Pass-Through vs. Closed Loop-Calculating Water Charges

On a pass-through system that is run 50 weeks a year for eight hours a day, assume water/sewage rates of 50 gpm at $5.00 per 1000 gallons. The annual cost for water and sewer charges for a pass-through system would be approximately $30,000. Typical Water and Sewage Rates chart (PDF)

On the other hand, for similar usage, the annual costs for operating a closed system with all motors combined at 62KW and an electric rate of $.10/KW would cost approximately $12,400. Typical Electrical Rates chart (PDF). The difference in annual operating costs would be at least $17,600 in utilities alone. In addition, the cost for maintenance and downtime on a closed system are practically eliminated as the absence of contaminants in the water make fouling of the heat exchanger non-existant.

Savings in operational costs realized from a closed system can support the purchase of new equipment within the year. 

Calculating Formulas

        COOLING LOAD FLUID STREAM:
           Water       GPM x (Temp. out – Temp. in) x 500 = BTU/hr
           Other       GPM x (Temp. out- Temp. in) x 8.33 x Specific Heat x Specific Gravity x 60 = BTU/hr
        COOLING LOAD RESERVOIR:
           Lb. of fluid x Temp. °F rise (first 15 min.) x Specific Heat of fluid x 4 = BTU/hr
        ENERGY:
           1 horsepower       = 745.7 Watts (0.7457 KW)
               = 2545 BTU/hr
           1 Kilowatt (KW)       = 1.341 horsepower
           3412 BTU       = 1 KWH = 1.341 HP/hr
        WEIGHT:
           1 US Gallon of Fresh Water       = 8.330 Lbs
           1 US Gallon of Salt Water       = 8.547 Lbs
        PRESSURE:
           1 lb/sq in (PSI)       = 2.31 Ft of water
           1 Atmosphere (at sea level)       = 14.7 lbs/sq in
               = 33.9 Ft of water

Other Technical Reference Data

Heat Load Requirements for Metal Removal chart (PDF)

Friction Loss chart (PDF)

Temperature Conversion chart (PDF)

Conversion Factors chart (PDF)

Dewpoint chart (PDF)