Dirt factor

Values include dirt factors of 0.003 and allowable pressure drops at 5—10 psi on the controlling steam. DOWTHERM fluid would be included as light organic. (Kern. Process Heat Transfer, 1 Ed., p. 840 1950.)... 

The performance of a heat exchanger depends upon the transfer surfaces being clean and uncorroded. The performance deteriorates with time due to accumulation of deposits on the heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer. In the design of heat exchangers, this added resistance is accounted for by a fouling factor or dirt factor, R. ... 

The overall heat transfer coefficient is a composite number. It depends on the individual heat transfer coefficients on each side of the tube and the thermal conductivity of the tube material. The individual heat transfer coefficient in turn depends on the fluid flow rate, physical properties of the fluid, and dirt factor. The temperature along the tube is not uniform. The hot and the cold fluids may flow in the same (cocurrent) or in opposite (countercurrent) directions. Generally the hot and cold fluids come in contact only once, and such an exchanger is called single pass. In a multipass exchanger, the design of the... 

R dw = combined resistance of tube wall and scaling or dirt factors, [Btu/(hXft2X°F)] 1 defined with Eq. (39). 

For simplicity, considering only the shell- and tube-side resistances (neglecting dirt factors), the overall heat transfer coefficient U is given by... 

---