Carrier 17EX specifications Capacity Overrides

2.The motor load ramp loading rate is an operator-configured value that limits the rate at which the compressor motor current or compressor motor load increases. (LOAD PULL- DOWN %/MIN on the CONFIG screen).

To select the ramp type, highlight the SELECT RAMP TYPE parameter on the CONFIG screen and select either 0 (TEMP) or 1 (LOAD). Motor load (1) is the default ramp loading control type.

Capacity Override (See Table 4) Ð Capacity over- rides can prevent some safety shutdowns caused by exceed- ing the motor amperage limit, refrigerant low temperature safety limit, motor high temperature safety limit, and con- denser high pressure limit. In all of these cases, there are 2 stages of compressor vane control.

1.The guide vanes are kept from opening further, and the status line on the LID displays the reason for the override.

2.The guide vanes are closed until the condition decreases below the ®rst step set point. Then, the vanes are released to normal capacity control.

Whenever the motor current demand limit set point is reached, it activates a capacity override, again using the 2-step pro- cess. Exceeding 110% of the rated load amps for more than 30 seconds initiates a safety shutdown.

The compressor high lift (surge prevention) set point causes a capacity override as well. When the surge prevention set point is reached, the PIC normally prevents the guide vanes from opening. See the Surge Prevention Algorithm section, page 37. If the chiller is equipped with the hot gas bypass option, the PIC opens the hot gas bypass valve instead of holding the guide vanes.

High Discharge Temperature Control Ð If the dis- charge temperature increases above 200 F (93 C), the guide vanes are proportionally opened to increase gas ¯ow through the compressor. If the leaving chilled water temperature drops 5° F (2.8° C) below the control point temperature, chiller enters the RECYCLE mode.

Oil Sump Temperature Control Ð The oil sump tem- perature control is regulated by the PIC which uses the oil heater relay when the chiller is shut down.

As part of the pre-start checks executed by the controls, the PIC compares the oil sump temperature to the evapora- tor refrigerant temperature. If the difference between these 2 temperatures is 50 F (27.8 C) or less, the start-up is delayed until the oil temperature difference is 50 F (27.8 C) or more. Once this temperature is con®rmed, the start-up continues.

The oil heater relay is energized whenever the chiller com- pressor is off and the oil sump temperature is less than 150 F (65.6 C) or the oil sump temperature is less than the cooler refrigerant temperature plus 70° F (39° C). The oil heater is turned off when the oil sump temperature is either

·more than 160 F (71.1 C)

·or the oil sump temperature is more than 155 F (68.3 C) and more than the cooler refrigerant temperature plus 75° F (41.6° C).

The oil heater is always off during start-up or when the compressor is running.

When a power failure to the PSIO module has occurred for more than 3 hours (i.e., initial start-up), the compressor guide vane opening is slowed down to prevent excessive oil foaming that may result from refrigerant migration into the oil sump during the power failure. The vane opening is slowed via temperature ramp loading to a value of 2° F (1.1° C) per minute.

Table 4 Ð Capacity Overrides

	OVERRIDE			FIRST STAGE SETPOINT		SECOND STAGE	OVERRIDE
						SETPOINT	TERMINATION
	CAPACITY
	CONTROL		View/Modify	Default Value	Con®gurable Range	Value	Value
			on LID Screen
HIGH CONDENSER			Equipment	125 psig	90 to 200 psig	.Override	,Override
						Set Point
	PRESSURE		Service1	(862 kPa)	(620 to 1379 kPa)		Set Point
						+ 4 psid (28 kPad)
LOW REFRIGERANT						<Trippoint	.Trippoint
	TEMPERATURE		Equipment	,3° F (1.6° C)	2° to 5° F	+ Override	+ Override
(Refrigerant Override			Service1	(Above Trippoint)	(1° to 3° C)	DT ±1° F	DT +2° F
Delta Temperature)						(0.56° C)	(1.2° C)
				Min: T1 Ð 1.5° F	0.5° to 15° F
				(0.8° C)	(0.3° to 8.3° C)		Within
HIGH COMPRESSOR				P1 Ð 50 psid	30 to 170 psid		Lift Limits
			Equipment	(345 kPad)	(207 to1172 kPad)		Plus Surge/
		LIFT				None
			Service1	Max: T2 Ð 10° F	0.5° to 15° F		HGBP
(Surge Prevention)
				(5.6° C)	(0.3° to 8.3° C)		Deadband
				P2 Ð 85 psid	30 to 170 psid		Setting
				(586 kPad)	(207 to 1172 kPad)
		MANUAL	Control				Release of
	GUIDE VANE		Algorithm	Automatic	0 to 100%	None	Manual
		TARGET	Maint01				Control
	MOTOR LOAD Ð					>5% of	2% Lower
		ACTIVE	Status01	100%	40 to 100%		Than
						Set Point
	DEMAND LIMIT						Set Point
	LEGEND
HGBP	Ð	High Gas Bypass
P1	Ð	Minimum Pressure Load
P2	Ð	Maximum Pressure Load
T1	Ð	Minimum Temperature Load
T2	Ð	Maximum Temperature Load

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