Gas Emission Control - Mathematics and Statistics for the Environment - page 483

1. The specific heat ( C p ) of a substance is dependent on temperature, and the temperature
throughout the condenser is constantly changing.

2. The dewpoint of a substance is dependent on its concentration in the gas phase, and because
the mass flow rate is constantly changing (vapors being condensed) the dewpoint tempera-
ture is constantly changing.

3. No provision is made for cooling the vapors below the dewpoint. An additional term would
have to be added to the left side of the equation to account for this amount of cooling.

16.6.2 s urFaCe C ondenser C alCulations
In a surface condenser or heat exchanger (see Figure 16.18), heat is transferred from the vapor stream
to the coolant through a heat exchange surface (USEPA, 1981, p. 6-3). The rate of heat transfer depends
on three factors: total cooling surface available, resistance to heat transfer, and mean temperature dif-
ference between condensing vapor and coolant. This can be expressed mathematically by

q = UA ∆ T m (16.22)

where
q = Heat transfer rate (Btu/hr).
U = Overall heat transfer coefficient (Btu/°F⋅ft 2 ⋅hr).
A = Heat transfer surface area (ft 2 ).
∆T m = Mean temperature difference (°F).

The overall heat transfer coeficient U is a measure of the total resistance that heat experiences
while being transferred from a hot body to a cold body. In a shell-and-tube condenser, cold water
flows through the tubes causing vapor to condense on the outside surface of the tube wall. Heat is
transferred from the vapor to the coolant. The ideal situation for heat transfer is when heat is trans-
ferred from the vapor to the coolant without any heat loss (heat resistance).
Every time heat moves through a different medium, it encounters a different and additional heat
resistance. These heat resistances occur throughout the condensate, through any scale or dirt on
the outside of the tube (fouling), through the tube itself, and through the film on the inside of the
tube (fouling). Each of these resistances are individual heat transfer coefficients and must be added
together to obtain an overall heat transfer coefficient. An estimate of an overall heat transfer coeffi-
cient can be used for preliminary calculations. The overall heat transfer coefficients shown in Table
16.3 should be used only for preliminary estimating purposes.

TABLE 16.3
Typical Heat Transfer Coefficients in Tubular Heat Exchangers

Condensing Vapor (Shell Side) Cooling Liquid U (Btu/°F ⋅ ft 2 ⋅ hr)

Organic solvent vapor with high percent of noncondensable gases Water 20-60

High-boiling hydrocarbon vapor (vacuum) Water 20-50

Low-boiling hydrocarbon vapor Water 80-200

Hydrocarbon vapor and steam Water 80-100

Steam Feedwater 400-1000

Naphtha Water 50-75

Water Water 200-250

Source: Adapted from USEPA, APTI Course 415: Control of Gaseous Emissions , EPA 450/2-81-005, U.S.
Environmental Protection Agency Air Pollution Training Institute, Washington, DC, 1981, p. 6-12.