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Air Heaters

Air Heaters

Posted by Adelle Webber on

Industrial Electric Air Heaters

Electric duct heaters are used for industrial, commercial, and residential applications by being placed in the airflow of HVAC ductwork to transfer heat into factory floors, offices, and homes to warm the air.  

The heating components placed into an output duct transfers heat into the air stream that is carried throughout the building. 

These components are typical finned or honeycomb structures that are effective at transferring the heat from heating elements into the air passing through components.  One of the safest and durable is the PTC heating elements.

 

PTC heating elements are used in duct air heating systems.

Ceramic PTC, positive temperature coefficient, heating elements will not cause the ignition of dust or other combustible materials in the ductwork since they do not use resistive wires.  Since they have a maximum set temperature where they automatically restrict current flow they cannot overheat and cause fires.

Both of these characteristics are valued since the typical air duct heating components are inside ductwork where they cannot be viewed without opening maintenance covers.  

The heating elements also isolate voltage and current from the finned components providing safety for maintenance workers and anyone coming in contact with the often metal ductwork.  The fin structure containing multiple PTC heating circuits is also insulated from the surrounding ductwork such that heat doesn’t conduct to the ductwork itself.

 

Finned structure for air heaters

finned heater technology

The structure for transferring heat to the air flowing through the ductwork needs to cause a minimum pressure difference across the elements.  While multiple types of fin structures are used the Delta fin provides the lowest pressure drop across the heater.

Pressure drops in ductwork, especially in large commercial HVAC systems can add up and adversely affect the performance of the air handlers.  Inline filters, transitions, and angles in the ductwork constrict airflow.  It is important that the heating components minimize the pressure drop across the structure to not impede airflow more than necessary yet provide efficient heat transfer.

 

Duct heater temperatures

Depending on the application there are both high and low-temperature duct heaters available.  Industrial process heaters often require high heat for a manufacturing operation requiring material heating.  While space heating for offices or homes requires lower temperature heating elements.

PTC heating elements are designed with a specific set-point of maximum temperature that they will not exceed.  For some applications, this automatically adjusting heater is ideal and will require no additional control circuitry.  Other applications where more control of the output temperature is required will have temperature limiters or thermostats installed.

Where are air heaters used?

Air heaters are used anywhere there is air being forced through a duct by fans to acclimatize an area requiring additional heating.  The applications can be construction, commercial, and passenger vehicles.  You will find these heaters in manufacturing plants, on ships, in airplanes, and on trains.  Almost all HVAC systems contain some type of air heater in the ductwork.

  • Industrial machinery operator compartments
  • Ventilation systems with heat recovery
  • Building climatization
  • Low energy residential housing
  • Cockpit heater
  • EV passenger compartments

  • What do duct air heaters look like?

    modern duct heaters

    Modern duct heaters are aluminum honeycomb or finned structures that fit either partially or completely across the inside of the square, rectangular, or round ductwork.  

    The air flowing through the duct has to pass through the heater.

    The structure you see here is a V fins sandwich between heat transfer channels that conduct heat from the PTC heating elements to the fins.  Air moving through the fin structure gets heated by the transfer of heat from the fins to the air moving through them.

    v fin heaters

    As the heat is drawn from the fins heat is continually transferred from the heated bars onto the fins and the air from the PTC heating elements positioned at the ends of each bar.  Current is fed into the PTC ceramic heating elements from electrical wiring connected to a heating circuit. 

     

    What typical features of PTC electric heaters?

    PTC electric airflow heaters come in multiple sizes, voltages, and wattages.  The dimensions of the ductwork and available voltage are primary factors.  The output wattage and temperature requirements are taken into account.


    DBKUSA (HRP) Air Heaters Features

    • 2 Types of Fin available, V fin for maximum power, Delta fin for reduced pressure drop across the heater.
    • 6 Independent Encapsulated elements in each heater
    • Mounting via 4.5mm T-slot (mounting sliders available) 
    • Optional temperature limiter (must be used as the signal only to switch heater via separate relay)

    Small duct air heaters are used for heating electrical vehicles, construction equipment, commercial trucks, trains, and planes.  From a small almost square dimension of 3.8 X 4.5 X 4.7 inches up to 9 X 8.8 X 8.  The difference comes from the mounting brackets and electrical leads on the two sides.

    An example of specifications comes from DBK USA’s HRP line of heaters shown below.  Go to the (HRP) air heater page to learn more about the 115 Volt HRP and 230 - 400 Volt HRP.


    HRP

    HRP S

    HRP M

    HRP L

    HRP S

    HRP M

    HRP L

    Fin shape

    Delta

    V

    Rated voltage (V)

    230 - 400

    230 - 400

    Heating capacity 230V | 400V at 20°C & 300m³/h (W)

    790 | 870

    1230 | 1270

    1550 |1600

    1110 | 1170

    2000 | 2050

    2570 | 2750

    Inrush current at 230V | 400V (A)

    7 | 14

    14 | 26

    18 | 31

    6 | 12

    12 | 22

    19 | 35

    Differential pressure at 20°C, 200m³/h (Pa)

    56

    26

    16

    237

    127

    72

    Protection class

    II

    II

    Length (L1) of heater (mm)

    99,6

    153,6

    210

    99,6

    153,6

    210

    Width (mm)

    116

    116

    Heating circuits

    6

    6

    Standard wire length (mm)

    170

    170


    These heaters are rated from 115 up to 400 Volts AC/DC with heating capacities ranging from 840 Watts to 2750 Watts.  Heating capacity is based on 68 degrees Fahrenheit & 175 cfm (cubic feet per minute)    


    What is differential pressure in the ductwork?

    If you ever changed your home air conditioning or heating system filter it is because a dirty filter has a higher differential pressure than a clean one.  Differential pressure is the resistance to airflow through a filter, heater, or cleaner placed in the air stream.  

    Finned air heaters are designed to provide a minimum amount of resistance to the airflow in ductwork.  As with your filter, they do have to be cleaned periodically to ensure heat transfer effectiveness and minimize resistance.

    Differential pressures of finned air flow heaters vary from 44 Pa to 650 Pa at the same standard airflow.  Before you go to Google what Pa stands for is Pascal, a unit of pressure.  A more useful measurement from pressure gauges is mmH20 or how many millimeters the pressure equals in a tube of water.

    Without going into the equations 44 Pa = 4.49 mmH20 where 650 Pa = 66.28 mmH20.  The latter is a much higher pressure loss across the heating elements or resistance to airflow.  For those millibar enthusiasts, 100 pascals = 1 millibar. 

    A technician measuring airflow across a device will measure on both sides and subtract the difference.  They have numerous handheld measuring devices at their disposal. In-process heating applications there might even be a permanently mounted device called a pitot tube similar to what is used in airplanes to monitor airflow.

     

    Questions on what type of air heater to use for your industrial applications?


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