The Efficiency of Electric Heaters

All electric air heaters are efficient users of energy. The losses exist in the power station generating system and distribution grid. You only need to record the input power to the heater with a kWh meter to see what heat is supplied to the building.

If an electric air heater is used to heat a room, it heats recirculated air until the thermostat is satisfied. The room fabric warms up and the thermostat cycles slowly, based on the loss of heat through the walls , ceiling etc.

Heat added to fresh input air requires a more sophisticated control method to hold the temperature to a set level. This is done using multi step control or smooth thyristor ‘pulse’ control. This is summarised in the chart:

Control type Accuracy Maintenance Energy Efficiency
3 or 4 step +/- 5-7 c After 2 years Approx. 92%
6 step +/- 3-4 c After 2 years Approx. 95%
Pulse control +/- 0.5 c After 10 years Approx. 99%

Fresh air input systems employing MVHR (air to air heat recovery) will use an electric duct air heater to trim the input temperature up by a few degrees to the comfort level required.

The pulse controlled heater will do this and turn off as soon as the recovery system is at working efficiency.

Neatafan pulse control heaters are sensitive down to low (trickle) airflows and can prevent heat damage caused by loss of flow.

Air Duct Heater Output Calculation

Having calculated the duct velocity and selected a suitable duct size, the next step is to determine the power input/heat output (apart from thermal losses in the ducting they are the same) of the duct heater from the air on & off temperatures required.

E.g. A typical frost heater would be required to raise ambient air from minus 5 deg.c to plus 5 deg.c and a typical re-heater would raise the air off temperature from a heat exchanger from 12 deg.c to 22 deg.c. Both of these heaters have a 10 deg.c rise and both will be similar in kW size.

Most HVAC heaters have a temperature rise between 5 and 25 deg.c.

The basic formula to calculate the heater size in kW is:

Airflow in m3/s x density factor (1.3) x temperature rise (deg.c) = kW

To calculate the airflow in cubic metres per second, convert the units given:

Litres/sec. Divide by 1000
 m3/hour Divide by 3600
 CFM Divide by 2119
 kg/hr Divide by 3600 (you do not need to use the density factor for kg/s).

kg/s x temperature rise (deg.c) = kW

Air duct velocity calculation

The velocity or speed of the air in a duct is proportional to the air flow rate. High velocity (6 m/s and above) is useful to transport particles in process industries but will create noise and pressure (energy) loss in ventilation ducting. Low velocity (below 1.5m/s) can result in uneven flow patterns and local overheating of materials near the electric heater. Low velocity can also make it difficult to detect airflow using pressure switches or sensors.

Neatafan electric heaters are designed to operate between 1.5 & 6m/s. outside these limits the life of the heater will be reduced significantly.

To calculate the airflow in cubic metres per second, convert the units given:

Litres/sec. Divide by 1000
 m3/hour Divide by 3600
 CFM Divide by 2119
 kg/hr Divide by 3600 and then divide by the density ( 1.3 for a typ. heater).

 

The basic formula for velocity (m/s) is: Airflow (in m3/s) / Duct Area (in m2)

EG:

  • For an airflow of 0.5 m3/s and an ideal velocity of 3m/s       0.5/3 = 0.17
  • The square root of 0.17 is approx. 0.4, so a 400mm square duct will flow at 3m/s.
  • The minimum duct size would be 0.5/6 = 0.083, which is approx. 300mm square (6m/s).
  • The maximum duct size would be 0.5/2 = 0.25, which is 500mm square or 400mm x 625mm.

The heater will create a static air pressure loss in the duct which will increases rapidly with duct velocity (see published chart). To reduce energy waste and noise it is best practice to avoid high duct velocities.

Do’s & Don’ts

DO

  • Use heaters with built in control. Save space and wiring costs.
  • Use supply duct sensing. To provide steady tempered fresh air.
  • Use the run on timer provided on larger heaters.
  • Link the heater to the BMS (if available) but use the local control provided.
  • Choose the MEMs velocity sensor with low pressure ducts.
  • Choose the MEMs sensor for speed controlled fans.
  • Fit a local isolator switch.

Don’t

  • Allow plastic, rubber or items easily damaged by heat to touch the heater casing.
  • Use a duct thermostat to control a heater, its wastes energy.
  • Use old fashioned step control. Its noisy, inaccurate and needs maintenance.
  • Use bi-metal overheat thermostat without checking its operation.
  • Use heater outside without fitting a weatherproof housing.
  • Forget to fix the duct sensor or airflow switch tubes (if applicable).

THE COMMON FORMULAE USED TO SELECT ELECTRIC HEATERS

1/  To determine heater size in kW for a specific airflow & temperature rise;

POWER   =   AIR VOLUME  X  DENSITY  X  (Air on – air off)

     kW      =           m3/s        x      kg/m3   x       Deg. C (rise)

units To obtain m3/s Mean Temp.(c) Density kg/m3
L/s Divide by 10001.3 0 1.3
M3/hr Divide by 3600 5 1.25
Kg/s Density = 1 10 1.23
15 1.2

2/  To determine the cable current in Amps;

kW x 1000   /    V (volts)    =   I (Amps)

For a single phase heater     Watts divided by 240v * = current in Amps.

*Its important that the actual voltage is used for these calculations and not the 230v/400v

EU harmonised voltage.          ( In the UK this is still  between 235v & 245v.)

For a three phase heater you need to divide the total Watts by three first (for each cable).

The cable voltage will be 415v divided by 1.732(root3) i.e.  240v. This is the same for star or delta heaters.

Example: What is the current in each cable feeding a 27 kW 3 phase heater?

   27 x 1000 / 3  =  9000W            9000W / 240V = 37.5 A

Heater Codes

CHC
Circular heater with built in control
CHN
Circular heater for external control
DHC
Duct heater with built in control
DHN
Duct heater for external control
SHC
Insertion heater with built in control
SHN
Insertion heater for external control
UHC
Slide in heater (AHU) with built in control
UHN
Slide in heater (AHU) for external control

Electrical Power Calculation

In the UK, single phase loads up to 9kW can be used. For accurate calculations it is necessary to use the correct rms supply voltage and not the nominal 230v ‘harmonised’ figure.

For a single phase heater:

Supply current in amps = input power in kW x 1000 / supply voltage (typically 240v)

 For a 3 phase heater, you will need the current drawn by each phase. Simply divide the heater kW by three and use the above formula to calculate this

 

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