Carrier 48AJ Operating And Service Manual page 41

H.C.ON
L.C. OF L.C. OF/2
L.C.ON
Lo Cool End
Cooling Setpoint (OCSP,UCSP)
Fig. 6 — Cool Mode Evaluation
C.TYP = 3 and 4 (Thermostat Cool Mode Selection) —
When a thermostat type is selected, the decision making pro-
cess involved in determining the mode is straightforward.
Upon energizing the Y1 input only, the unit HVAC mode will
be LOW COOL. Upon the energizing of both Y1 and Y2 in-
puts, the unit HVAC mode will be HIGH COOL. If just input
G is energized the unit HVAC mode will be VENT and the
supply fan will run.
Selecting the C.TYP = 3 (TSTAT – MULTI) control type
will cause the control to do the following:
• The control will read the Configuration
configuration parameter to determine the number of
cooling stages and the pattern for each stage.
• An HVAC mode equal to LOW COOL will cause the
unit to select the Setpoints
to. An HVAC mode equal to HIGH COOL will cause the
unit to select the Setpoints
to. Supply air reset (if configured) will be added to either
the low or high cool set point.
• The control will utilize the SumZ cooling algorithm and
control cooling to a supply air set point. See the section
for the SumZ Cooling Algorithm section for information
on controlling to a supply air set point and compressor
staging.
Selecting the C.TYP = 4 (TSTAT – 2 STG) control type
means that only two stages of cooling will be used. On unit siz-
es 020, 025 and 027 (with three compressors), an HVAC Mode
of LOW COOL will energize one compressor in Circuit A; an
HVAC Mode of HIGH COOL will energize all three compres-
sors. On unit sizes 030 and larger (with four compressors) an
HVAC Mode of LOW COOL will energize both compressors
in Circuit A; an HVAC Mode of HIGH COOL will energize all
four compressors. Refer to the section on Economizer Integra-
tion with Mechanical Cooling for more information.
2-Stage Cooling Control Logic (C.TYP = 4 and 6) — The
logic that stages mechanical cooling for the TSTAT and SPT
2-Stage cooling control types differs from that of the multi-
stage control types. This section will explain how compressors
are staged and the timing involved for both the Low Cool and
High Cool HVAC Modes.
There are either three or four compressors divided among two
refrigeration circuits. Circuit A always contains two compressors
→ →
(Outputs COOL A1 and A2). Circuit B has either one com-
→ →
pressor (Outputs COOL
→
compressors (Outputs COOL
units. For 2-stage cooling control, regardless of configuration,
there is no minimum load valve (MLV) control. The decision as
to which compressor should be turned on or off next is decided
by the compressor's availability and the preferred staging order.
The preferred staging order is either A1, A2, B1, B2 (B2 on
030-060 only) or A2, A1, B1, B2 (B2 on 030-060 only). Either
A1 or A2 may start first as there is a built-in lead/lag logic on
compressors A1 and A2 every time the unit stages to 0 com-
pressors. Also, based on compressor availability, it should be
noted that any compressor may come on. For example, on a
Hi Cool Start
Lo Cool Start
Hi Cool End
A48-7701
→ →
UNIT SIZE
→
SA.LO set point to control
→
SA.HI set point to control
B1) on size 020-027 units or two
→
B1 and B2) on size 030-060
3 compressor unit, if no compressors are currently on,
compressor A2 is currently under a minimum off compressor
timeguard, and 2 compressors are to be turned on, then com-
pressors A1 and B1 will be turned on immediately instead of
A1 and A2.
Low Cool Versus High Cool Mechanical Staging — The num-
ber of compressors to be requested during a cooling mode are
divided into 2 groups by the control, HVAC mode = Lo Cool
and HVAC mode = Hi Cool.
If the economizer is not able to provide free cooling (Run
→ →
Status ECON ACTV = NO) then:
• Lo Cool Mode mechanical stages = 2
• Hi Cool Mode mechanical stages = 3 (for 020 through 027
size units)
• Hi Cool Mode mechanical stages = 4 (for 030 through 060
size units)
If the economizer is able to provide free cooling (Run Sta-
→ →
tus ECON ACTV = YES) then:
If the economizer's current position is less than Configura-
→ →
tion ECON EC.MX – 5 and mechanical cooling has not
yet started for the current cool mode session then:
• Lo Cool Mode mechanical stages = 0
• Hi Cool Mode mechanical stages = 0
During the first 2.5 minutes of a low or high cool mode
where the economizer position is greater than Configuration
→ →
ECON EC.MX – 5% and mechanical cooling has not yet
started:
• Lo Cool Mode mechanical stages = 0
• Hi Cool Mode mechanical stages = 0
If the economizer position is greater than Configuration
→ →
ECON EC.MX – 5% for more than 2.5 minutes but less
than 5.5 minutes and mechanical cooling has not yet started
then:
• Lo Cool Mode mechanical stages = 1
• Hi Cool Mode mechanical stages = 1
If the economizer position is greater than Configuration
→ →
ECON EC.MX – 5% for more than 5.5 minutes but less
than 8 minutes and mechanical cooling has started then:
• Lo Cool Mode mechanical stages = 2
• Hi Cool Mode mechanical stages = 2
If the economizer position is greater than Configuration
→ →
ECON EC.MX – 5% for more than 8 minutes but less
than 11.5 minutes and mechanical cooling has started then:
• Lo Cool Mode mechanical stages = 2
• Hi Cool Mode mechanical stages = 3
If the economizer position is greater than Configuration
→ →
ECON EC.MX – 5% for more than 11.5 minutes and
mechanical cooling has started then:
• Lo Cool Mode mechanical stages = 2
• Hi Cool Mode mechanical stages = 3 (for 020 to 027 units
only)
• Hi Cool Mode mechanical stages = 4 (for 030 to 060 units
only)
NOTE: If some compressors are not available due to being
faulted, the Hi Cool Mode number of compressors are affected
before the Lo Cool Mode number of compressors. For exam-
ple, if a 4 compressor unit has one compressor faulted, and the
economizer is not active, then an HVAC mode Hi Cool
requested number of compressors is changed from 4 to 3. If
another compressor faults, then both Lo Cool and Hi Cool
requested number of compressors are set to 2. In addition,
compressors cannot be brought on faster than one every
30 seconds. If the control needs to bring on 2 compressors at
once, the first compressor will come on followed by the second
compressor 30 seconds later.
Staging of compressors is shown in Tables 49-54.
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