HEAT EXCHANGERS

Epcon’s unique patented heat recovery systems and our commitment to innovation keep us on the cutting edge of technology. Epcon is one of the few companies to have its own state-of-the-art heat exchanger manufacturing facility on site. Our 250,000 square foot manufacturing operations shop includes an array of fabrication equipment including lathes, drilling machines, presses and state-of-the-art Computer Controlled Milling Machines.

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 to create a completely integrated Combination Oven/Oxidizer System.

The price of the fuel necessary to run fired equipment (Ovens, Oxidizers, etc) 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 wasted. In the various systems manufactured at Epcon, heat exchangers are used for the following purposes:

  • 1. Pre-Heating the process gas as it enters the thermal oxidizer (also called Primary heat exchanger). 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.
  • 2. Using the gas for Secondary heating systems, such as process heating of the manufacturing facilities and heating boilers.

Regenerative & Recuperative Heat Exchangers

The two major types of primary heat recovery are Recuperative and Regenerative heat exchanger systems.

  • Recuperative systems typically employ 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 efficiency.
  • 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 to 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 the following equation expressed.

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, the equation below applies.

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 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 built up a database of heat exchanger designs based on 100’s of applications which accelerates concept development and insures successful system sizing. Epcon utilizes advanced computer modeling software, supported by our database of installations, for analyzing and determining the number of tubes required for designing the heat exchanger. Epcon always allows for a sufficient factor of safety and provides more than required area for heat transfer. Thus, we always prioritize system reliability and robustness to insure customer satisfaction for the life of the system.

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:

E = (Thi-The) / (Thi-Tci)
Where E = Effectiveness
Thi = Temperature of incineration
The = Temperature exiting heat exchanger
Tci = Temperature cold entering heat exchange

Heat required to incinerate:

E = (Thi-The) / (Thi-Tci)
Where E = Effectiveness
Thi = Temperature of incineration
The = Temperature exiting heat exchanger
Tci = Temperature cold entering heat exchange

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!

From theoretical to the practical, understanding the thermal stresses imparted upon a heat exchanger as well as the factors which influence heat exchange efficiency and pressure drop within the unit and overall system are critical to designing a capable system.

Heat Exchanger design and fabrication is a job that requires specialized tools/software, knowledge-base of practical applications, highly qualified engineers, skilled craftspeople, all with the experience and passion to produce world class products. Epcon has proven its capabilities and excels in all these categories.

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 effectiveness of approximately 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 effectiveness of 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 transference indicated by the arrow. The transference of heat allows the burnerless 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.

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 transference indicated by the arrow. The transference of heat allows the burnerless 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

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.

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.

As compared to Recuperative systems, Regenerative systems are not steady state, 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 ceramic, 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 then 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 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 which 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 includes; Cutting the tubes and side sheets, Punching holes in the side sheets, Welding the tubes, expansion joint and side sheets. The processes and procedures developed at Epcon for the critical welds are one of the keys to producing the high quality, high performance heat exchangers employed on all of our Air Pollution Control, Ovens and Finishing Systems.

Manufacturing in-house allows Epcon to control quality and performance from concept through execution. Delivering standard and custom engineered Air Pollution Control, Ovens and Furnaces, Finishing Systems and Specialty Systems that meet and exceed our cutomer’s requirements at prices they can afford.

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