Here in Houston, we engineers get so carried away with shell/tube exchangers, TEMA types, HTRI/BJAC simulations, air coolers, etc, that we forget electric heaters are also available. Sometimes a pilot plant can be run in an industrial park that is located in a residential neighborhood. The only utility available for heating might be electricity only. That is where an electric heater comes in.
For me, I always got hung up on the concept of “watt density”. Where do I find that? GPSA Handbook? Well, Gaumer has got that covered for you. Please read their press release below and visit their website: http://www.gaumer.com
To heat any process, selecting the proper heater is paramount to success. Several factors must be reviewed to select the appropriate material, watt density, and heater duty.
Heater Duty Calculation
Selecting the duty of the electric heater requires three components:
Q = Required Duty in BTU/Hr
M = Mass to be heated (lb/hr, kg/hr)
C = Specific Heat of the fluid
ΔT = Temperature difference (Ending Temperature – Beginning Temperature)
For Example, to heat water @ 50 gpm from 50 – 100°F
Water weighs 8.34 lbs/gallon, so 50 * 8.34 = 417 Lb/Minute * 60 = 25,020 Lbs/Hr with a specific heat of 1 btu/lb °F
25,020 Lb/Hr * 1 btu/Lb/°F * 50°F = 1,251,000 btu/Hr
Since there are 3412 btu / kW, 1,251,000 / 3412 = 366.6 kW.
This is a very simplistic sizing procedure for a circulation style heater. Other factors include heat loss and voltage/element resistance fluctuations. High temperature heaters need to be insulated properly for both energy conservation and personnel protection.
Watt Density Selection
We are concerned with three things when determining the proper watt density.
- Specific Heat
- Thermal Conductivity
Specific Heat is the amount of energy to raise 1 unit of fluid 1 °F. It is measured as Btu/lb °F.
Viscosity is a fluid’s resistance to flow. Fluids may flow by either forced flow (pump/blower) or by natural flow (tank heating applications). Water has a low viscosity while crude oil has a high viscosity. The ability of the fluid to flow around the heater is critical to determine the watt density. Fluids with a low viscosity can be heated with a higher watt density than a more viscous fluid.
Thermal conductivity is the ability of a fluid to transfer heat. It is measured as btu/hr – Ft – °F. Heat will transfer at a higher rate when a fluid has a high thermal conductivity.
The combination of these three properties determine the proper watt density. Gaumer uses a proprietary calculation program to predict the heater element temperature.
Material selection is important to prevent corrosion and premature failure. Fluid properties such as temperature, pH, and component mixture must be reviewed. There is no one material good for all applications. Hydrogen sulfide content, deionized or demineralized water, sodium or acid content will play an important role in determining material selection. Galvanic corrosion may also play an important role. Alloy 800 (UNS N08800) is the preferred choice for many applications, while Alloy 825 (UNS 08825) is preferred for fluids with hydrogen sulfide content. For deionized water, Alloy 800 (UNS N08800) elements with passivation is a good choice.
Heating elements typically come in one of three forms:
- As Welded Tube
- Welded and Drawn Tube
- Seamless Tube.
If price is a concern, then as welded tube is the least expensive. Welded and drawn tube is welded tubing that has been drawn over a mandrel to remove the weld joint and has seamless like properties. True seamless elements are the most expensive but have the best corrosion resistance.
Gaumer Process reviews all of these important factors to select the proper heater for each application.