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Introduction
Flue gases from large boilers are typically 450 - 650°F. Â Stack Economizers recover some of this heat for pre-heating water. Â The water is most often used for boiler make-up water or some other need that coincides with boiler operation. There is a class of economizers that are designed to condense the flue gases and/or have the water in direct contact with flue gases. I have called them 'Flue Gas Condensers'. Stack economizers and Condensers should be considered as an efficiency measure when large amounts of make-up water are used (i.e.: not all condensate is returned to the boiler or large amounts of live steam is used in the process so there is no condensate to return) or there is a simultaneous need for large volumes of hot water.
The application difference between an economizer and condenser is that economizers are primarily used to heat a smaller volume of water to a high temperature for boiler feed water, and condenser units heat a larger volume of water to a lower temperature. Condensers can be more efficient because they can have a lower outlet exhaust temperature and take advantage of the energy in condensed flue gasses (the Latent Heat of Vaporization).
The savings potential is based on the existing stack temperature, the volume of make-up or hot water needed, and the hours of operation. Economizers are available in a wide range of sizes, from small coil-like units to very large waste heat recovery boilers. Condensers are available as small as 50 hp and a single condenser can be used on multiple boilers.
Some condensers have water in direct contact with the flue gases and others use heat exchangers. In some applications the water that has been in direct contact with the flue gases can be directly used; in other applications, the water must be passed through a heat exchanger before it can be used.
The key to the successful application of heat recovery is the ability to put the recovered heat to use. Uses include industrial process water heating, clean-up/wash-down water heating, laundry wash water, domestic water heating, space heating, snow melt and district heating systems. Potential Applications include: Greenhouses, Hospitals and Health Centers, Food processing, Schools and Universities, Laundries, Breweries, Hotels, Wineries, Government Buildings, and Swimming pools.
Operation
In a direct contact unit, water is sprayed in contact with the flue gases, causing condensation and extracting most all of the heat. In units that have a heat exchanger, condensation is likely when there is enough water flow -or- at a cold enough inlet temperature to remove enough heat to cause condensation on the flue gas side of the heat exchanger. See the Manufacturer's descriptions below for more details.
Economics
The savings potential is a function of how much heat can be recovered, which is a function of how much cold water needs to be heated. A generally accepted "rule of thumb" is that about 10% of boiler input capacity can be recovered with a properly sized condenser. This is a higher percentage than what can be recovered with an economizer because the water temperature is lower. However, there is also a lot more volume of water involved, assuming there is a need for enough cold water to condense all of the flue gas that is available. Therefore, for 'ball parking' purposes, start by comparing boiler input capacity with the need to heat water.
For example: consider a 500 hp boiler with a gas input of 20 million BTUs per Hour.
20,000,000 BTUs x 10% = 2,000,000 BTUs (100% Load Factor)
2,000,000 BTUs / (900 BTUs per Gallon of 160F water) = 2,200 Gallons per Hour
(2,000,000 BTUs / 80% efficiency) = ~2.4 MCF x $7.00 per MCF Natural Gas = $16.80 per Hour Value
Savings is reduced by 50% for a 50% Load Factor, etc.
If there is a need for that much hot water, the savings potential of $16.80 per hour would be multiplied by the number of boiler run hours, or the number of hours that the hot water can be used. In each application, be sure to consider the boiler Load Factor, the efficiency that the hot water is otherwise produced at, the cost of natural gas, and the installation cost of the equipment.
Condensers are generally NOT used to heat boiler make-up water. Use an Economizer for that application.
A condenser that recovers 10% of boiler input should easily have a 1 year payback in a year-round application. |
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