Heatcraft Refrigeration Products HCS manual Electric Heater Selection Example, Maximum KW

Accessories- Electric Heat

The standard electric heater section construction			Typical options for the electric heater
includes:			section are:
1.	Automatic thermal cutout		1.	Fiberglass insulation in cabinet
2.	Heat limiters		2.	At least one magnetic contactor per step of
3.	Air flow switch			heating (two or three pole break)
			3.	Three phase fuse blocks and fuses
			4.	Transformer with primary fusing
			5.	Manual reset thermal cutout in control circuit
	Maximum KW per				Maximum KW
	contactor and per fuse		Component	208 V	240 V	460 V	480 V
	block (at least one	Low amps contactor		9.0	10.4	19.9	20.8
	contactor is required per
		High amps contactor		17.3	19.3	38.2	39.9
	step of heating):
			Fuse block	17.3	19.9	38.2	39.9

Electric Heater Selection Example

A.How many KW is needed to increase temperature of 17000 CFM by 27 °F for Air Handler Model Size 34?

17000 X 27	is:	153 KW IS REQUIRED
3000

B.How many steps required at 10°F rise per step?

300 X 153	is:	2.7 STEPS ROUNDS UP TO 3 STEPS
17000

C.How many steps required at 460 volts with low amps contactors?

153	is:	7.7 STEPS ROUNDS UP TO 8 STEPS
19.9

D.How many steps required at 460 volts with high amps contactors?

153	is:	4.0 STEPS
38.2

E.How many fuse blocks required?

153	is:	4.0 FUSE BLOCKS
38.2

°F. Order the 153 KW to get the 27 degree rise.

Throw out the 3 steps (figured at 10°F. rise in part B), because it is smaller than the two values figured per contactor size (parts D & E). Decide whether you want high amps or low amps contactors, if you want low amps contactors, round the 7.7 to 8 and order 8 of the low amps contactors (you now have a possible 8 steps of control). If four steps of control are satisfactory, order the high amps contactors. In either case, the application requires four fuse blocks (part E).

Notes:

1.You must order the split shipment of the fan and coil sections to get electric heat on unit size 03 through unit size 18.

2.All electric heat controls are built into the heater control box.

3.Specify two pole break contactors or three pole break contactors. Two pole contactors only de-energize, while three pole contactors disconnect.

4.Be aware of requirements for heater circuits rated more than 48 amps. According to N.E.C. Code and U.L. you must subdivide the heater elements of such circuits. N.E.C. Code requires that the sub-circuit fuses be furnished by the heater manufacturer.

Commonly Used Control Systems:

A.For small loads (up to 20 KW),consider a multi-stage thermostat.

B.For medium loads (up to 200 KW), consider a modulating thermostat that operates a step controller.

C.For fine control of large loads (above 75 KW), consider a combination step control and SCR vernier system. This scheme has 80% of heater capacity controlled by conventional step controller, with the other 20% on a solid state SCR controller.

D.For 100% proportional control of any size, consider a total SCR control.

E.Heatcraft does not supply : multi-stage thermostats modulating thermostats

But you will want to order contactors and fuse blocks from Heatcraft that are appropriate for for the control scheme specified in the field.

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