|Optimal Boiler Size and its Relationship to Seasonal Efficiency - Optimal Sizing|
|Written by Henry Manczyk|
|Wednesday, 09 June 2010 08:30|
Page 2 of 3
Boiler systems must be optimally sized to meet the maximum facility demand during the normal heating season. Essentially, the system must provide the heat output required to meet the facility's total demand at the lowest expected temperature of the heating season. Systems that are sized beyond the optimal output capacity, (oversized) boilers, will have lower seasonal efficiency. Properly sized boilers will also reduce maintenance costs by starting and stopping less frequently. Oversized boilers waste fuel and, because of short cycling, ultimately shorten the life of the system. (The "Oversized Boiler Equations" section of this article shows calculations that permit one to determine the extent to which a boiler is oversized.)
Optimally sized equipment operates more efficiently by cycling properly, thus saving fuel. A U.S. Department of Energy Office of Industry Technoloyies "Energy Tips" report discusses this (see "Minimize Boiler Short Cycling Losses". (http://www.oit.doe.gov/bestpractices ).
A boiler cycle consists of a firing interval, a post-purge, an idle period, a pre-purge, and a return to firing. Boiler efficiency is the useful heat provided by the boiler divided by the energy input (useful heat plus losses) over the cycle of duration. Boiler "short cycling" occurs when an oversized boiler prematurely satisfies space heating demands and then shuts down until heat is again required. Efficiency decreases when short cycling occurs because heat demand is smaller than the boiler output.
This decrease in efficiency occurs in part because fixed energy losses are magnified under lightly loaded conditions. For example, if the radiation loss from the boiler enclosure is 1% of the total heat input at full load, at half load the losses increase to 2%; and at one quarter load, the loss is 4%.
Standby Losses Report published by Tri-State Generation and Transmission Association, Inc (http://tristate.apogee.net/et/ehubsbl.htm) states that about 1.5% to 2.0% of rated boiler fuel input is lost to the boiler room. While this "standby loss" is small in comparison to useful output when boilers operate at or near their rated capacity, it can be significant where boilers operate frequently at low loads. For example, imagine a boiler rated at 10 million Btu/hr fuel input, but operating at a 2 million Btu level. The standby loss of 2% of 10 million Btu is 200,000 Btu/hr, or 10% of the 2 million Btu operating output level. This is the reason why plants with large summer to winter variations in steam use install small boilers to operating during the summer rather than operate large boilers year round.
One can also avoid short cycling by adding small boilers to a boiler facility to provide better flexibility and high efficiency at all loads. (This strategy is discussed in "Modular Boiler Systems, below.) Consider when one boiler with a seasonal efficiency of 73% (E1) is replaced with three modular boilers resulting in a seasonal efficiency of 79% (E2).
The Annual Fuel Savings
AFS=(E2 -E1)/E2 (100%)
E1=Seasonal Efficiency with One Boiler
E2=Seasonal Efficiency with Three Boilers
AFS=Annual Fuel Savings
If the original boiler used 100,000 MMBtu of fuel annually, the savings from switching to smaller boilers given a fuel cost of $5.00/MMBtu is calculated as follows:
Annual Cost =
(Annual fuel consumption)(Annual Fuel Savings)(Cost/MMBtu)
(100,000 MMBtu)(0.076)($5.00) = $38,000
Savings at this level can yield a payback period for the new or modified boiler system of less than one year which could be financed directly from operating budget savings resulting from the new system.
MODULAR BOILER SYSTEM
Seasonal efficiency can be increased by replacing a single boiler with a network of smaller modular boilers, as shown below. Since modular boilers can be fired independently each module would be fired on demand at 100 percent capacity with load fluctuations being met by firing more or less boilers. When the first boiler can no longer keep up with the heat demand, a second boiler picks up the extra heat load. Also, modular boilers have low thermal inertia which provides rapid response and low heat-up and cool-down losses.
Outdoor temperature fluctuations during the heating season reduce the seasonal efficiency of even optimally sized boilers and boiler systems. There are relatively few periods during the heating season when it will be running at its rated output or point of maximum efficiency.
The following demonstrates that the percentage of run-time for a optimally sized boiler is less if there is a temperature set-back during the unoccupied hours than if the building were to be kept at a constant temperature throughout the heating season.
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