Dunham-Bush HRSC - D, Operating & Maintenance Instructions

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not have to be earlier than the turn off time.
Schedules turn on by time and day, but turn off
by time alone.
For example, a schedule from
1900 to 0700 Saturdays would turn on at 7:00 PM
Saturday (time and day) and turn off at 7:00 AM
Sunday (time only).
Example: If a unit is to operate at all times except
between the hours of 1:00 AM and 6:00 AM, the
following schedule would be entered:
CP 17 SCHEDULE GRP:1 SCH:1 0600 0100
DAYS: *** ALL DAYS ***
Another example: A typical building may require
cooling from 6:00 AM to 7:00 PM Monday - Friday
and from 7:00 AM - 3:00 PM on Saturdays. The
schedules would be entered as follows:
CP 17 SCHEDULE GRP:1 SCH:1 0600 1900
DAYS: MTWRF
CP 17 SCHEDULE GRP:1 SCH:2 0700 1500
DAYS: A
4. HRSC Package Control
In order to start a unit, the following conditions must
be met:
--
chilled water pump running
-- chilled water flow switch made
-- customer control contact closed
-- control switch and compressor switch on
-- main unit power on
-- all safety conditions satisfied
-- reset pressed on microcomputer keypad
-- the compressor has not started within the last 15
minutes
-- leaving chilled water temperature 20°F or more
above set point
The NC25 computer system performs the following
functions on small screw compressor packages:
1. Capacity control of compressors
2. Staging of compressors
3. Compressor current limiting
4. Ramp control
5. Manual lead/lag
6. Compressor start delay
7. Power loss reset
8. Anti-recycle timing
9. Low pressure safety
10. High pressure safety
11. Freeze safety
12. High Oil Temp Safety
13. Alarm output
14. Fan control
15. Customer control interlock
16. Hot Gas Bypass option control
17. Compressor power control (No-stop alarm)
18. Sensor alarm shutdown
19. External shutdown indication (No-run alarm)
20. Fast Unload Solenoid Control (HRSC 50 thru 225)
21. Low suction / discharge differential pressure alarm
22. High Motor temperature safety
23. Low chiller flow alarm
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A description of each of the functions follows:
4.1. Capacity Control of Screw Compressors
The capacity of a compressor can be controlled
manually or automatically.
The status of a compressor can be observed by
displaying the compressor control point (1/CP, 4/CP,
7/CP or 10/CP). One of the following messages will
be displayed where # is 1, 4, 7 or 10 for compres-
sors 1, 2, 3 or 4:
CP# COMP LOAD COM Computer load
CP# COMP CURRENT STATUS: LOAD Manual load
CP# COMP HOLD COM Computer hold
CP# COMP CURRENT STATUS: HOLD Manual hold
CP# COMP UNLD COM Computer
unload
CP# COMP CURRENT STATUS: UNLD Manual unload
CP# COMP OFF COM Off on a normal
control shutdown
CP# COMP COFF COM Off on timer
(clock off)
CP# COMP LOFF MAN Manual off or
safety shutdown
4.1.1 Automatic Control
The computer calculates the operating per-
cent capacity of a compressor by measuring
discharge pressure and amps. This operating
percent capacity is then compared to a tar-
get percent capacity. If the operating
capacity is outside of a + 3% deadband, the
load or unload solenoids are pulsed to
match the operating and target percent
capacities. Since all compressors have
the same target, their percent capacities are
balanced.
The target percent capacity is given a fixed
value when a compressor starts or stops.
This value is then increased or decreased
based on how far the leaving water tem-
perature (TLW) is from setpoint and also on
how fast the TLW is approaching setpoint.
The target percent capacity will not change
if the temperature is within a temperature
deadband around the setpoint.
Some packages may have logic that prevents
loading if the leaving water temperature is
falling (negative derivative) at a certain rate.
The desired leaving water temperature is
typically stored in setpoint 1 A.
Calibrations of Amps Calculation at Full Load
The amps calculation is calibrated at the
factory and does not normally need to be
altered. If a compressor is replaced or
improper operation is observed, the amps
values can be calibrated as follows:
1. Check calibration of discharge pressure,
and amps. NOTE: Calibrate amps with
potentiometer on filter board.