Download HVAC Design Criteria and Guidelines ELECTRIC HEATING COILS

HVAC Design Criteria and Guidelines
ELECTRIC HEATING COILS
Coil computations and selections may be documented by using the Heating Coil Selection Work Sheet.
Select electric resistance heating coils on the basis of heating requirement as follows:
kW = Heating Load (Btuh) / 3413 (Btuh/kW)
To determine electrical load (amps) imposed by a heating coil, the following equations can be used:
Single Phase
Amps = (kW x 1000) / (volts x pf)
3-Phase
Amps = (1000 x kW) / (1.732 x volts x pf)
where pf is the electrical service power factor, typically 0.9-0.95.
To prevent hot spots, airflow must be uniformly distributed across the coil face. The coil’s UL Listing
requires that it not be installed closer than 48” downstream or upstream from a fan outlet, abrupt transition,
or other obstructions. Elbows or ogees must be located at least 48” from inlet of the heater and 24” from
outlet of the heater.
Sufficient minimum airflow must be provided to prevent overheating and nuisance tripping of the thermal
cutouts. The minimum required velocity is determined from the following figures on the basis of entering air
temperature and KW per square foot of cross sectional duct area. The maximum air inlet temperature for
open coil heaters is 100°F and 80°F for finned tubular heaters.
HVAC Design Guidelines
ELECTRIC HEATING COILS
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HVAC Design Criteria and Guidelines
If electric heating coils are installed in a coastal location, in a variable air volume AHU or in a system
utilizing an economizer cycle, utilize finned tubular construction, coated for corrosion protection.
For coils installed at an elevation exceeding 1000 feet above sea level, the coil performance must be
corrected for elevation.
Determining the number of stages of electric heat requires consideration of the total ΔT of the heating coil
and the degree of control required. For “good” space temperature control, the ΔT for each stage should be
generally no more than 5ºF, though 10ºF per stage may be “adequate” for noncritical comfort control. For
typical overhead supply air, the air temperature leaving the heating coil should not exceed 95ºF in order to
avoid air buoyancy problems. Based on a room (heating) temperature setpoint of 70ºF, the total ΔT would
be 25ºF. Thus, a 3-stage heating coil would provide adequate control,” while a 5-stage coil would provide
good control.
Control of a multi-stage electric heating coil requires some method of switching to turn power on/off to each
stage and three methods are commonly applied, as follows:
The most common method for doing this is via use of a mechanical relay or contactor to control
power delivery to the electrical circuit of each coil stage, resulting in “on/off” control of each stage.
Mercury displacement relays (MDR) can cycle faster than mechanical relays. However, installation
of MDRs can be an issue, since they have to be installed perfectly vertical, and overheating of the
relay, due to excessively fast cycling or overloading, can cause it to explode, a situation which
creates a hazardous materials problem. In addition, shipping and disposing of MDRs is becoming
increasingly more difficult due to more stringent federal environmental regulations. The use of
MDRs is not recommended.
The solid-state relay (SSR) is a popular alternative to mechanical power control. A common
characteristic of all solid-state devices (including SSRs) is that they generate heat that needs to
removed. Solid-state relays generate more heat than SCRs. Almost all solid-state relays are rated
for maximum output at a temperature of 25°C (77°F). However, in real world operating conditions
where temperatures inside electrical enclosures exceed 40°C (104°F), a solid-state relay will fail if
used at full output. Most SSR manufacturers have a de-rating chart for their product to
compensate for this discrepancy. Unfortunately, many vendors will use the advertised maximum
rating when selecting a solid-state relay, resulting in early failure. Another drawback to SSRs is
cost, since they are significantly more expensive than mechanical relays.
When more precise temperature control is required (e.g., hospitals, laboratories, etc.), a solid-state silicon
controlled rectifier (SCR) controller can be used to provide fully proportional control of heating output. An
SCR controller is a time proportioned controller that modulates the heater to supply the exact amount of heat
required to satisfy the temperature requirements by modulating the time the electric heater is powered on,
not the kW of the heater. An SCR can cycle as fast as 0.08 seconds and, with proper selection and use,
can cycle on and off 1,000,000,000 times without any operating problems. Unlike a mechanical relay, an
SCR controller has no mechanical parts to wear out and it will not arc and is not affected by dirty contacts.
SCR controllers are typically rated for ambient operating temperatures of around 50°C (122°F) at full power,
significantly surpassing SSRs. But, as for all solid-state controllers, protection against over-temperature,
voltage spikes, and short circuits must be taken into account at the outset of a system design.
HVAC Design Guidelines
ELECTRIC HEATING COILS
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