Carrier 30GN040-420 Example 4 Ð Reading and Changing Chilled Fluid Set Point, Reset Amounts

Example 4 Ð Reading and Changing

Chilled Fluid Set Point

KEYPAD	DISPLAY	COMMENTS
ENTRY	RESPONSE
	SET POINT	System set points
	CSP1 44.0	Present occupied chilled ¯uid
		set point is 44.0 F
	CSP1 42.0	Press the	.
		Display shows new occupied
		chilled ¯uid set point is 42.0 F
	CSP2 44.0	Present unoccupied chilled ¯uid
		set point is 44.0 F
	CSP2 50.0	Press the	. Display
		shows new unoccupied chilled
		¯uid set point is 50.0 F
	RESET	Displays the maximum reset
		set point. The minimum and
		maximum reference reset
		set points can also be displayed.
		These set points are not
		accessible when reset type has
		been con®gured for NONE in
		the service function.

Subfunction displays temperature reset set points.

Temperature Reset Based on Return Fluid Temperature Ð The control system is capable of providing leaving ¯uid tem- perature reset based on return ¯uid temperature. Because the temperature difference between leaving and return tempera- ture is a measure of the building load, return ¯uid tempera- ture reset is essentially an average building load reset method.

Under normal operation, the chiller maintains a constant leaving ¯uid temperature approximately equal to chilled fluid set point. As building load drops from 100% down to 0%, entering cooler ¯uid temperature drops in proportion to load. Thus, temperature drop across the cooler drops from a typi- cal 10 F (5.5 C) at full load to a theoretical 0° F (0° C) at no load. See Fig. 5.

At partial load, leaving chilled ¯uid temperature may be lower than required. If this is allowed to increase (reset), the efficiency of the chiller increases. Amount of reset can be de®ned as a function of cooler temperature drop, as shown in Fig. 5. This is a simple linear function that requires 3 pieces of input data for the set function that will vary de- pending on measurement method used as follows (see Table 11):

NOTE: Reset set points are not accessible unless the reset function is enabled ®rst. This is done as a ®eld

con®guration. Select one of the 4 choices for type of reset: Return Fluid Reset, External Temperature Reset, 4-20 mA External Signal Reset, or 4-20 mA Internal Signal Reset.

If dual set point control is enabled (see Field Wiring sec- tion on page 69), the amount of reset is applied to whichever set point is in effect at the time.

Examples 5A-5C demonstrate how to activate reset. Ex- ample 6 demonstrates how to change the type of reset. As- sume that reset is to be based on return ¯uid temperature, the desired reset range is to be 0° to 10° F (0° to 5.5° C) and full load is a 10° F (5.5° C) drop across the cooler. See Fig. 5.

Activating reset based on external temperature or 4-20 mA signal is done the same way, except the reference set point range is ±40° to 240° F (±40° to 115° C), or 4 to 20 mA depending on which method was selected at the ®eld con®guration step.

Example 5A Ð External Reset

In this example, the unit set point is reset from full load at 90 F (32 C) to a maximum reset value of 10 F (5.5 C) at 20 F (±6.7 C) outdoor ambient.

NOTE: All temperatures given in this example are in F.

KEYPAD	DISPLAY	COMMENTS
ENTRY	RESPONSE
	FLD CFG
	CRTYP 0	Scroll past to reset type
	CRTYP 2	External reset selected
	ERTYP 0	Scroll past to space
		thermistor sensor selected
	ERTYP 1	OAT selected
	RESET
	CRT2N 0	Temperature for no
		reset is 0
	CRT2N 90	Temperature for no
		reset is 90
	CRT2F 0	Temperature for maximum
		reset is 0
	CRT2F 20	Temperature for maximum
		reset is 20
	CRT2D 0	Maximum reset is 0
	CRT2D 10	Maximum reset is 10

Table 11 Ð Reset Amounts

			MEASUREMENT METHOD
INPUT DATA DESCRIPTION	4-20 mA		OAT/Occupied Space		Return Water
			or Internal/External
	Variable	Limits (F)	Variable	Limits (F)	Variable	Limits (F)
Maximum Reset Amount Ð Allowable range for	CRT1	±30 to 30	CRT2D	±30 to 30	CRT3D	±30 to 30
maximum amount which LWT is to be reset.
Maximum Reset Reference Ð Temperature at	Ð	Ð	CRT2F	±40 to 240	CRT3F	0 to 15
which maximum reset occurs.
Minimum Reset Reference Ð Temperature at	Ð	Ð	CRT2N	±40 to 240	CRT3N	0 to 15
which no reset occurs.
LEGEND
OAT Ð Outdoor-Air Temperature
LWT Ð Leaving Fluid Temperature

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