Posted by Bill Brown on Tue, Jul 06, 2010 @ 10:03 PM
Many factors influence the reliability of an electric duct heater, the use of quality parts being the most significant. Contactors, transformers, manual resets and automatic limit switches are all essential to a heater's operation, but are fairly easy to replace if they happen to fail.
This is not always the case with a heater's resistive elements. Elements are the most critical component of an electric heater, and can be difficult to replace.
HEATER ELEMENTS
Open coil heating elements are made of a nickel/chromium alloy that radiate energy through their outer surface, or skin. The elements' skin can reach temperatures as high as 1700 degrees fahrenheit. When operated at such extreme temperatures, oxidation will begin to degrade the elements. To retard this process, and extend element longevity, surface temperatures of the elements should be kept as low as possible while still obtaining the wattage needed for the application.
WATT DENSITY
"Watt Density" is the rated wattage per unit of an element's surface area, i.e., "watts per square inch". To lower the "watt density", use longer and/or heavier gauge wire to increase surface area. The result will be elements that operate at cooler temperatures and therefore, last longer.
TEMPERTATURE RATINGS
High quality resistive heating elements are designed to operate below their maximum rated temperature. When operating at 100% capacity, they should appear to be dark or black in color, with almost no red glow anywhere on the element.
A heater that is made with low quality materials, or that does not receive adequate air flow, will operate closer to its maximum rated temperature. In that case, the resistive elements will appear to be dark orange to bright red in color, an indication that the elements are degrading and will need to be replaced earlier than usual.
ENSURE THAT YOUR HEATER HAS A LONG, REILABLE LIFESPAN
A well designed heater can be expected to deliver 15-20 years of reliable service. Ensure that this is the case by consulting with an HVAC design engineer prior to placing an order for equipment.
- Request that low "Watt Density" elements be specified in the heater's construction.
- Insist on the use of the highest quality parts
- Ensure that the CFM is correct for the heater's application
- Ensure that placement of the heater provides for adequate air flow
- Keep the air filters clean
Posted by Bill Brown on Wed, Feb 24, 2010 @ 08:11 AM
When specifying an electric duct heater an HVAC professional can help you save $$ and aggravation by designing the heater correctly the first time. The design engineer will need answers to a number of questions.
Information about the site
- Does the application allow open coil electric heat? Most HVAC installations do.
- What will the heat load be under the worst case conditions?
- Is "quiet operation" required? (ie: A classroom or doctor's office)
- Is the power to operate the heater available in the area where the conditioning is needed?
- What type of power is available? (both phase and voltage)
Duct system
To determine the appropriate kilowatts for the heater, the design engineer will need to know the duct size, the CFM and the temperature rise needed for the air exiting the duct. You may also want to consult with the engineer regarding staging. He can recommend the optimum number of stages to help minimize surging of the building's power for minimum cost.
How to save $$
Consulting with the design engineer regarding the number of kilowatts required for the job can often result in cost savings. For examlpe, a 20 KW heater at 240 Volts and 3 Phase can cost up to twice as much as a 19.6 KW heater with the same phase and voltage. This is simply because the 20 KW load is over 48 amps and UL requires circuit fusing for any load over 48 amps. You will save $$ if a 19.6 KW heater is adequate to do the job!
Other Considerations
- What is the control voltage and will it be supplied by the heater or another source?
- Will an Air Pressure Switch or a Fan Interlock be required? (to prevent operation of the heater in the event of low or no air flow)
- To ensure the safety of service personnel, the duct heater control box can be configured with a door interlocking disconnect switch. This saves on installation cost since a separate service disconnect may not be required in the area.
- Duct heaters are available with many options including: fan motor interlock relays, additional fan motor fusing, plenums, square to round construction, single point line connections, quiet SCR or mercury contactors and step controllers to name a few.
Make sure to consult with an HVAC professional before making any decisions about your heating requirements.
