Boiler Facts... George R. Carey, Jr. Vice President, Fluid Industrial Associates gcarey@fiainc.com twitter: @Ask_GCarey Understanding the Importance of GPM in a Hydronic System Of all the formulas that are used in hydronic system design, there is one in particular that once understood, allows you to understand what’s going on in any hydronic system. Flow in a hydronic system is used to carry heat from the boiler or chilled water from the chiller out to the system. An accurate load calculation is the basis for any system design. The following formula is used to calculate the required flow in any hydronic system: GPM = Heat Load/ 500 ^T Where: GPM is the volume flow rate, gallons per minute Heat load is in British thermal units/hr (BTU)/Hr or BTUH ^T = Temperature difference between the supply and return °F Five hundred is the constant for standard water properties at 60°F Density, 8.33 lbs. per gal. Specific heat, 1BTU/lb°F The complete calculation is then: GPM = BTU/hr 8.33lb./gal x 1 BTU/lb°F x 60 min/hr x ^t°F Where 8.33 x 60 x 1 = 500 You will notice in this formula that the specific heat and density are referenced to 60°F water. However, since 60°F water is too cool for a typical heating system and too warm for a typical cooling system, you would think that the flow should be calculated by taking into account the following changes: • Specific heat and density changes caused by water temperature changes. 24 ICM/May/June 2016
ICM May-June 2016
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