{"id":15885,"date":"2023-07-05T15:13:12","date_gmt":"2023-07-05T07:13:12","guid":{"rendered":"https:\/\/aircondlounge.com\/?p=15885"},"modified":"2024-04-22T15:54:34","modified_gmt":"2024-04-22T07:54:34","slug":"chilled-water-flow-rate-calculation","status":"publish","type":"post","link":"https:\/\/aircondlounge.com\/chilled-water-flow-rate-calculation\/","title":{"rendered":"Chilled Water Flow Rate Calculation (IP & SI)"},"content":{"rendered":"\n

When designing a chilled water system, the chilled water flow rate is one of the parameters to calculate. Most of the time, it is estimated through the rule of thumb (gpm per ton). However, it can also be calculated by hand.<\/p>\n\n\n\n

To calculate chilled water flow rate, determine the chilled water supply-return design temperature and chiller capacity first. Then, use the heat transfer formula (Q=mc\u03b8) to calculate the required chilled water flow rate. The result is ft3<\/sup>\/hr for IP unit and m3<\/sup>\/s for SI unit.<\/strong><\/p>\n\n\n\n

For many years, a standard chilled water gpm per ton has been widely practiced. However, modern chilled water systems are switching to a lower gpm per ton to save energy.<\/p>\n\n\n\n

Chilled Water GPM per Ton<\/h2>\n\n\n\n

In a chilled water system<\/a>, the flow rate “gpm per ton” is referring to how much water flow for every refrigeration tonnage (RT). The gpm stands for gallons per minute and it is based on the Imperial unit.<\/p>\n\n\n\n

For SI unit, the flow rate is expressed in cubic meter per second (m3<\/sup>\/s). However, litre per second (L\/s) is more commonly used by the HVAC industry as it is easier to look at the numbers.<\/p>\n\n\n\n

Now, a typical 600 RT chiller may have an evaporator flow rate (chilled water flow rate) of 1439 gpm at full load. If we take the flow rate and divide it by the chiller capacity, we get:<\/p>\n\n\n\n

1439 gpm \u00f7 600 RT = 2.398 gpm per ton<\/p>\n\n\n\n

Therefore, the standard value for chilled water flow rate is about 2.4 gpm per ton<\/strong>.<\/p>\n\n\n\n

To get the chiller evaporator flow rate, you normally have to get the chiller selected first. But, if you don’t, how can you calculate the required chilled water flow?<\/p>\n\n\n\n

Chilled Water Flow Rate Calculation (IP)<\/h2>\n\n\n\n

When designing a new chilled water system, the chilled water flow rate has to be determined for pump selection. If the chiller is not yet selected, you can calculate the chilled water flow rate using the heat transfer formula.<\/p>\n\n\n\n

Q = mc\u03b8<\/p>\n\n\n\n

where,
Q = chiller capacity, btu\/hr
m = mass flow rate, lb\/hr
c = specific heat of water, 1.0 btu\/lb.\u00b0F
\u03b8 = chilled water temperature difference, \u00b0F<\/p>\n\n\n\n

Since mass flow rate is the product of fluid density and volumetric flow rate (m=pq), the heat transfer formula can be rearranged as follows:<\/p>\n\n\n\n

Q = pqc\u03b8<\/p>\n\n\n\n

where,
Q = chiller capacity, btu\/hr
p = water density, 62.4 lb\/ft3<\/sup>
q = chilled water flow rate, ft3<\/sup>\/hr
c = specific heat of water, 1.0 btu\/lb.\u00b0F
\u03b8 = chilled water temperature difference, \u00b0F<\/p>\n\n\n\n

Before we can calculate the chilled water flow rate, we must determine the chiller capacity and the chilled water supply-return design temperature.<\/p>\n\n\n\n

As discussed in my post about high delta T chilled water system<\/a>, the standard chilled water design temperature is supply 44\u00b0F and return 54\u00b0F. Assume that the cooling load is 600 RT, the required chilled water flow rate can be calculated as follows:<\/p>\n\n\n\n

Q = pqc\u03b8
600 x 12000 = 62.4 x q x 1 x (54-44)
q = 7200000 \u00f7 624
q = 11538.46 ft3<\/sup>\/hr<\/p>\n\n\n\n

Then, convert the chilled water flow rate from ft3<\/sup>\/hr to gpm by dividing 11538.46 by 8.021 and we get a chilled water flow rate of 1438.5 gpm or 1439 gpm.<\/p>\n\n\n\n

What we know from the calculation is that the standard 2.4 gpm per ton is based on the standard chilled water design temperature of supply 44\u00b0F and return 54\u00b0F, or anything that results in a delta T (\u0394T) of 10\u00b0F.<\/p>\n\n\n\n

As I touched on it earlier, modern chilled water systems use a greater delta T, resulting in a lower chilled water gpm per ton that saves pump energy.<\/p>\n\n\n\n

To convert from IP to SI unit, use my HVAC Online Unit Converter<\/a>.<\/p>\n\n\n\n

Chilled Water Flow Rate Calculation (SI)<\/h2>\n\n\n\n

When designing a new chilled water system, the chilled water flow rate has to be determined for pump selection. If the chiller is not yet selected, you can calculate the chilled water flow rate using the heat transfer formula.<\/p>\n\n\n\n

Q = mc\u03b8<\/p>\n\n\n\n

where,
Q = chiller capacity, kW
m = mass flow rate, kg\/s
c = specific heat of water, 4.187 kJ\/kg.\u00b0C
\u03b8 = chilled water temperature difference, \u00b0C<\/p>\n\n\n\n

Since mass flow rate is the product of fluid density and volumetric flow rate (m=pq), the heat transfer formula can be rearranged as follows:<\/p>\n\n\n\n

Q = pqc\u03b8<\/p>\n\n\n\n

where,
Q = cooling capacity, kW
\u03c1 = water density, 997 kg\/m3<\/sup>
q = chilled water flow rate, m3<\/sup>\/s
c = specific heat of water, 4.187 kJ\/kg.\u00b0C
\u03b8 = chilled water temperature difference, \u00b0C<\/p>\n\n\n\n

So, before we can calculate the chilled water flow rate, we must determine the chiller capacity and the chilled water supply-return design temperature.<\/p>\n\n\n\n

As discussed in my post about high delta T chilled water system<\/a>, the standard chilled water design temperature is supply 6.7\u00b0C and return 12.2\u00b0C. Assume that the cooling load is 2110 kW, the required chilled water flow rate can be calculated as follows:<\/p>\n\n\n\n

Q = pqc\u03b8
2110 = 997 x q x 4.187 x (12.2-6.7)
q = 2110 \u00f7 22959.41
q = 0.0919 m3<\/sup>\/s<\/p>\n\n\n\n

Then, convert the chilled water flow rate from m3<\/sup>\/s to L\/s by multiplying 0.0919 by 1000 and we get a chilled water flow rate of 91.9 L\/s or 92 L\/s.<\/p>\n\n\n\n

What we know from the calculation is that the standard 2.4 gpm per ton is based on the standard chilled water design temperature of supply 44\u00b0F (6.7\u00b0C) and return 54\u00b0F (12.2\u00b0C), or anything that results in a delta T of 10\u00b0F (5.5\u00b0C).<\/p>\n\n\n\n

As I touched on it earlier, modern chilled water systems use a greater delta T, resulting in a lower chilled water gpm per ton that saves pump energy.<\/p>\n\n\n\n

On a side note, if you want to quickly learn about chilled water system, you can get my Chilled Water System (eBook)<\/a>. If you’re into design, you can enroll in my Chilled Water System Design Course where I teach you various design procedures with tons of examples.<\/p>\n\n\n

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Chilled Water System Design Course<\/p><\/div>\n

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Learn how to design a chilled water system with AHU\/FCU selection, chiller sizing, cooling tower sizing, pump sizing, piping design, ductwork design and more.<\/p><\/div>\n

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Enroll Now<\/p><\/a><\/div><\/div>\n\n\n

ASHRAE 90.1 Standard GPM per Ton<\/h2>\n\n\n\n

The ASHRAE 90.1 standard requires chilled water cooling coils to be designed for at least 15\u00b0F delta T (temperature difference between supply and return). The chilled water return temperature must be equal or above 57\u00b0F.<\/p>\n\n\n\n

If the cooling load of 600 RT is now designed based on a chilled water delta T of 15\u00b0F, the required chilled water flow rate becomes 959 gpm. By dividing 959 gpm by 600 RT, we can see that the new standard for chilled water flow rate is 1.6 gpm per ton<\/strong>.<\/p>\n\n\n\n

With the new standard, chilled water systems see a much lower chilled water flow rate and this helps reduce the pump power significantly as I explain why in the following.<\/p>\n\n\n\n

How Reduced Chilled Water Flow Rate Saves Energy<\/h2>\n\n\n\n

A chilled water pump will see 2 times power consumption reduction if the chilled water flow rate is reduced by 1 time. This is due to the Affinity Laws.<\/p>\n\n\n\n

As mentioned earlier, if the required chilled water flow rate for 600 RT is calculated based on 15\u00b0F instead of 10\u00b0F, the result is 959 gpm instead of 1439 gpm, a 33% reduction.<\/p>\n\n\n\n

Now, the Affinity Laws when applied to a pump state that:<\/p>\n\n\n\n