When to use electric steam boilers
Packaged electric boilers, in the right applications, are an effective way to offset rising fuel prices and help maintain profitability. They can also serve as a backup should an unforseen shutdown of the main plant boiler occur. Paul Rannick, of Chromalox, describes some typical situations where an electric boiler can be economically used and explains how to determine boiler size.
By Paul Rannick, Chromalox, Pittsburgh, PA
- Electric steam boilers can supply process needs when large central boilers are shutdown
- When central boiler turndowns are too high, electric boilers are economical to operate
- Electric boilers can deliver steam in one-half hour
Fossil fuel producers are again turning up the heat on industrial and commercial operators of large fossil-fueled steam boilers. Since there is little boiler operators can do to control energy costs, the obvious answer is to find ways to control consumption — without curtailing operations or compromising the productivity of steam-heated processes.
Fortunately, as the weather warms, reduced demand for space heating allows large central boilers to be turned down. But this still leaves operators literally “blowing off steam” when the same boiler provides steam for processes. Typically, even at the maximum turndown ratio, a boiler produces more steam than processes need, and this excess is vented to the atmosphere. The wasted energy represents a significant fuel cost on the input side.
A packaged electric steam boiler may be a more economical solution (Fig. 1). It can keep steam-consuming operations running at full speed, while the big fossil-fueled boiler takes the summer off or has to be maintained or repaired.
Fig. 1. Packaged electric steam boilers only require electricity and feed water.
What about electricity costs?
Although electric utilities are some of the largest operators of fossil-fueled steam boilers, they suffer less from rising oil and gas prices because of coal usage. Those that use oil and gas have more clout with suppliers because of their size and usage volume. Most industrial and commercial boiler operators probably pay a premium for electric energy compared to oil and gas for equivalent Btu quantities. A total-cost comparison points out the advantages of using an electric steam boiler to generate process steam when overall plant steam requirements are reduced. (See Table 1.)
Table 1. Boiler operating costs1.
|Boiler type||Fuel cost, $/hour|
|10 bhp electric boiler (~100 kW)|
|150 BHP2fossil||Gas: |
1. See example below for assumptions and calculations used for these figures.
2. Fossil-fueled boilers are turned down to a minimum operating ratio of 0.2 or 30 bhp
Example: Electric Boiler Payback
The length of time it takes to recover the investment in a new electric boiler depends on the fuel that fires the existing central boiler, fuel cost, the minimum operating level, and how the boilers are used. The calculations for the figures in Table 1 assume that only 10 boiler horsepower (bhp) is needed for process heating and the central boiler has a nominal 150 bhp rating. This is the energy output of the boiler at rated capacity, and one bhp is equivalent to 34.5 pounds of steam per hour or 33,446 Btu/hour (mean) at 212 ° F and higher.
The full 150 bhp output equates to 5,017,000 Btu/hr. With an assumed efficiency of 80%, the input at full power is 6,271,000 Btu/hr. Turned down to 20% output, the unit will put out 30 bhp or 1,003,000 Btu/hr at a somewhat lower efficiency, which is assumed to be 75%. At this turndown ratio and efficiency, the input will be 1,338,000 Btu/hr. For a gas fired boiler, the energy input is 1,000,000 Btu per mcf of gas. The boiler in Table 1 will use 1.338 mcf/hr to generate 30 bhp, the minimum operating level. Since only 10 bhp is needed, 20 bhp of steam goes up the stack as wasted energy.
For Table 1, a delivered gas cost of $7.00/mcf is assumed. The cost to operate the central boiler at 30 bhp is 1.338 mcf/hr x $7.00/mcf = $9.37/hr. Piping losses are ignored since the boiler has plenty of excess capacity to overcome these losses at the minimum turndown ratio.
For oil fired boilers, the situation is similar — fuel prices vary by locale and supplier, and are prone to future increases. Table 1 assumes a price of $1.40/gal. and an energy content of 138,000 Btu/gal. Dividing that into the input of 1,338,000 Btu/hr (the same as the gas-fired boiler), the boiler is consuming 9.7 gal./hr. At $1.40/gal., the operating cost is $13.58/hr. Piping losses are ignored.
The electric boiler operates at full capacity, which for simplicity is assumed to supply process requirements and 3% losses in the short piping run between the boiler and the process. It is assumed that the electric energy cost is $0.05/kWh. Since one bhp equates to about 9.81 kW, and electric energy conversion in the boiler is about 98% efficient, the 10 bhp output requires 100 kW at the input. The operating cost is $5.00/hr.
The energy per unit of fuel is based on charts published by the Institute of Gas Technology, Chicago, IL.
As suggested in Table 1, the fuel cost savings from using a 10 bhp electric boiler, compared to keeping the 150 bhp main boiler in operation, amounts to at least $4.37 per hour in the case of the gas fired boiler. This means that the typical $20,000 installed cost of this electric boiler can be recovered in less than 4600 hours of operation. This equates to 27 weeks of 24/7 service, or about two summers of operation. For an oil-fired boiler, the payback period would be much shorter. Also, payback periods tend to be shorter when maintenance costs are considered.
Packaged electric steam boilers are available to produce low and high-pressure steam at rates up to at least 165 BHP. Some manufacturers will custom build larger electric boilers for specific installation requirements. Electric boilers are suitable for a wide range of processes, including those used in the manufacture of chemicals, paints, paper, textiles, petroleum products, pharmaceuticals, plastics and rubber; as well as food and beverage processing and many other facilities where heat, humidification, and sterilization are required. Specific applications include supplying steam for storage tanks and jacketed vessels; reaction and distillation vessels, retorts, and autoclaves; heat rolls for paper coating, calendaring, laminating, corrugating and embossing; platens, dies and molds used in the processing of plastics and elastomers (Fig. 2).
Fig. 2. Typical closed loop boiler system
Although the main plant boiler usually supplies steam and hot water for comfort heating and humidification, there may be cases where it is cost effective to install an electric boiler for localized heating in a plant expansion. Similarly, electric boilers are ideal choices for new process facilities where large, fossil-fueled boilers are impractical or not required. With a properly sized electric boiler, there are no onsite combustion stack losses, stacks, or stack emissions. Compared to oil fired boilers, there are no fuel storage tanks to install, keep filled or worry about leaking. Also, there is no maintenance required for burners and heat exchanger surfaces.
Packaged electric boilers are delivered with all controls and ordered accessories in place and ready to operate. Custodial personnel with a minimum of training can operate most electric boilers. They start quickly, delivering steam within one-half hour. They are safe, compact, quiet-running units that can be installed close to the site of steam consumption, eliminating the need for long pipe runs and pipe heat losses that can run as high as 10%. Electric boilers are well insulated, so they don’t add significant heat to the surrounding area (Fig. 3). They operate on existing distribution voltages and the only additional requirement is a feed water supply.
Fig. 3. Typical vertical electric boiler construction.
Maintenance on electric boilers is minimal beyond routine inspections of water levels and monthly inspections of wiring. As with all boilers, they do require scale control measures and periodic blowdowns to maintain efficiency. Heating element replacement, when required, is easily accomplished through the boiler door.
Sizing and selection
Since packaged electric steam boilers are pre-engineered and assembled, and installation requirements are similar for most units, sizing and selection from a list of standard products generally is a straightforward process. Most heating applications involve only four steps to determine the correct BTU capacity of the boiler.
1. Determine the number of BTU/hr required to bring the application up to the operating temperature in the desired time, including piping.
2. Determine the number of BTU/hr required to maintain the operating temperature in the process and the piping.
3. Taking the largest result from Step 1 or Step 2, convert the BTU/hr into pounds of steam per hour, using standard saturated steam data (Table 2). Divide the BTU/hr required by the latent heat in Btu/lb at the process working or gauge pressure.
4. Convert the pounds of steam per hour into the boiler kW requirements. Since saturated steam tables are based on feed water at 32 deg F, a simplified kilowatts-per-pound-of-steam table is used to determine a correction factor for feed water at some higher temperature (Table 3). Multiply the required pounds of steam per hour from Step 3. by this correction factor.
Table 2. Saturated Steam — Thermodynamic Properties
|Gauge Press. (psig)||Temp. (°F)||BTU/lb.||Sat. Vapor (ft3/lb)||Gauge Press. (psig)||Temp. (F)||BTU/lb.||Sat. Vapor, (ft3/lb)|
|Liquid heat||Latent heat||Steam total||Liquid heat||Latent heat||Steam total|
Table 3. Boiler feed water temperature vs. kilowatts required per pound of steam
|Feed water |
|Steam gauge pressure, psig|
The steam pressure should be the lowest pressure that will give the required output at a temperature higher than the final desired product or process temperature. To determine if the increase in the product or process temperature is sufficient, determine if there is sufficient heat transfer surface area in the heat exchanger to satisfy the formula:
Q = U x A x D T.
Q = Heat transfer, Btu/hr
U = Overall heat transfer coefficient for the specific product being heated, type of heat source [saturated steam; water], and manner of transfer [free convection; forced convection; clamp-on]
A = Heat exchanger heat transfer surface area, ft2
D T = Change in temperature, deg F
D T can be increased, if needed, as long as the maximum pressure rating of the boiler and the maximum allowable skin temperature of the heat transfer surface in contact with the product are not exceeded.
Boiler sizing example
A chemical company uses a shell-and-tube heat exchanger to heat 10 gpm of water from 140 F to 185 F for a continuous process. The heat exchanger is supplied with 50-psig steam from a large fossil-fueled central boiler. The company wishes to shut down the large boiler in the summer months. What size electric boiler is needed to replace the central steam supply during the shutdown? (Condensate is mixed with feed water, returning to the boiler at 50 deg F.)
The heat energy required for this example can be calculated from the following formula:
Q = [(C) ( C p ) (SG) (V) (D T ) (K) x (SF)] / [H], Where:
Q = Heat required, kW/hr
C = Conversion factor for gpm to lb/hr, 8.345
(8.345 lb/gal x 60 min/hr) = 500 lb-min/gal-hr
C p = Specific heat of water, 1 Btu/lb/F
SG = Specific gravity of liquid, 1 for water
V = Volumetric liquid flow, 10 gpm
D T = Temperature change of liquid, deg F
(185 deg F – 140 deg F) = 45 deg F
K = Boiler kW per pound of steam
(at 50 psig for 50 F feed water, from Table 3) = 0.3401 kW/lb.
H = Latent heat of steam at 50 psig operating pressure (from Table 2) = 912 Btu/lb
SF = Safety factor of 20% = 1.2 (for unknown heat loss, loss of condensate due to flashing, etc.)
This calculation can be broken down into the following steps:
1. Convert gallons per minute of flow to pounds per hour:
10 gal/min x 500 lb-min/gal-hr = 5000 lb/hr
2. Based on the pounds per hour, calculate the Btu’s required by using the specific heat of water and the temperature rise (D T):
5000 lb/hr x 1 Btu/lb/F x 45 F = 225,000 Btu/hr
3. Using the latent heat content at 50 psig from Table 2, calculate the pounds of steam per hour needed to deliver the required Btu’s:
225,000 Btu/hr / 912.2 Btu/lb = 246.66 lb/hr
4. Convert the pounds of steam per hour to boiler kW using the factor 0.3401 from Table 3:
246.66 lb./hr x 0.3401 kW/lb = 83.89 kW
To compensate for unknown heat loss and possible loss of heated condensate due to flashing, a 20% safety factor is commonly applied in boiler sizing. The calculated boiler size is:
1.20 x 83.88 kW = 100.67 kW
Based on this calculation, select an electric steam boiler. Depending on the manufacturer, such a boiler could be equipped with a variety of additional performance options including automatic blow down, blow down separators where steam and hot water cannot be discharged directly into a drain, electronic water level controls and electronic controls for modulated pressure installations.
-Edited by Joseph L. Foszcz, Senior Editor,
The author is available to answer questions on electric steam boilers. He can be reached at 412-967-3886.
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