Heat Recovery Systems
METHODS OF HEAT RECOVERY
There are four general methods of heat recovery that can be used with either catalytic or straight thermal oxidizers:
- Standard heat exchanger
- Waste heat boiler to generate steam.
- Waste heat boiler to generate steam which in turn is used with the Servel principal to produce chilled water.
- Waste heat boiler to generate steam which can produce electricity using a steam turbine generator.
ADVANTAGES OF THE STANDARD HEAT EXCHANGER
The major advantages of the standard heat exchanger as a method of recovering the heat exiting from an oxidizer, is that it is a passive device. There are no moving parts, and it can be used in a wide variety of flow rates and temperatures.
In essence, the heat exchanger has hot gas passing through one side of a medium which conducts heat to a cooler gas on the other side of the medium. This medium can be any structual metal in the form of plates or tubes.
HEAT EXCHANGER LIMITATIONS
Due to superior design and choice of materials, the heat exchangers that we supply have only a few limitations as noted below:
- For applications where the two streams are not equal: for a plate type exchanger, the maximum difference in flow rates from cool side to hot side is 3 to 1, for shell and tube exchangers, the ratio can be increased to 5 to 1.
- Normal maximum temperature is 1600°F using type 304 and type 309 stainless steel. In individual cases, the working temperature can be increased by using more exotic metals such as Inconel 600.
- Typical operating pressures are in the range of 10" to 15" water column. However, we can supply exchangers with working pressures up to 7 psi.
ACCEPTABLE EFFLUENT STREAMS
One question that is frequently asked concerns the types of effluent streams that are compatible with heat exchangers. A stream consisting only of fumes is the easiest stream to process through a heat exchanger.
On applications where there is any particulate matter, we always install a differential pressure transducer across the exchanger. We have developed an algorithm which allows us to post on the operator's screen the probable hours remaining until a clean-out session will be necessary. This permits the operator to schedule a clean-out period at a convenient time.
In cases where down time can not be scheduled, we equip the heat exchanger with a by-pass. When it becomes necessary to clean the particulate accumulation out of the exchanger, the by-pass gate diverts the effluent stream from the process directly to the atmosphere if that is permitted by your local EPA Permit To Operate. If not, we use a double by-pass system so that the effluent goes through the oxidizer before being discharged to the atmosphere.
Therefore, with our early warning system and by-pass arrangement, all effluent streams, regardless of particulate matter, justify a heat exchanger analysis.
HEAT EXCHANGER CONFIGURATIONS
There are several ways that a heat exchanger can be configured. Pictured below are some typical flow arrangements.
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| (Please refer to "Fuel Cost Savings" chart below) | |||||
FUEL COST SAVINGS
As the cost of natural gas and propane has increased, it has become easier to justify the additional cost of a heat exchanger in any oxidizer application. Following are two typical examples of heat exchanger economics. In both cases, the heat exchanger was used to heat the incoming effluent prior to entering the thermal oxidizer.
In the examples below, we have used a typical cost of $6.00 per million BTU for natural gas and a 2,000 hour year.
| ITEM | CASE I | CASE II | |||
|---|---|---|---|---|---|
| System Flow Rate | 6,000 scfm | 3,700 scfm | |||
| Cool side incoming temperature | A-from process 100°F |
A-from process 600°F |
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| Cool side outlet temperature | B-to oxidizer 660°F |
B-to oxidizer 995°F |
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| Hot side incoming temperature | C-from oxidizer 1000°F |
C-from oxidizer 1450°F |
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| Hot side outlet temperature | D-to atmosphere 462°F |
D-to atmosphere 1075°F |
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| Hourly BTU savings | 3.696 MMBTU/HR | 2.072 MMBTU/HR | |||
| Annual cost savings | $44,352.00 | $24,864.00 | |||
| Heat exchanger capital cost | $41,250.00 | $27,400.00 | |||
| Capital recovery period | 12 months | 13 months | |||
| (Please refer to "Air Flow Schematics" diagram above) | |||||
Please Call Us At:
(800) 503-4121 or (510) 887-7891
Fax us at: (510) 887-7894
E-mail us at: jlm@conversionproductsinc.com
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