuit cooling

capacity—when the first stage of fans and compressors come on. The SpeedTrol control has a throttling range

of 170 psig to 230 psig. Thus it will modulate the fan to zero speed at 170 psig and full speed at 230 psig. The

proper "MinLift35%" setting is determined by the formula:

SpeedTrol Typical Deadband

MinLift-35% = top throttling - evaporating - pressure at 35°

range pressure circuit capacity

A typical evaporating pressure for chilled water mode is 65 psig. The dead band pressure at 35% circuit

capacity is shown to be 50 psig in Figure 12. The formula becomes:

MinLift-35% = 230 psig - 65 psig - 50 psig

In this example, the formula yields a recommended "MinLift35%" setting of 115 psig. This setting can be used

for virtually all SpeedTrol applications except for "Ice" mode.

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Microtek MHP1-HD Installation Manual Page 30