Thermal Energy Recovery (TER) Efficiency
Rodney L Pennington, P.E.
Oxidizer installations are often installed as end of the pipe air emission control solutions
without an overall consideration for the total system thermal energy efficiency. Both the
selection of the thermal efficiency of the oxidizer and the potential for secondary heat
recovery can substantially reduce the energy operating costs for the system.
Exploiting energy recovery and increasing the overall efficiency of air emission control
systems also contributes to a sensible reduction of greenhouse gases released through
electric or mechanical power production.
Energy recovery includes any technique or method of minimizing the input of energy to
an overall system by the exchange of energy from one sub-system of the overall system
with another. The energy can be in any form, but most energy recovery systems
exchange thermal energy in either sensible or latent form.
http://en.wikipedia.org/wiki/Energy_recovery
Latent heat is the heat released or absorbed by a substance during a change of
state (or phase) that occurs without a change in temperature. (i.e. liquid to vapor)
Sensible heat is the energy exchanged during a process that has as its sole
effect a change of temperature.
Thermal Energy Recovery (TER) Efficiency in Oxidizers varies from zero (0)
to a maximum of 96%.
• Obviously, the direct fired thermal oxidizer is zero (0%),
• the recuperative oxidizer ranges from 35% to 70 %,
o In a recuperative oxidizer both air streams are separated by a wall through
which heat is transferred from one stream to the other.
• the regenerative thermal oxidizer (RTO) is 70 to 96%.
o the regenerator oxidizer utilizes a heat exchange media to store and
release heat energy on a continuously counter cyclic flow basis with direct
contact with each of the air streams.
Enthalpy Transfer efficiency is the most accurate approach to establishing the true
oxidizer efficiency and actual energy /fuel requirements. The enthalpy transfer efficiency
of a heat recovery unit or system can be expressed as:
μe = (h2 - h1) / (h3 - h1), …….. (1107-311.5)/ (1149-311.5) = 0.95………95% TER
where:
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μe = enthalpy transfer efficiency
h1 = enthalpy of the process air before the oxidizer heat exchanger
T1 =100 OF…………………………………………...h1= 311.5 KJ/Kg
h2 = enthalpy of the process air after the oxidizer heat exchanger, prior to the
combustion chamber. T2= 1435 OF …………..…h2 = 1107 KJ/Kg
h3 = enthalpy of the combustion chamber exhaust air before the heat exchanger
T3 = 1500 OF ………………………………………..h3 = 1149 KJ/Kg
h4 = enthalpy of the combustion chamber exhaust air after the heat exchanger
T4 = 175 OF …...h4 = 353.5 KJ/Kg
The following site provides enthalpy calculations for pressure and temperature
conditions:
http://www.peacesoftware.de/einigewerte/luft_e.html
In all RTOs, the inlet (preheat) delta enthalpy (h2-h1) must equal = the outlet (recovery)
delta enthalpy (h3-h4)…….. (h3-h4) = (h2-h1)
(1149-h4) = (1107- 311.5) ….. h4 = 353.5 KJ/Kg
Since the preheat temperature ( T2 ) is not typically measured or provided, RTO
manufactures use a TER calculation based on the outlet temperature (T4), however,
many do not correct for balanced mass flow, which is essential to determine the true
operating thermal energy recovery (TER) efficiency .
TER Eff. = (T3-T4) / (T3-T4)…. (1500-170)/ (1500-100) = 0.95 ...95% (balanced
mass).
This is the design efficiency TER which is only achieved with:
1. Balanced mass flow (equal mass flow in and out)…….see NESTEC News Letter
October 2013 for details.
2. And uniform air flow and temperature distribution. ……see NESTEC News
Letter April 2014 for details.
The un-balanced mass flow in an RTO is due to the added combustion products
introduced into the combustion chamber and exhausted through the outlet heat
recovery chamber. As can be seen from the attached graph below, the added fuel
requirement can be substantial at very low solvent loads, less than the self sustaining
level (virtually zero auxiliary fuel).
NESTEC offers specific RTO features to address both mass balance and uniform air
flow and temperature distribution to insure the RTO Thermal Energy Recovery
(TER) Efficiency is maximized.
Secondary heat recovery can provide additional savings through a variety of different
types of systems.
The most cost-effective use of waste heat is to improve the energy efficiency of the heat
generating process itself, or the oxidizer.
The efficiencies shown below are useful in comparing the range of technologies
available for secondary heat recovery:
• Recuperative heat exchanger: ..typically 55%- 65% …….maximum 80%
• Thermal (heat) wheel: ……….…typically 65%-75%, …….maximum 80%
• Air to liquid circulation coil: …...typically 45%-50%,……..maximum 55%
• Heat pump: …………………..….typically 35%-50%, ……..maximum 60%
• Heat pipes: ……………………....typically 50%- 65%, …….maximum 75%
• Regenerative heat exchanger….typically 85% - 95% ……..maximum 95%
The operational efficiency of a boiler is measured by the percentage of the fuel input
energy that is eventually delivered as useful heat output. Not all of the heat released
when the fuel is combusted can be used; some potential heat is never released due to
incomplete combustion and some is lost.
Major sources of heat loss from steam boilers are through the flue gas, blow down and
radiation to the boiler’s surroundings. For a shell type steam boiler the losses are:
• Flue losses ~18%
• Heat transfer gas and water side losses ~2%
• Insulated chamber radiation losses ~2%
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Water outlet blow down losses ~3%
Common uses for recovered heat include:
• pre-heating of combustion air for boilers, ovens, furnaces, and/or recirculation of
oxidizer exhaust for drier, oven or furnace process
• waste heat boilers
• preheating fresh air used to ventilate the building
• Hot oil generation for process use
• hot water generation, including pre-heating of boiler feed water
• space heating
• Drying
• other industrial process heating/pre-heating
• power generation
• Absorption chillers
NESTEC's RTO / RCO designs have optimized the benefits of the heat recovery media,
simplified the flow control and has compacted the configuration into a smaller
prepackaged unit, greatly improving on earlier RTO designs.
The majority of NESTEC’s staff has over thirty plus years (30+) of individual experience
which has resulted in RTO / RCOs designs that offer:
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a smaller, more compact unit for a lower capital investment
a 30-percent to 50-percent reduction in energy consumption
increased destruction efficiency
shop-assembled modules or skid mounted units with a shorter field-installation
time
increased reliable
lower maintenance
direct RTO internet service/ maintenance interface with remote service and
monitoring support capabilities.
many of these features can be retrofitted to existing systems.
Together, NESTEC’s improvements have made it possible for industry to remain in
regulatory compliance while continuing to turn a profit.
Contact NESTEC for a complete evaluation of your oxidizer, RTO unit and how we can
possible incorporate some of the proven unique features and benefits into your system
to maximize performance and up time reliability.