Standardizing Electric Process Heaters
Advantages of using electric process heaters include high outlet temperatures, accurate control, small footprint, low weight, low maintenance costs, no emissions, no heat transfer fluid and no steam. Electric process heaters convert electricity into heat with 100 percent efficiency. One disadvantage of using an electric process heater is the high cost of electricity vs. the cost of natural gas or steam.
A Closer Look. The three components of an electric process heater are the heater control panel, the heater bundle and the heater vessel. Electricity passes from the main circuit breaker to a power distribution block, where the electricity is divided into subcircuits. Electricity in each subcircuit flows through a magnetic contactor to semiconductor fuses and then through a silicon-controlled rectifier (SCR). The purpose of the magnetic contactor is to cut power to the heater bundle if an overtemperature situation occurs. The electricity leaves the panel, goes through conduit or cable, and into the heater bundle terminal enclosure. The electricity then flows through the heater bundle, where it is converted into heat.
Almost all industries use electric heaters -- oil and gas, refinery, petrochemical, pipeline, power and other general industries. Common applications include:
- Water heating to prevent freezing.
- Diesel fuel-oil heating to prevent formation of waxy paraffins.
- Fuel gas heating to prevent hydrocarbon condensation.
- Glycol reboiling to remove water from glycol.
- Heating transfer fluids.
- Air preheating for ammonia and air mixing.
- Nitrogen heating to regenerate adsorption beds.
Of course, that is just a partial list; electric heaters are used in almost any application you can imagine.
- Control panel: NEC, NEMA, UL.
- Heater bundle: NEC, NEMA, ASME, UL, TEMA.
- Heater vessel: ASME, SPCC.
- Skid: AWS, SPCC.
- Conduit and wire: NEC.
Electric process heating has been accepted by all industries, yet there has not been an industry-standard specification. The lack of a specification creates confusion for end users, engineering companies and suppliers. Generally on a project, one engineer (electrical, mechanical or process) will write a "job" specification for an electric process heater. This practice causes many discrepancies from job to job and engineer to engineer. Until now, no single specification that addresses all aspects of an electric process heater has existed.
One heater manufacturer has developed a working draft of such a standard. At present, the company is seeking comment on "IFS-607A," the working title of the standard. If adopted industry-wide, this standard would ensure that every electric process heater has several features. The following are just a few.
Finger-Safe Protection. Finger-safe protection inside the control panel is important to prevent accidental electrical shock.
Overtemperature Control. Operators want alarm notification and reset inside of the control room. Some electric process heaters have one over-temperature controller. The standard would specify that all electric process heaters have at least two overtemperature controllers: one that can be remote reset and a higher one that must be reset at the control panel. Furthermore, an additional overtemperature trip is on the heater vessel. It also calls for a 4 to 20 mA signal available for the operator for both the heater elements and heater vessel.
Magnetic Contactor Failure Sensor. Magnetic contactors are the first line of defense against an overtemperature situation. If a magnetic contactor had fused closed, then it is not able to stop the flow of current to the heater. The standard calls for magnetic contactor failure sensing and a shunt trip circuit breaker.
Semiconductor Fuses. Generally speaking, there are slow blow, fast blow and semiconductor protection fuses. If the control panel has an SCR, the standard specifies that semiconductor fuses are used to protect the SCR. Slow blow and fast blow fuses will not react quickly enough to protect the SCR.
Flange-Mounted Disconnect. A flange-mounted disconnect is specified. Through-the-wall disconnects can break more easily in the field.
Shunt Trip Circuit Breaker. This required breaker provides a fourth level of overtemperature protection.
Epoxy Sealing of Elements with Ceramic Insert. About half of all heater failures are due to moisture in the magnesium oxide. One way to prevent this is to seal the end of the heater element. Many methods exist, but the standard specifies that the end of the element is filled with epoxy squeezed in place with a ceramic plug.
Blocked Control. If magnetic contactors are used for power control, an additional set of magnetic contactors should be used for overtemperature protection. (This also is defined in NEC 424-73.)
Cascade Process Control. Using the flow diagram in figure 1, temperature control is primarily on the heater element. The setpoint is adjusted by the outlet temperature controller. This allows a maximum element temperature to be specified at a lower temperature than the overtemperature controller setpoint. Control can further be enhanced with a flow transmitter.
Safety Factor. A safety factor should be applied for voltage variations and manufacturing wattage tolerances. If the flow rate already has a safety factor, then it is up to the engineer to decide if additional safety factor is required. Actual jobsite voltages can vary due to transformer tolerances and branch loading. If a heater is sized for three-phase 480 V and the actual voltage at the jobsite is three-phase, 462 V, then the power available at the heater is
(4622/4802) x 100
The result is 93 percent less than design. In this example, a safety factor of 1.07 should be used unless the flow rate already has an excess of 7 percent.
ASME Code Stamp and TEMA. The American Society of Mechanical Engineers' Pressure Vessel Code, Section VIII, has identified that the heater immersion bundle is required to have a partial code stamp. Any immersion heater going into a pressure vessel should have a partial code stamp. Likewise, the Tubular Equipment Manufacturers Association does an excellent job specifying the manufacturing tolerances of shell and tube exchangers. Electric process heaters should be within the same tolerances.