WSP00451

• File Number: 8273.100
• Description: Colorado River Basin Salinity Control - Federal Agency Reports - BOR
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 11/1/1969
• Title: Cost Analysis of Six Water Desalting Processes
• Water Supply Pro - Doc Type: Report/Study

The first eConomic analysis concerned the separate effect of changes

in steam costs and condenser capital costs on water costs. Figure 2

indicates the differential increase or decrease in water costs resulting

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from varying the steam cost and the performance ratio (PR). For example,
if the PR is held at 14 and the steam cost is reduced to 1~/106 Btu, a
savings in water costs of 11~/kgal will result. However, as will be de-

tailed later, optimization studies on the combined effect of steam costs

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and evaporator capital charges on the water conversion cost have indicated

that it is economically advisable to reduce the number of evaporator

stages as the, unit steam cost is decreased. As a result, the differential
savings in steam costs will then follow the dotted line labeled "locus
of economic PR.lI This decrease in PR will result in less savings on

steam costs, but the steam cost will be more than offset by lower capital

charges on the smaller evaporator.

The separate effect of condenser surface costs and required surface

area on water costs is shown in Fig. 3. For example, if the required
surface area is held constant at 0.106 ft2/gpd and the tube cost is re-
duced from $2.75 to $2.00/ft2, there would be a differential savings of
2~/kgal in water costs.

As was stated previously, there is an interdependence of the impor-

tant process variables and resulting economic combination of these vari-

abIes that will yield the lowest water costs, The amount of steam used

in a distillation process and the reqUired surface area in the evaporator

are best interrelated with the PR, Increasing the PR means that more

water will be produced per pound of steam used, thereby reducing the steam

cost portion,of water cost. But increasing the PR also requires more

stages and heat transfer surface, which increases the fixed cost portion

of conversion costs. Thus the lowest water cost results from selecting

a PR that minimizes the total of energy charges plus the fixed costs.

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