5 Sizing Steps for Chillers in Plastic Process Cooling


No matter what your application, there is a single formula for determining the size of chiller you need. Before you begin, you must know three variables:

  1. The incoming water temperature
  2. The chilled water temperature required
  3. The flow rate

    For our example, we will calculate what size chiller is required to cool 40 GPM (gallons per minute) from 70 °F to 58 °F? Use the following five steps and general sizing formula:

    1. Calculate Temperature Differential (ΔT°F)

    ΔT°F = Incoming Water Temperature (°F) - Required Chilled Water Temperature. 

    • Example: ΔT°F = 70°F - 58°F = 12°F

    2. Calculate BTU/hr.

    BTU/hr. = Gallons per hr x 8.33 x ΔT°F

    • Example: 40 gpm x 60 x 8.33 x 12°F = 239,904 BTU/hr.

      3. Calculate tons of cooling capacity

        Tons = BTU/hr. ÷ 12,000

        • Example: Ton capacity = 239,904 BTU/hr. ÷ 12,000 = 19.992 tons

        4. Oversize the chiller by 20%

         Ideal Size in Tons = Tons x 1.2

        • Example: 19.992 x 1.2 = 23.9904

        5. You have the ideal size for your needs

        • Example: a 23.9904 (or 25-Ton) chiller is required

         

        A 20 ton cooling-capacity chiller from Cold Shot Coolers

        A 20 ton cooling-capacity chiller from Cold Shot Coolers

         

        Plastic Process Cooling Applications

        There also industry-specific, rules-of-thumb for chiller sizing. These may vary depending upon the application. The below guidelines and formula may be used for sizing chillers for plastic process cooling applications. In our example, we will calculate what size chiller is needed for a polypropylene molding operation that incorporates a 6oz. "Shot Size" and a 18 second cycle time with a 3 H.P. hydraulic motor. We will use Charts 1 and 2 as references.

          1. Calculate the pounds of material per hour being processed.

            • Example: 6 oz. / 18 sec. = 19.99 oz/min (20.00 oz./min)
            • 20 oz. / min x 60 min. = 1200 oz/hr
            • 1200/16 = 75 lbs. / hr

              2. Determine how many pounds per hour are required for each ton of cooling capacity using Chart 1.

                • Example: Polypropylene requires 1 ton of cooling capacity for every 35 lbs./hr processed
                • 75 lbs. ÷ 35 lbs. = 2.14 tons of cooling

                  3. Determine if the extruder or any auxiliary equipment will require chilled water using Chart 2. If not, go to step #5.

                    • Example: A hydraulic motor requires 0.1 ton/HP of cooling capacity
                    • 3 HP x 0.1 ton/HP = 0.3 ton of capacity

                    4. Combine the process and auxiliary equipment cooling requirements.

                    • Example: 2.14 tons + 0.3 ton = 2.44 tons

                    5. Size your chiller by rounding up to the closest standard unit.

                    • Example: This application will require a 3-ton unit

                     

                    Chart 1: Plastic Material Process Cooling Requirements

                    Injection Molding

                    30#/hr H.D. Polyethylene

                    1 ton

                     

                    35#/hr L.D. Polyethylene/Polypropylene

                    1 ton

                     

                    40#/hr Nylon

                    1 ton

                     

                    50#/hr Polystyrene or ABS

                    1 ton

                     

                    65#/hr PVC or Polycarbonate

                    1 ton

                     

                    70#/hr P.E.T

                    1 ton

                    Extrusion

                     

                     

                     

                    50#/hr Polyethylene/Polypropylene

                    1 ton

                     

                    75#/hr Polystyrene

                    1 ton

                     

                    80 #/hr PVC

                    1 ton

                    Blow Molding

                    35#/hr Polyolefins

                    1 ton

                    Source: www.waterchillers.com

                     

                    Chart 2: Auxiliary Equipment and Extruder Cooling Requirements

                    Extruder Cooling

                    Gear box cooling

                    1 ton/100 hp

                     

                    Feed throat: 3” screw or less

                    1 ton

                     

                    Feed throat: larger than 3” screw

                    2 ton

                     

                    Barrel or screw cooling (per inch of screw diameter)

                    1 ton/inch

                    Auxiliary Equipment Cooling

                     

                     

                     

                    Air compressor (no aftercooler)

                    0.16 ton/hp

                     

                    Air compressor (with aftercooler)

                    0.2 ton/hp

                     

                    Vacuum pump

                    0.1 ton/hp

                     

                    Hydraulic cooling

                    0.1 ton/hp

                     

                    Hot runner mold

                    0.1 ton/hp

                     

                    Water pump in circuit

                    0.1 ton/hp

                     

                    Feed throat: less than 400 ton

                    0.5 ton

                     

                    Feed throat: greater than 400 ton

                    1 ton

                    Source: www.waterchillers.com

                     

                    About Cold Shot Chillers®

                    Based in Houston, Texas, Cold Shot Chillers® manufactures economical, ruggedly dependable industrial air cooled chillers, water cooled chillers, portable chillers and central chillers. Our industrial water-cooled chillers and air-cooled chillers serve a variety of different industries and applications.

                    Cold Shot Chillers® began in the late 1970s as an HVAC repair company in Houston, Texas. In 1980, the company began manufacturing new chillers for the plastic process industry and refurbishing used chillers for an assortment of industries. As our new chiller sales grew the company emphasis shifted from service to 100% manufacturing. Primary industries served include plastic processing, food & beverage, and metal finishing.

                     

                    For more information contact Bob Casto, Business Development Manager, Cold Shot Chillers®.

                    To read more articles about Chiller Technology, visit www.coolingbestpractices.com/technology/chillers.