Accurate chiller GPM calculations are critical when sizing chillers, pumps and maintaining optimal cooling performance. Incorrect flow rates can shorten equipment lifespan, reduce heat transfer and increase energy consumption.
In this guide, we will explain how to calculate chiller GPM and how you can successfully implement it in your system for optimal cooling.
What Is Chiller GPM?
Chiller GPM is the metric used to gauge the rate at which water flows through a chiller. This metric is directly linked to heat transfer, given that it quantifies the amount of water flowing through the evaporator at any given time. The flow directly impacts the cooling capabilities, temperature stability, and safety of the equipment.
For applications involving a small temperature rise, more water flow is recommended, and conversely, a high rise in temperature requires less flow to process the same load. Identifying suitable GPMs elevates system reliability, minimizes machine failures, and safeguards valuable chiller machines.
Commercial and industrial facilities that have high cooling requirements require proper water flow not only to save energy but to ensure the temperature is kept constant.
Variables Required before Calculating Chiller GPM
· Cooling Capacity
This is the point of origin of the GPM because this metric determines the quantity of heat that the selected chiller can remove. Depending on the scale of the machine and the industrial chiller design, this variable can be given using different inputs.
- Tons of Refrigeration (TR): Typically featured in HVAC and process cooling, this type of chiller-sizing unit is the most commonly used.
- BTU/hr: This unit is mainly used to represent the heat removal load in calculations. An example of a formula with this unit is GPM = BTU/hr ÷ (500 × ΔT)
- kW. Typically featured in large-scale industrial plants, this unit is used in situations when the cooling load is given in electrical or metric terms, making it optional.
Having accurate data in terms of capacity is essential to various contractors, suppliers, and other key personnel, as it allows them to properly size the accessories required to keep the systems running.
· Temperature Difference (ΔT)
An important metric in calculating chiller GPM is generating the Delta Temperature, which is obtained by subtracting the temperature of the supply water from the return water. This value allows you to accurately gauge the amount of heat picked up as the water flows.
A larger difference translates to more heat carried by the water per gallon, which means a lower GPM. A smaller difference means that less heat is transferred to the water per gallon, meaning a higher GPM for the system.
Calculating GPM using this unit can be expressed as
GPM = BTU/hr ÷ (500 × ΔT)
| Standard ΔT | Use |
| 10°F | Used as a traditional baseline in numerous designs |
| 12°F | Minimizes required flow |
| 15°F | Reduces pumping energy |
| 16°F to 20°F | Energy-optimized systems |
· Fluid Type
Fluid type is a key variable that factors into chiller GPM, as different types of fluid have varying capabilities when it comes to carrying heat. It also determines properties such as temperature load, fluid concentration, and the design of the systems. The main types utilized include water and glycol mixtures.
Water
Featuring both superior specific thermal and heat conductivity, water is the most efficient, economical, and easily available fluid. Standard HVAC chiller design calculations have constant values determined by the delta T and the given load. Using water minimizes pumping energy and fluid costs, with reduced compatibility issues allowing it to be used for a wide range of devices.
Water-glycol mixture
For applications executed in cold climatic conditions, this type of fluid mixture is best as it features freeze protection. This fluid has elevated viscosity, which increases pumping resistance, requiring elevated pump power. While it features enhanced freeze protection, it has a lower heat-carrying capacity.
Chiller GPM Calculation Formulas
· Formula 1: GPM from Cooling Capacity (Tons)
This is the formula used when tons of refrigeration (TR) is the unit used to quantify the chiller load. It is a quick and one of the most common ways to calculate a standard water system’s chilled water flow. For buyers and contractors, you can quickly gauge an estimate for the system requirements
To properly calculate, follow these steps:
- First, determine the capacity of the chiller in TR
- Ensure that the fluid in the system is water
- Confirm that the temperature variance is about 10°F.
- The last step is to multiply TR by 4
The formula is expressed as:
GPM=TR×2.4.
Therefore, if your water-based chilled-water system’s capacity is 250 TR, then the GPM will be 250 × 2.4, bringing it to 600.
· Formula 2: GPM from BTU/hr
If the cooling load of your device is given in BTU/hr, this second formula is a suitable method of determining GMP. This is therefore best suited for equipment that is gauged in terms of the thermal load, making it suitable for accurate engineering applications. Using this formula, both suppliers as well as designer accurately align the capacity of the chiller.
Calculating it involves the following steps:
- First, gauge the cooling load
- To establish the design delta temperature
- Then multiply the delta temperature by 500.
- Divide the cooling load by the obtained result.
This formula is expressed as GPM = BTU/hr ÷ (500 × ΔT)
If your device’s cooling load is 50,000 and the delta T is 10, then you will divide 600,000 by 500 × 10, making the GPM 120.
· Formula 3: GPM for Glycol Systems
For systems that rely on a water-glycol mixture, this formula is the one to use. The addition of glycol gives the mixture freeze protection, which directly impacts its thermal properties. Therefore, if you are in a cold region or the device operates under low temperatures, then this calculation will ensure the safety of your equipment.
The formula is calculated as follows:
- The first step is to get the devices’ cooling load
- Confirm the mixture’s properties as well as the concentration of glycol incorporated.
- Establish accurate fluid-specific heat data.
- And then use the new formula, which is expressed as
GPM=BTU/hr (500 × correction factor × ΔT)
Therefore, if the cooling load is 250,000 BTU/hr, Delta T is 10°F with a correlation factor of 0.9. Then it will be 250000 divided by 500 × 0.9 × 10, making the GPM 55.55.
Quick Reference Chiller GPM Calculation Table
| Cooling Capacity (Ton) | Delta T (ΔT) | Required GPM |
| 20 | · 10°F | · 48 |
| 50 | · 10°F | · 120 |
| 100 | · 10°F | · 240 |
| 200 | · 10°F | · 480 |
| 500 | · 10°F | · 1,200 |
It is essential to calculate the accurate chiller GPM as it will allow you categorize machines according to the intended applications. Best chiller GPM varies depending on the application requirements.
FAQs
What is the standard GPM per ton?
2.4 GPM per ton at about 10°F delta T is the standard GPM.
What happens when chiller GPM is too high?
Issues such as elevated pump energy, increased drop in pressure, and in some cases limited heat transfer.
What happens if chiller GPM is too low?
It may cause poor cooling as a result of reduced heat transfer, freezing, and tripped alarms.
Which flow rate is best for my chiller?
The best flow rate varies depending on the manufacturer’s minimum and maximum flow limits.
Can improper GPM reduce equipment lifespan?
Setting an incorrect GPM can induce stress, which in turn limits the service life of the equipment by increasing water wear.
How does flow affect energy efficiency?
Flow rate affects pump energy as well as chiller efficiency, which directly impacts energy efficiency.
Final Thoughts
Calculating GPM is critical when choosing the right industrial chiller for optimal heat transfer and maintaining system stability. At ChillerOne, our engineers will help you choose the right chiller systems for your industrial cooling needs – get a custom quote now.
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