Microtek MHP1-HD Installation Manual download pdf (Page 47)

OMRCPMICRO 47

Lvg Evap = Desired Ice Temp + 1/2 "CntrlBand+" + "ShutDn D-T"

Lvg Evap = 21°F + 1/2 (4) + 1.5°F

Lvg Evap 21 + 2 + 1.5

Lvg Evap = 24.5

For day operation the maximum chilled water reset (Menu 14 "MaxChWRst=") is added to the "Lvg Evap" as

calculated above to reach the desired day operating leaving chilled water temperature. In Ice mode, do not

unload, the MicroTech will shutdown whole circuits as the load drops off. This is necessary to maintain

evaporator velocity for motor cooling and air return.

When the Ice option is selected, the resetting of the leaving chilled water set point (during day operation) via

the 4-20mA input is not a functional option. To retain the option, the set points in Menu 14 must be changed

by a Building Automation System through our MicroTech Open Protocol Monitor Software.

The "Network" reset option allows a signal to be sent reflecting 0-100% reset of the "Lvg Evap" set point

(chillers) or "Lvg Cond" set point (THRs) based on the Maximum Chilled Water Reset set point. This function

acts much like the 4-20 milliamp reset option as described above.

The Maximum Pull Down Rate set point ("MaxPullDn") will prevent overshooting the active water set point

during initial start-up and normal operation. The controller will limit the rate at which the chilled water loop

temperature is reduced and the rate at which the heated water loop (THRs) is increased based on an adjustable

set point (default of 0.5°F/min). Every minute, the controller checks the leaving water temperature and

compares it to the last reading. If the pulldown rate has been exceeded, the controller will delay additional

stages.

Figure 15, Delta-T set points for chillers

The Control Band ("CntrlBand") set point

defines the temperature range on either side

of the active water set point that must be

exceeded in order for a stage up or stage

down to occur. The Start Up Delta-T

("StartUpD-T") and Shut Down Delta-T

("ShutDn D-T") define when the unit will

start and shut down. In chiller mode, the

unit will cycle on from a shut down mode

once the leaving chilled water temperature

has reached the Leaving Chilled Water Set

Point plus half of the Control Band plus the

Start Up Delta-T. After the chiller starts, it

will stage up and down according to the

leaving chilled water temperature ± half the Control Band. Once the load is met and the chiller has staged

down to stage 1, the unit will shut completely off once the leaving chilled water temperature has reached the

Leaving Chilled Water Set Point minus half of the Control Band minus the Shut Down Delta-T. The THR,

controlled from leaving condenser water, stages up and down in the reverse manner of a chiller. For more

information on staging, see the Normal Sequence Of Operation section of this manual.

Menu # 15: Softload Set Points

—The controller can limit the number of available stages when the unit is initially started to prevent excessive

power consumption and help control overshoot of the water temperature set point. The maximum number of

stages will equal the Soft Load Maximum Stages for the time period defined in the softload timer set point. If

the softload timer is set to zero, no soft loading will take place. Any time remaining in the softload timer will be

displayed when the soft load option is enabled. During morning start-up, the controller will run the chilled

water pump and sample the loop water temperature for a time equal to the Load Delay set point. If cooling or

heating is required at the end of this time delay, the first compressor will be started.

Menu # 16: Compressor Set Points

—This menu is used to set the lead-lag order of the refrigerant circuits. The lead compressor may be manually

set to circuit #1 or circuit #2 or the Automatic mode may be enabled. In Automatic mode, the MicroTech

controller will select the refrigerant circuit with the lowest number starts as the lead. The interstage timer set

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Microtek MHP1-HD Installation Manual Page 47

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