Heat Exchanger Fouling FactorsCompressed air is a dirty gas service and it is very difficult to avoid the effects of the impuritiescontinuously ingested by the compressor. These impurities in the air and coolant eventually depositon heat exchanger tube surfaces, impeding heat transfer.A heat exchanger uses the principle that heat transfer occurs when there is a difference in temperaturebetween the two fluids. In an air compressor heat exchanger, there is a coolant stream and a hot airstream. The two streams are separated by a thin, solid wall. The wall must be thin and conductive inorder for the exchange of heat to occur. Yet, the wall must be strong enough to withstand anypressure by the fluids. Copper is a common choice for construction.Here is a simple flow diagram showing how heat transfers in a heat exchanger.

This flow arrangement is called co-current. If the direction of one of the streams is reversed, thearrangement is called counter-current flow. Air compressor heat exchangers are typicallycounter-current flow.Here are the temperature profiles along the heat exchanger. Note that the temperature profiles aredifferent for co-current flow and for counter-current flow.

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The area between the curve is the heat transfer rate (Q). We can see that the heat transfer rate forcounter-current flow is larger than the rate for co-current flow. Counter flow heat exchangers providemore effective heat transfer. Most of the air compressor heat exchangers we encounter arecounter-current flow design.

A perfectly clean exchanger will have three components that make up the total thermal resistance (R):R1 = Thermal resistance of inside flowR2 = Thermal resistance of exchanger tubeR3 = Thermal resistance of outside flow

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Therefore: R = R1 + R2 + R3

When your new compressor is being tested in the factory the exchangers can come close to thisperformance, but it does not accurately represent real world operation. During normal heat exchangeroperation, surfaces are often subject to fouling by fluid impurities, rust formation, scale or otherreactions between the fluid and the wall material. The subsequent deposition of a film or scale on thesurface can greatly increase the resistance to heat transfer between the fluids. This effect can beconsidered during the design phase by introducing an additional thermal resistance, termed as foulingfactor (Rf). Its value depends on the operating temperature, fluid velocity and length of service of theheat exchanger. Fouling also impacts fluid temperature. To account for the fouling that will occur therewill be two additional components in the total thermal resistance (R):

R4 = Thermal resistance of fouling on inside surfaceR5 = Thermal resistance of fouling on outside surface

You can plan for the effects of fouling by requiring that a fouling factor be used when the compressormanufacturer quotes compressor performance. If the effect of fouling is not considered, exchangerswill be undersized when placed in a real world, dirty environment.Now rather than deriving heat transfer coefficients, we are going to give the fouling factors you shoulduse based on the fluid. Please follow this link to the Fouling Factor table. Remember, you need tospecify a fouling factor for both the coolant side and the air side of the heat exchanger. If you areusing a fluid not listed in the table, please post a request in the Open Air Forum. We will try to get youan answer.

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Fouling Factors For Heat ExchangersWater Temperatures 125 F (51.7 C) or less

Types of Water Water Velocity 3 ft/sec (1m/sec) and lessWater Velocity over 3 ft/sec

(1 m/sec)

h - ft2 - F /

Btu m - C / Wh - ft2 - F /

Btu m - C / W

Seawater 0.0005 0.000088 0.0005 0.000088Distilled 0.0005 0.000088 0.0005 0.000088Treated boilerfeedwater 0.001 0.00018 0.0005 0.000088

Engine jacket 0.001 0.00018 0.001 0.00018Great Lakes 0.001 0.00018 0.001 0.00018Cooling Tower and Spray Pond - Treatedmakeup 0.001 0.00018 0.001 0.00018

- Untreated 0.003 0.00053 0.003 0.00053Boiler blowdown 0.002 0.00035 0.002 0.00035Brackish water 0.002 0.00035 0.001 0.00018River Water - Minimum 0.002 0.00036 0.001 0.00018 - Mississippi 0.003 0.00053 0.002 0.00035 - Delaware /Schuylkill 0.003 0.00053 0.002 0.00035

- E. River / NewYork Bay 0.003 0.00053 0.002 0.00035

- ChicagoSanitary Canal 0.008 0.00141 0.006 0.00106

Muddy or silty 0.003 0.00053 0.002 0.00035Hard (over 15grains/gal) 0.003 0.00053 0.003 0.00053

Type ofFluid

h - ft2 - F /Btu m - C / W

Air 0.002 0.00035Nitrogen 0.0005 0.000088

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