VAV WITH ELECTRIC HEATER

Transcription

1 VAV WITH ELECTRIC HEATER

2 Electric Heater Electric heating coils are applied on VAV terminal units as terminal reheat devices. Electric heat coil capacity is rated in kilowatts (kw). Coils are available with the total capacity divided into one. two, or three stages. Electric heat coils are available in single-phase or three-phase models. This refers to the type of power source connected to the coil. Single-phase models have resistance elements internally connected in parallel. The current drawn for the electric coil will depend whether it is single phase coil or a three phase coil. The current drawn is necessary for determining what size wire should be used to power the electric coils and how big the primary power fusing should be. The equations for current draw for these coils are: 1 ö amps = kw x 1000 / Primary Voltage 3 ö amps = kw x 1000 / Primary Voltage x 3 It is important to note that these coils have certain minimum airflow rates for each amount of kw heat the coils can supply to operate safely. These airflow values are based upon a maximum rise across the electric heat coil of 50 F (28 C). The equation that relates the airflow across an electric coil to the temperature rise and the coil change in temperature is: CFM = kw x 3145 /?T Where: CFM = Minimum airflow rate across the coil kw = The heating capacity of the electric coil 3145 = A constant.?t = The maximum rise in air temperature across the coil (usually 50 F (28 C)) 3

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4 SELECTION GUIDELINES: 1) Once the design air flow has been determined refer to the appropriate air flow limits to select the size of unit required to deliver the specified air flow. 2) With size and air flow known, electric reheat capacity can be calculated. Refer to the electric coil selection procedure below for details. 3) With the heating capacity known, confirm: a. Power Supply requirements from coil selection charts. b. The minimum air flow requirements are met (at least 70 cfm/kw), and that the discharge air temperature does not exceed 120 F. 4) Select coil options. 5. Static pressure loss of the coil elements is negligible, therefore minimum pressure is equal to the basic unit values. 6. For stable operation of heater controls minimum discharge static pressure of 0.2 in. w.g. is recommended. 4

5 How to Use the Chart Electric Coil Selection Procedure The selection of an electric coil for a single duct terminal unit requires the determination of the two comp onents of the heat loss. One component is the heat required to satisfy the space load. The second component is the heat required to raise the temperature of the primary air to that of the space. This can be determined as follows, using the Electric Reheat Selection Chart and the equation as given below. Reheat Selection Chart 1) Locate the room heat loss on the MBH scale on the far left side of the chart. Convert to kw by moving horizontally to the right to the kw scale (1 kw = MBH). 2) Calculate the kw required to heat the primary air to room temperature using the following equation: kw = ( cfm x 1.08 x?t / 3413 ) 5

6 3) Add the kw value obtained in step 2 to the kw scale at the left side. Move horizontally to the right to the point where the kw value and the air flow volume intersect. 4) With the point of intersection from step three, the air temperature rise (ATR), can be obtained by interpolating between the air temperature rise lines on the graph. 5) To verify the selection, sum the air temperature rise and the temperature of the primary air. The sum total should be less than 120 F. Selection Example Select electric coil for a size 12 in. with a minimum air flow of 700 cfm. Space heat loss is estimated at 20 MBH and space design temperature is 70 F. The temperature of the primary air flow is 58 F. 1) Space heat loss (20 MBH) = 5.9 kw. 2) Heat required to raise the temperature of the primary air: kw = (cfm x 1.08 x?t / 3413) kw = (700 x 1.08 x 12 / 3413) kw = 2.65 kw 3) Total heat required = = 8.6 Kw. 4) Air temperature rise (ATR) = 40 F (From graph) 5) Leaving air temperature = 58 F + 40 F = 98 F. Since the leaving air temperature is less than the recommended maximum limit of 120 F, the selection is satisfactory. 6) Select a suitable power supply from the Coil Selection Charts. 7) Verify minimum air flow requirements are met. (Minimum 70 cfm/kw) 700 cfm / 8.6 kw = 81 cfm/kw 6

7 Electric data Single duct VAV-PI-EH Electric coil kw guideline Minimum to maximum

8 Minimum and maximum Airflow per Inlet size and kw x x 8

9 Performance data - Electric data Single duct

10 Performance data - Electric data Single duct

11 Performance data - Electric data Single duct

12 Welding Robot