HeatSponge Economizer Tutorial

We are pleased to provide this brief tutorial on what an economizer is, does, and some of the information that is important to selecting the proper unit.

 

An economizer is a heat exchanger that is used to transfer energy from an air stream into a liquid stream. Most typical economizers use the flue gas exiting the boiler to preheat boiler feedwater being delivered to the boiler. Most natural gas or fuel oil fired boilers will utilize economizers with extended surface finned tubes. The reason for this is that water has much better ability to absorb heat than a flue gas can transfer it. The use of finned tubes increases the total amount of heating surface on the air side which tends to equalize the different heat transfer rates. this is called being "air-side limited", almost all economizers are. Fuels that could foul a finned tube economizer such as coal of heavy fuel oil fired boilers may not be able to take advantage of a finned tube unit requiring a physically larger and more expensive economizer. Since all HeatSponges are design for natural gas or #2 fuel oil only, two fuels that have little fouling concern, no further discussion is warranted here.

 

The primary design consideration for any economizer is the flue gas side pressure drop. Pressure drop is critical to the design of an economizer. Heat transfer calculations depend on many variables, one of the more important is velocity. The higher the velocity typically the higher the overall heat transfer rate (known as the U value) meaning a given amount of heating surface will perform better. The higher the pressure drop across the unit, the better the unit will perform. Pressure drop however comes at the expense of forced draft fan pressure. Too excessive of a gas side pressure drop can restrict the forced draft fan too much resulting in reduced boiler capacity or combustion problems. Most typical firetube boilers can withstand a pressure drop in the 0.20 to 0.50 inch water column range and watertubes can typically withstand up to a full inch water column. The HeatSponge product lines were designed to maximize heat transfer within those ranges. Although economizers are air-side limited the water side pressure drop is also very important to performance. We designed the HeatSponge with 0.75 inch diameter tubes to increase water side pressure drop greatly improving the heat transfer capabilities of the HeatSponge.

 

Economizers are very important because the flue gas exiting boilers contains a great deal of energy that cannot be recovered inside of the boiler itself. The reason is the Laws of Thermodynamics. The gases inside of the boiler cannot be lowered to a temperature less than the temperature of the saturated water inside of the boiler. What this means is a boiler that is generating 150 psig steam is filled with water that is at a corresponding temperature of 366 deg F. The flue gases exiting the furnace of the boiler are typically in the range of 2,000 deg F. These gases travel through the boiler transferring energy into the boiler water. As the gas temperature lowers so does the Log Mean temperature Differential (LMTD) through the unit. LMTD is another critical component of heat transfer calculation. Basically what happens is a boiler of a certain amount of heating surface area (known as A) will find it more difficult to transfer energy as the temperature of the flue gases approaches the temperature of the saturated water. On typical firetube boilers the boiler can lower the gas temperature to approximately 75 deg F above the saturated steam temperature and watertubes are usually upwards of 150 deg F over saturation. This means the 150 psig boiler discussed earlier operating at 366 deg F will have flue gas exiting the boiler in the range of 441 deg F if a firetube and 516 deg F if a watertube. There is still a great deal of energy to be recovered in this gas, the key is to find a heat sink that can accommodate it.

 

The heat sink used on an economizer is water, for most typical applications it is boiler feedwater from a deaerator or feedwater tank being sent to the boiler. Most pressurized deaerators operate at 5 psig, or 228 deg F while atmospheric feedwater systems can range from 140 deg F to 200 deg F. This feed water enters an economizer to be heating using flue gas exiting the boiler. A rule of thumb is water temperature will be raised 1 degree for every 4 degrees that the flue gas loses temperature. The water from the deaerator is raised using the heat remaining in the discharged flue gas by use of the economizer. The hotter the water entering the boiler, the lower the amount of energy the burner must input into the boiler to heat that incoming water up to saturation. This is where the efficiency increase is realized.

 

Using an economizer in a dearated boiler feed water application is the easiest and most straight forward. Because the water is deaerated the economizer can use inexpensive and effective carbon steel boiler tubing. Not all installations have a deaerator however. The HeatSponge was engineered to be able to be utilized in a variety of installations. For boilers that use an atmospheric feedwater system a HeatSponge with stainless steel tubes and fins will be supplied. In the process of heating water dissolved gases, primarily oxygen and carbon dioxide, are liberated. Carbon steel tubes are subject to attack from liberated dissolved oxygen, known as oxygen pitting. Utilizing a carbon steel economizer tube without deaerated water will result in a very short lifespan for the tube. Another advantage to the use of stainless steel tubes is the ability to utilize cold water sinks for the water. Remembering back to the discussion of LMTD across the boiler; the same Laws of Thermodynamics apply to the economizer. If your facility has a high amount of cold water such as high make-up rates or use of cold water for use in a process not directly related to the boiler feedwater circuit then a stainless steel tube economizer can provide a significant increase in heat recovery for an economizer of the same physical size as a unit used for deaearted boiler feedwater. Let's consider both a conventional deaerated application and a 100% make-up application: the flue gas exiting the boiler for either application will be the same, let's use 441 deg F flue gas discharge temperature for a typical firetube application. The deaerated application with 228 deg F water has a temperature difference of 213 deg F, but the 100% make-up application, assuming city supply temperature water of 50 deg F, has a temperature differential of 391 deg F, an improvement of over 50%. The capital cost increase of a stainless steel unit for cold water compared to a carbon steel unit for deaerated water is usually insignificant when compared to the significant increase in heat recovery.

 

Boiler feedwater does not have to be used. Processes with high use of heated water such as wash down water, glycol heating loops, or many other applications can utilize a HeatSponge to transfer energy from the boiler flue gas into the liquid stream. All of the same rules apply.

 

When installing an economizer we have already discussed the importance of understanding flue gas backpressure. It is also important to understand the importance of a constant water flow rate on the tube side of the unit. If water flow stops yet flue gas is still going through the unit the water is subject to steam inside of the economizer. Steaming can damage economizer tubes or create hammering that can damage other equipment in the water circuit. Provision should always be made to either insure constant water flow or design the system for periodic stagnant flow conditions. Your local HeatSponge distributor can assist with these issues.

 

We have designed to HeatSponge to be the most innovative economizer available today. We are very proud of our product. Should you have any questions never hesitate to contact a local distributor or e-mail us directly at sales@heatsponge.com. We appreciate your consideration.