Carrier 48HE003---006 Enthalpy Changeover Set Points, Demand Controlled Ventilation DCV

48HE,HJ

																						85		90	95	100		105	110
																						(29)		(32)	(35)	(38)		(41)	(43)
																			46
CONTROL	CONTROL POINT																4
CURVE APPROX. deg. F (deg. C)																	4
													42							80
	AT 50% RH
A	73 (23)									AIR										(27)
												40
B	70 (21)																										(%)
C	67 (19)									DRY	8														HUMIDITY
D	63 (17)							POUND		3
									36
																			75
							PER											(24)
								4
								3
				BTU																				RELATIVE
							32
																70
					30
														(21)
			28
															0
	ENTHALPY												1	0			0
		26										65				9		80
	4											(18)							70
	2																				0
	2																			6
	2									60											50
	0									(16)
	2										A											4	0
8								55
1								(13)		B														30
6
1							50
4								C
						(10)
1																										20
2			45				D
1			(7)
		40
																													0
		(4)
																													1
	35
	(2)
																			D C		B	A
																													HIGH LIMIT
																													CURVE
	35	40	45				50	55		60		65				70			75		80	85		90	95	100		105	110
	(2)	(4)	(7)				(10)	(13)		(16)		(18)				(21)			(24)		(27)	(29)		(32)	(35)	(38)		(41)	(43)
	APPROXIMATE DRY BULB TEMPERATURE--degrees F (degrees C)

C06037

Fig. 45 ---Enthalpy Changeover Set Points

Typically the maximum ventilation rate will be about 5 to 10%		OA		+ (TR x	RA
more than the typical cfm required per person, using normal
	(TO x 100		)			) =TM
outside air design criteria.				100

Demand Controlled Ventilation (DCV)

When using the EconoMi$er IV for demand controlled ventilation, there are some equipment selection criteria which should be considered. When selecting the heat capacity and cool capacity of the equipment, the maximum ventilation rate must be evaluated for design conditions. The maximum damper position must be calculated to provide the desired fresh air.

A proportional anticipatory strategy should be taken with the following conditions: a zone with a large area, varied occupancy, and equipment that cannot exceed the required ventilation rate at design conditions. Exceeding the required ventilation rate means the equipment can condition air at a maximum ventilation rate that is greater than the required ventilation rate for maximum occupancy. A proportional-anticipatory strategy will cause the fresh air supplied to increase as the room CO2 level increases even though the CO2 set point has not been reached. By the time the CO2 level reaches the set point, the damper will be at maximum ventilation and should maintain the set point.

In order to have the CO2 sensor control the economizer damper in this manner, first determine the damper voltage output for minimum or base ventilation. Base ventilation is the ventilation required to remove contaminants during unoccupied periods. The following equation may be used to determine the percent of outside-air entering the building for a given damper position. For best results there should be at least a 10 degree difference in outside and return-air temperatures.

TO = Outdoor-Air Temperature OA = Percent of Outdoor Air TR = Return-Air Temperature RA = Percent of Return Air TM = Mixed-Air Temperature

Once base ventilation has been determined, set the minimum damper position potentiometer to the correct position.

The same equation can be used to determine the occupied or maximum ventilation rate to the building. For example, an output of 3.6 volts to the actuator provides a base ventilation rate of 5% and an output of 6.7 volts provides the maximum ventilation rate of 20% (or base plus 15 cfm per person). Use Fig. 44 to determine the maximum setting of the CO2 sensor. For example, a 1100 ppm set point relates to a 15 cfm per person design. Use the 1100 ppm curve on Fig. 47 to find the point when the CO2 sensor output will be 6.7 volts. Line up the point on the graph with the left side of the chart to determine that the range configuration for the CO2 sensor should be 1800 ppm. The EconoMi$er IV controller will output the 6.7 volts from the CO2 sensor to the actuator when the CO2 concentration in the space is at 1100 ppm. The DCV set point may be left at 2 volts since the CO2 sensor voltage will be ignored by the EconoMi$er IV controller until it rises above the 3.6 volt setting of the minimum position potentiometer.

Once the fully occupied damper position has been determined, set the maximum damper demand control ventilation potentiometer to this position. Do not set to the maximum position as this can result in over-ventilation to the space and potential high-humidity levels.

28