Carrier 48/50AJ Run Status Cool Display, Expansion Range Units CCN Point Write Status

Table 62 — Run Status Cool Display

ITEM	EXPANSION	RANGE	UNITS	CCN POINT	WRITE STATUS
COOL	COOLING INFORMATION
C.CAP	Current Running Capacity		%	CAPTOTAL
CUR.S	Current Cool Stage			COOL_STG
REQ.S	Requested Cool Stage			CL_STAGE
MAX.S	Maximum Cool Stages			CLMAXSTG
DEM.L	Active Demand Limit		%	DEM_LIM	forcible
SUMZ	COOL CAP. STAGE CONTROL
SMZ	Capacity Load Factor	-100 – +100		SMZ
ADD.R	Next Stage EDT Decrease		^F	ADDRISE
SUB.R	Next Stage EDT Increase		^F	SUBRISE
R.PCT	Rise Per Percent Capacity			RISE_PCT
Y.MIN	Cap Deadband Subtracting			Y_MINUS
Y.PLU	Cap Deadband Adding			Y_PLUS
Z.MIN	Cap Threshold Subtracting			Z_MINUS
Z.PLU	Cap Threshold Adding			Z_PLUS
H.TMP	High Temp Cap Override			HI_TEMP
L.TMP	Low Temp Cap Override			LOW_TEMP
PULL	Pull Down Cap Override			PULLDOWN
SLOW	Slow Change Cap Override			SLO_CHNG

Slow Change Cap Override (SLOW) — With a rooftop unit, the design rise at 100% total unit capacity is generally around 30° F. For a unit with 4 stages, each stage represents about 7.5° F of change to EDT. If stages could reliably be cycled at very fast rates, the set point could be maintained very precisely. Since it is not desirable to cycle compressors more than 6 cy- cles per hour, slow change override takes care of keeping the PID under control when “relatively” close to set point.

SumZ Operation — The SumZ algorithm is an adaptive PID style of control. The PID is programmed within the control and the relative speed of staging can only be influenced by the user through the adjustment of the Z.GN configuration. The capaci- ty control algorithm uses a modified PID algorithm, with a self adjusting gain which compensates for varying conditions, in- cluding changing flow rates across the evaporator coil.

Previous implementations of SumZ made static assump- tions about the actual size of the next capacity jump up or down. This control uses a “rise per percent capacity” technique in the calculation of SumZ, instead of the previous “rise per stage” method. For each jump, up or down in capacity, the control will know beforehand the exact capacity change brought on. Better overall staging control can be realized with this technique.

SUM Calculation — The PID calculation of the “SUM” is evaluated once every 80 seconds.

SUM = Error + “SUM last time through” + (3 * Error Rate) Where:

SUM = the PID calculation, Error = EDT – Cooling Control Point, Error Rate = Error – “Error last time through”

NOTE: “Error” is limited to between –50 and +50 and “Error rate” is limited to between –20 and +20.

This “SUM” will be compared against the “Z” calculations in determining whether cooling stages should be added or subtracted.

Z Calculation — For the “Z” calculation, the control attempts to determine the entering and the leaving-air temperature of the evaporator coil and based upon the difference between the two during mechanical cooling, and then determines whether to add or subtract a stage of cooling. This is the adaptive element.

The entering-air temperature is referred to as MAT (mixed-air temperature) and the leaving-air temperature of the evaporator coil is referred to as EDT (evaporator discharge temperature). They are found at the local display under the Temperatures→CTRL sub-menu.

The main elements to be calculated and used in the calcula- tion of SumZ are:

1) the rise per percent capacity (R.PCT)

2)the amount of expected rise for the next cooling stage addition

3)the amount of expected rise for the next cooling stage subtraction

The calculation of “Z” requires two variables, Z.PLU used when adding a stage and Z.MIN used when subtracting a stage. They are calculated with the following formulas:

Z.PLU = Z.GN * (10 + (4*(–ADD.R))) * 0.6

Z.MIN = Z.GN * (–10 + (4*(–SUB.R))) * 0.6 Where:

Z.GN = configuration used to modify the threshold levels used for staging (Configuration→COOL→Z.GN)

ADD.R = R.PCT * (C.CAP – capacity after adding a cooling stage)

SUB.R = R.PCT * (C.CAP – capacity after subtracting a cool- ing stage)

Both of these terms, Z.PLU and Z.MIN, represent a thresh- old both positive and negative upon which the “SUM” calculation must build up to in order to cause the compressor to stage up or down.

Comparing SUM and Z — The “SUM” calculation is com- pared against Z.PLU and Z.MIN.

•If “SUM” rises above Z.PLU, a cooling stage is added.

•If “SUM” falls below Z.MIN, a cooling stage is subtracted.

There is a variable called SMZ which is described in the reference section and which can simplify the task of watching the demand build up or down over time. It is calculated as follows:

If SUM is positive: SMZ = 100*(SUM/Z.PLU)

If SUM is negative: SMZ = –100*(SUM/Z.MIN)

Mixed Air Temperature Calculation (MAT) — The mixed- air temperature is calculated and is a function of the economiz- er position. Additionally there are some calculations in the con- trol which can zero in over time on the relationship of return and outside air as a function of economizer position. There are two configurations which relate to the calculation of “MAT.” These configurations can be located at the local display under Configuration→UNIT.

ITEM	EXPANSION	RANGE	CCN	DEFAULTS
			POINT
UNIT	UNIT CONFIGURATION
MAT.S	MAT Calc Config	0 - 2	MAT_SEL	1
MAT.R	Reset MAT Table	Yes/No	MATRESET	No
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