Heat Exchanger
From Specialty Systems
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Our Shell and Tube Heat Exchangers are designed to...
Our Shell and Tube Heat Exchangers are designed to transfer heat efficiently and effectively while reducing fuel consumption. A Heat Exchanger can be combined with any of our other systems or as a retrofit. In addition, Epcon has several designs that combine burnerless ovens with Thermal Oxidizers.
The price of the fuel necessary to run a Thermal Oxidizer can be exorbitant. In order to lower operating costs, a heat exchanger can be used. Heat Exchangers are used for transferring heat from one fluid (such as air) at higher temperature to another fluid at a lower temperature, thus making use of heat which would otherwise be expelled. In the various systems manufactured at Epcon, heat exchangers are used for the following purposes:
- Pre-Heating the process gas as it enters the thermal oxidizer (also called Primary heat exchange). The heat is transferred from the hot gas leaving the combustion process to the cold air entering it. The two basic methods are Regenerative and Recuperative.
- Using the gas for Secondary heating systems, such as process heating of the manufacturing facilities and heating boilers.
The two types of heat exchangers most used are the...
The two types of heat exchangers most used are the Pre-heat systems, the Recuperative and Regenerative heat exchange type systems. Recuperative systems are also known as shell and tube type heat exchangers. In these heat exchangers, a stream of cold process gas passes through a series of tubes and is heated by another stream of gas which passes over the tubes on the shell side. These types of systems are generally used for thermal oxidizers with low to medium process flow rate, and generally can provide up to 80% thermal energy recovery effeciency.
Regenerative type heat exchangers use a media to absorb heat given off by one hotter fluid and transfer it to another colder fluid. Typical medias used are packed towers of ceramic material with required gaps for the gases to pass through them. The operation of regenerative heat exchangers is cyclic. In the first cycle hot gases/fluids passing through the media heat up the media. In the following cycle, the cold gases pass through the media and they are heated by the already hot media. Regenerative systems can operate with process flow rates in the low or high range, and can yield a thermal energy recovery efficiency between 80% and 95%+.
Heat Exchanger Effectiveness
At Epcon, these heat exchangers are used to transfer heat from hot gases at 1500°F to cold gases which are in the range of -10°F to 500°F. Effectiveness is a measure of the performance of a heat exchanger. Effectiveness is defined as the ratio of rate of heat transfer to the maximum possible heat transfer. The mathematical expression for the effectiveness of heat exchanger is given by,
E = (Rate of heat transfer in heat exchanger)
(Maximum possible heat transfer rate)
E = mc Cpc (Tce - Tci) / (m Cp)s (Thi-Tci).
When the heat from the hot gases is used to preheat the gases going to the combustion chamber, the mass flow rates and specific heat of incoming and outgoing gases remains the same. We use the following expression for finding the effectiveness of heat exchanger,
E = (Thi-The) / (Thi-Tci).
Where E = Effectiveness
Thi = Temperature of incineration
The = Temperature exiting heat exchanger
Tci = Temperature cold entering heat exchanger
As the effectiveness of the heat exchanger increas...
As the effectiveness of the heat exchanger increases, the heat recovery from a given stream of hot gases also increases. Generally regenerative heat recovery offers greater heat recovery options, where recuperative recovery has plateaued at 80%, regenerative heat recovery can have an effectiveness of almost 95%, or even more in some situations.
In a Recuperative system, the surface area of the heat exchanger tubes is the surface area available for heat transfer to take place. For higher effectiveness, this area needs to be increased. In other words, the number of tubes in the heat exchanger have to be increased. Longer heat exchanger obviously has more initial costs, but these are quickly repaid through fuel efficiency.
Heat Exchangers are designed to achieve a predetermined amount of effectiveness. The quantity of air flow on the shell and tube sides of the heat exchanger is known. Also known is the temperature range in which the heat exchanger is expected to operate. The design parameters which are important are heat exchanger tube lengths and number of tubes.
Through years of experience and experimentation, Epcon has selected an outer diameter of heat exchanger (1.5 inches) which is optimum from many considerations. Epcon uses a software system called BJAC for analyzing and determining the number of tubes required for designing the heat exchanger. Epcon always allows for a sufficient factor of safety and provide more than required area for heat transfer. Thus, we give more importance to the reliability and robustness of our systems.
Example of the advantage of using a Heat Exchanger
Compare the mass flow rate of natural gas required to heat 10,000 scfm of an exhaust process gas from 70 F to 1400 F, for a system without a heat exchanger and system with a 70% effective heat exchanger. Assume that the available heat of the natural gas is 950 Btu/scf and that there is no heat loss. The average heat capacity Cp over this range may be assumed to be 7.5 Btu/lb mol.
Molar flow rate of process gas:
N = (10,000 scfm) (1 lb mol / 379 scf)
= 26.4 lb mol / min
Heat required to incinerate:
Q = NCpT
= (26.4) (7.5) (1400-70) = 2.33 x 105 Btu/min
Amount of Natural Gas required to incinerate:
NG = Q/HA = 2.33 x 105 / 950 = 245 scfm
With a 70% effective heat exchanger the temperature of the air entering the burner is raised:
E = 70% = (Tin - 70oF) / (1330 oF)
Tin = 1001 oF
Which changes the total heat required to:
Q = (26.4) (7.5) (1400-1001) = .79 x 105 Btu/min
Making the total Natural Gas required:
NG = Q/HA = (.79 x 105 / 950) = 83.15 scfm
Which is a savings of almost 300% in natural gas!
Heat Exchanger design and fabrication is a job tha...
Heat Exchanger design and fabrication is a job that requires skill, high engineering capabilities, and experience. Epcon has proved its capabilities and excelled all the three areas.
Heat ExchangerIn order to increase area available for heat transfer, we attach additional fins to the heat exchanger tubes.
Other important areas which are important in design of heat exchangers is the expansion of the heat exchanger due to heat. Thermal expansion of heat exchanger takes place in the direction of the tubes as well as in the longitudinal direction. Expansion joints are located in the roof, bottom, and sides of the heat exchangers. When the heat exchanger is heated up, its expansion in longitudinal direction is absorbed by the compression in the expansion joint. Likewise, expansions in perpendicular to directions are absorbed by expansion joints in those directions. As the heat exchanger cools down, all expansion joints get their original positions back. The arrangement of tube rows also has an effect on performance of heat exchangers. The in line arrangement of the heat exchanger tubes, gives less turbulence in the shell side gas flow and hence less efficiency. Conversely, the staggered tube arrangement of heat exchangers, gives higher pressure drop in the shell side stream of the heat exchanger.
In the design of Thermal Oxidizers, it is very important to keep the streams of hot and cold gases separate. This is done by leak proof welding and using very good materials to avoid cracks in the walls of the heat exchanger. The waste heat in the flue gases is also used to heat another stream of gases. Thus, the waste heat can be used to carry out some other part of the process.
Primary Heat Exchangers
Primary Heat exchangers are heat exchangers which use the recently combusted hot gas to Pre-heat the process gas entering the Oxidizer. These kinds of heat recovery systems save energy costs by reducing the amount of fuel required to maintain combustion.
There are two main kinds of Primary heat exchangers:
Recuperative systems, which are generally air-to-air exchange systems which heat the process gas through a shell and tube system, which directly heats the air through convection, these systems can have heat recovery effectivenesses of around 80%.
Regenerative systems, which uses a ceramic media to conduct the heat from the hot gas to the process gas, these systems can have heat recovery effectivenesses up to 95%+.
Recuperative Systems
Recuperative heat exchange systems are the most common systems available. The technology for this kind of system has been around for more than 15 years. These systems can yield up to 80% thermal energy recovery (effectiveness). These systems rely on a shell and tube type heat exchange, where the hotter gas passes over the shell, heating up the cool gas passing through the tubes, using convection.
This is an example of a recuperative heat exchanger used in conjunction with a Thermal Oxidizer. The two segments for the heat exchanger represent the shell, and then the tube sections, with the heat tranference indicated by the arrow. The tranference of heat allows the burner less requirement for thermal input, and so uses less fuel.
Shell and tube type of heat exchangers are further divided on the basis of their operation. In parallel type heat exchangers, cold gas which is required to be heated passes through tubes, which are arranged in several passes. Hot gases flow on the shell side of the heat exchanger in a straight line. This heat exchanger is counter type of heat exchanger, where the hot gases incoming the heat exchanger come in contact with cold gases leaving the heat exchanger. Conversely, hot gases entering the system, which have the highest temperature in the system, heat up the cold gases which are already heated up. Counter type of heat exchangers give the highest efficiency for heat exchangers. Majority of heat exchangers used at Epcon fall into this category.
In series type heat exchangers, hot gases coming out of the thermal oxidizer pass through tubes and cold gases pass on the shell side. A number of passes are arranged on the shell side for the cold gases to pass over much larger area of tubes. The hot gases pass through the tubes, which are arranged longitudinally.
Regenerative Systems
Regenerative technology is a newer technology than recuperative, however these systems are more efficient than it's precursor. Regenerative systems are not steady state, like recuperative, rather they rely on a cycling gas which flows between at least two fixed packed beds.
In this case, at least one bed would be an inlet, while the other would be an outlet bed. A common combustion/retention chamber connects the two. These vessels are connected by inlet and outlet control valves to direct air flow through the different beds. The hot air from the combustion chamber would flow through one of the beds, heating it up, while the other discharges, then cold process air would flow through the heated cermaic, reaching near the temperature needed for combustion. Then after being combusted, would flow through the cold ceramic again to heat it back up. This would be a continuous cycle of retention and purging.
Here is the operation sequence of a three chambered Regenerative Thermal Oxidizer. The most important aspect of this cycle is the way which each chamber is purged of it's old air, re-heated, and tyhen heats the next system. Continued preheating of the process gas causes the ceramic media canister to cool down, at the same time the ceramic media in the next chamber is heated. The inherent nature of the Regenerative Systems involves discontinuous or cyclic operation.
The general process for a Regenerative Heat Exchanger is as follows
- The process gas is brought through the pre-heated ceramic bed and is combusted. It then flows out over another ceramic bed, which is subsequently heated.
- After an allotted time the valves are closed and the process gas flows through the ceramic bed which had just been heated, thus pre-heating the gas, combusted and flows over another ceramic bed.
- The three ceramic beds alternate the duty of pre-heating and being heated, as their valves are opened and closed.
The entire process, although more complicated that recuperative heat exchange, is far more efficient, yielding an effectiveness of over 95%. A few photographs depicting Regenerative systems also accompany the schematics.
Secondary Heat Exchange Systems
Secondary heat exchange systems use the hot gas from the combustion process to run other processes within the manufacturing facility. There are many different applications for secondary systems. At Epcon we have produced many of these. The two most commonly used are boilers, which can be used to heat water for washers, or heating systems within the factory. Another use is process heating, where the air can be used for systems like ovens or other curing devices. These systems are custom to particular customers, but the general idea remains the same. The gas leaving the oxidizer will generally heat a pre-heat exchanger, then it will continue on to the secondary exchanger. The secondary exchanger will use this heat in a manner specified to the particular device, and then will release the air to the atmosphere. A schematic of a Secondary Heat Exchanger wich was used to heat water for a washer is detailed on the following page, as well as photographs of several projects which used these kinds of heat exchange.
Manufacturing Considerations
Fabrication of shell and tube type heat exchangers consists of the following operation:
Cutting the tubes and side sheets, Punching holes in the side sheets, Welding the tubes, expansion joint and side sheets.
The procedure followed for welding has a great influence on the quality and life of heat exchanger. The weld gets stronger as it gets thicker, but a thicker weld greatly increases the chance of burning the back of the tube sheets. Dissipation of heat during the process of welding is also of prime importance. The welding procedure employed at Epcon is certified and the welders go through the qualification tests.
Welding Procedure Certification --
The welding procedure followed at Epcon is given below:
- Clean and debur all tube ends. Tag weld some tubes on side sheets at four places.
- Coat the backside of the punch sheets with flux, which reduces the danger of oxidizing backside of the punch sheets with flame cutting.
- For welding a tube end to a punch sheet a continuous weld is made around the tube.
- Tubes are welded in a random order to avoid concentration of heat in a certain area.
Every weld between punch sheet and tube is inspected. The inspection is done by pouring penetrant testing liquid on the weld. After twenty minutes, compressed air is sprayed over the weld to remove the red penetrant. Spot check developer is applied on the tube welds. Using magnifying glasses we check and mark cracks, pinholes, etc.
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