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Chiller Selection Guide

Size an Advantage Chiller From Process Numbers, Not Guesswork

Heat load, fluid temperature, ambient, flow rate and glycol

Overview

Heat load, fluid temperature, ambient condition, flow rate and glycol - in that order.

Every chiller selection starts with the actual heat load, in tons or BTU/hr, coming off the process - not the horsepower of the motor, the clamp tonnage of the press, or the size of the machine sitting next to where the chiller will go. A small, high-duty-cycle process can load a chiller harder than a large machine that runs light; the only reliable number is watts or BTU/hr measured or calculated from the process itself.

Next is the required leaving fluid temperature, and it matters more than most buyers expect. A chiller's nameplate tonnage is normally rated at a moderate temperature, often 50°F leaving fluid - the same physical unit delivers meaningfully less capacity at 34°F, because the colder, thinner suction gas at a lower evaporating temperature lets the same compressor move fewer pounds of refrigerant per minute. Quoting off a nameplate tonnage without stating your actual required temperature is how undersized chillers end up on the floor.

For air-cooled units, the ambient design condition - the hottest air temperature the condenser will actually see, not the average - sets how much heat the unit can reject before capacity falls off. A chiller sized for a 95°F ambient will fall short in a mechanical room that regularly sees 105°F. Flow rate (GPM) and the pump head available at the point of use matter just as much: too little flow starves the process of cooling even when the tonnage is correct, and too much head loss in long or narrow piping runs can leave the pump unable to deliver the flow the process needs.

Finally, decide whether the loop needs glycol. If the fluid has to run below 32°F for any part of the process, or the unit sits outdoors somewhere the fluid could freeze in winter, glycol is a selection input, not an accessory - see the Glycol & Low-Temperature Guide for how percentage affects performance. Once you have these five numbers, request a quote and we'll match a standard Advantage portable, central or glycol chiller rather than working backward from a guess.

  • See Portable Chillers and Central Chillers for Advantage configurations by capacity
  • See Glycol Chillers if your process needs sub-freezing fluid or outdoor freeze protection
  • Send us these five numbers on the Quotes page and we'll size the right standard model
Heat loadnoun

The heat, in tons or BTU/hr, that the process itself gives off - not the motor horsepower, not the clamp tonnage of the machine beside it. It is the first number every chiller selection starts from.

What a Quote Needs

Five inputs set the selection; the table says why each one matters.

What We Need Why It Matters
Heat load (tons or BTU/hr) The actual number the chiller has to remove - not machine size or motor horsepower
Required leaving fluid temperature Lower temperatures reduce a given chiller's effective tonnage
Ambient design condition Sets condenser capacity for air-cooled units at the hottest expected air temperature
Flow rate (GPM) and available pump head Confirms the pump can deliver adequate flow at the point of use
Glycol requirement, if any Needed below 32°F fluid temperature or for outdoor freeze exposure

ONE COMMON MISTAKE TO AVOID

Sizing Off the Wrong Number

The single most frequent selection error we see is a heat load and a required temperature quoted independently, as if either one alone were enough to pick a model. They aren't - a chiller's effective tonnage changes with leaving fluid temperature, so the same heat load number can point to two different model sizes depending on whether the process needs 50°F fluid or 34°F fluid. Send both numbers together, not one now and the other "once we confirm."

A related mistake: sizing an air-cooled unit off average ambient temperature instead of the hottest design condition. A unit that performs fine in spring will fall short during a summer heat wave if it was sized to the average rather than the peak the mechanical room or outdoor pad will actually see.

Getting It Right

  • Report heat load and leaving fluid temperature together, not separately
  • Use peak ambient design condition, not seasonal average
  • Measure or calculate flow rate rather than assuming existing piping is adequate
  • Confirm glycol requirement before, not after, a model is selected

Arithmetic behind the numbers on the quote form

Divide by 3.517, or multiply by 3,412 to get BTU/hr first. An instrument whose data sheet lists 18 kW of heat to water is about 61,400 BTU/hr, or 5.1 tons. The mistake is grabbing the electrical input instead: input power includes energy that leaves as light, motion or heat to the room rather than going into the cooling water. Use the heat-rejection or heat-to-water line where the sheet gives one, and if it lists only input, say so rather than converting it as if every watt lands in the loop.

It follows from the temperature rise you allow through the process: GPM = BTU/hr divided by (500 x rise). A 10-ton load, 120,000 BTU/hr, needs 24 GPM if the water warms 10°F, and 48 GPM if it warms only 5°F. Ask for a tight 3°F rise and it becomes 80 GPM - a bigger pump, bigger pipe and more pump heat. Most process loops are designed somewhere around a 5 to 10°F rise, with precision work at the low end. Glycol needs more flow than water for the same load. Check the process equipment's minimum flow as well, since some exchangers lose turbulence and foul below it.

Add up the pressure drops along the path at design flow: the process equipment itself, supply and return piping with fittings, valves, hoses and quick-disconnects. Convert psi to feet of head by multiplying by 2.31, so a 15 psi mold plus 10 psi of piping and couplings is about 58 feet. In a fully closed loop the height of the building largely cancels, since what goes up comes back down; where the return drains to an open reservoir, the lift to the highest point counts. The usual miss is small fittings - a few undersized quick-disconnects can take more pressure than 100 feet of pipe.

Because the loop volume, not the compressor, takes the first minutes. A 10-ton step is 2,000 BTU per minute. Dropped into a loop holding 50 gallons - about 417 pounds of water - it raises the temperature nearly 5°F a minute until the compressor loads up and control settles. Double the volume and the rate halves. That is why tank size matters as much as tonnage on batch processes, quench tanks and machines that start cold-loaded. Give us how fast the load arrives as well as how big it is; a larger reservoir or a pump tank station can cost less than more tons.

Yes, and on small loads it can be a real share. Pump horsepower that goes into the fluid ends up as heat, about 2,545 BTU/hr per hp. A 5 hp booster pump on a long distribution run adds around 12,700 BTU/hr - just over a ton - to a load the process sheet never listed. On a 4-ton process that alone is a quarter of the job.

Often, and it makes a good cross-check. Multiply the pounds per hour of product by its specific heat and the temperature drop you need: 2,000 lb/hr of steel parts (specific heat about 0.11) cooled 150°F is 33,000 BTU/hr, or 2.75 tons. Add whatever the machine itself puts into the water - hydraulics, motors, friction - since the product figure covers only the material. Where the product changes phase, as molten plastic or metal does, the latent heat adds to that and often dominates. If the product number and a measured water-side number disagree badly, one of them is wrong, and it is worth finding out before an Advantage unit is chosen.

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