Products

Convective Heaters

ETI designs and fabricates convective heaters for use in gas, adsorptive regeneration, process heating, and heat transfer industries.

 


5.5 MM BTU/HR Convective Regeneration Gas Heaters located in Pennsylvania.

 

Reasons for Convective Heating
Fired heaters usually transfer heat by two methods: radiation and convection. Transfer by radiation is hard to control compared to convection, since the amount of radiation emitted varies with the type of fuel, flame emissivity and vessel configuration. Radiant section geometry and control of flame pattern are very important, yet not always predictable. Convective transfer, on the other hand, can easily be controlled by modulating the flue gas velocity or temperature. The flue gas can be transferred in ducts to the process coil exchanger and the temperatures involved are relatively low. The lower temperatures are desirable to avoid refractory problems, thereby eliminating a common maintenance problem. High temperature refractory stores large quantities of heat, which is of little consequence unless an emergency shutdown occurs. The stored heat may “coke” the tubes if the fluid flow stops before the refractory cools. Also, high temperatures can lead to tube failures.

Some heaters have only a radiant section and are relatively inexpensive but quite inefficient. An economizer section may be added to raise the efficiency by transferring heat by convection as the flue gases pass from the radiant section. A power burner is often required in order to overcome the pressure drop across the convection section.  Therefore, the operating cost and initial investment relative to a radiant heater are increased. This expense must be overcome by either the fuel savings created by the economizer or safer operations.

ETI’s Model HTE (High Thermal Efficiency) is unique in that it has no radiant section. Instead, the heater is all economizer.  In order to understand the design principle, one must keep in mind that the efficiency of any fired heater is only a function of the stack gas temperature and the amount of excess air used (assuming complete combustion). The excess air can be best controlled with a power burner and is usually set at 15% to 20% with gaseous fuels. A good burner doesn’t require the excess air for complete combustion, rather the excess is set to compensate for barometric and ambient temperature changes so that the air injected will never drop below the stoichiometric quantity required for complete combustion.

The ETI HTE heater introduces cool ambient air or re-circulated flue gas into the combustion chamber in order to reduce the combustion gas temperature to the desired level. Typically, the temperature is reduced from a flame temperature of 2500°F to 1100°F, although it may be reduced to a much lower value if sensitive or heat reactive materials are being processed. The high temperature energy is traded for a lower temperature gas with greater volume. Quench air is introduced to the combustion chamber with a control valve on the air or flue gas recirculating (FGR) fan.

It is usually desirable to maintain a constant temperature of the mixed gases as the firing rate varies. This is done by modulating the amount of quench air or FGR. The resulting heat transfer is brought under complete control so that tube wall and film temperatures can also be predicted and controlled.

ETI’s HTE heater has fast response and very little stored heat. If fluid flow stops, the burner is shut off by a low flow detector, or a high process or tube wall temperature controller and the process coil bundle is not damaged. Because a higher volume of convection gas is circulated, the outside transfer rate may be increased on the stack side of the exchanger, resulting in higher efficiency with fewer tubes. The stack gas temperature is usually set above the flue gas dew point at 250°F to 350°F. The cost increase associated with the additional controls and fan sizes are offset by the ease in handling and safety (as compared to a salt bath heater).

The Process
The HTE heater is controlled by two (2) temperature control loops.

The process outlet temperature set point controls the burner firing rate by positioning the micro-ratio (parallel positioning fuel and combustion air) valves. The combustion chamber temperature set point (hot flue gases) controls the position of the inlet damper on the quench air/FGR. A temperature higher than the set point causes the damper to open allowing more quench air/FGR to cool the flame and bring the combustion chamber temperature back to the set point.

ETI’s HTE heater is a forced-draft design and requires a burner management system that automatically brings the burner on and off safely. A pre- and post-firing purge are required by most codes affecting these heaters.

Several automatic shutdowns are incorporated in the burner controls. These include:

  • Flame Failure
  • Exchanger Inlet High Temperature
  • Process High Temperature
  • Combustion Air Blower Failure
  • Stack High Temperature
  • Process Low Flow
  • Tube Wall High Temperature

 

Advantages of the ETI HTE Heater

  • Separate Combustion and Heat Transfer Sections
  • No Direct Flame Impingement on the Heat Exchanger Surfaces
  • Near Complete Convective Heat Transfer
  • A Uniform Heat Flux Density in the Heat Exchanger Section
  • Modulated Quench Air/Fgr Flows and Temperatures
  • Low Process Heating Temperatures
  • Light Weight, Low Density Refractory and Ceramic Fiber Insulation Little Stored Heat
  • Stack Gas Temperatures +100°F Above Dew Point Desirable–250°F To 260°F Possible
  • Skid-Mounted Design (Minimal Site Assembly Work)
  • Fast Thermal Response
  • Rapid Startup and Cool Down
  • Controlled Film Temperatures
  • No Process Fluid Degradation or Coking
  • High Overall Operating Efficiencies
  • Counter Current Flow
  • Fully Automated
  • Unattended Operation
  • Replaces Unsafe Salt Bath Heaters
  • Extended-High Surface Area Coils
  • High Turndown
  • No Exotic Alloy Tube Hangers