The flow rate necessary to deliver the full output of the heat source at a specific temperature drop can be found using equation below:
Q = H / (8.01 x ρ x c x ▲T)
Where:
• Q= Water volume flow rate (GPM)
• H = Heat load (Btu/hr)
• ▲T = Intended temperature drop (°F)
• ρ = Fluid's density at the average system temperature (lb/ft3)
• c = the fluid's specific heat at the average system temperature (Btu/lb/°F)
• 8.01 = a constant
In small to medium size hydronic systems, the product of (8.01 x ρ x c) can be taken as 500 for
water, 479 for 30% glycol, and 450 for 50% glycol. The total heat removed by air condition chilled-
water installation can thus be expressed as
H = 500 x Q x ▲T
Where
• H = total heat removed (Btu/h)
• Q = water flow rate (gal/min)
• ▲T = temperature difference ( 0F)
Evaporator Flow Rate
The evaporator water flow rate can be expressed as
Qe = Htons x 24 / T
Where
• Qe = Evaporator water flow rate (GPM)
• Htons = Air conditioning cooling load (tons)
• T = Temperature differential between inlet and outlet (°F)
Condenser Flow Rate
The condenser water flow rate can be expressed as
Qc = Htons x 30 / ▲T
Where
• Qc = Condenser water flow rate (GPM)
• Htons = Air conditioning cooling load (tons)
• ▲T = Temperature differential between inlet and outlet (°F)
Note the equation above assumes 25% heat of compression.
CONDENSATE GENERATION
Condensate generation in an air condition system where specific humidity before and after are known can be expressed as
Q Cond = Q air x W Lb / (SpV x 8.33)
Q Cond = Q air x W GR / (SpV x 8.33 x 7000)
Where
• Q Cond = Air Conditioning condensate generated (GPM)
• Q air = Air Flow Rate through the air-handling unit cooling c oil (Cu-ft / minute)
• SpV = Specific Volume of Air (Cu-ft per lb of dry air)
• W Lb = Specific Humidity diff. between inlet and outlet of air stream across coil (lb-H2O per
lb of dry air)
• W GR = Specific Humidity diff. between inlet and outlet of air stream acros s coil (Gr. H2O per lb of dry air)
FLOW RATES IN HEATING SYSTEMS
The volumetric flow rate in a heating system can be expressed by the basic equation:
Q = H / (Cp x ρ x ▲T)
Where
• Q = volumetric flow rate (GPM)
• H = heat flow rate (Btu/hr)
• CP = specific heat capacity (Btu/lb-°F)
• ρ = density (lb/ft3 )
• ▲T = temperature difference (°F)
The basic equation can be expressed for water with temperature 600F flow rate as:
Q = H (7.48 gal/ft3 ) / ((1 Btu/lb- 0F) (62.34 lb/ft3 ) (60 min/h)▲T)
Or
Q = h / (500 x ▲T)
Where
• Q = Water flow rate (GPM)
• H = Heat flow rate (Btu/hr)
• ▲T = Temperature difference (0F) (usually 20ºF)
For more exact volumetric flow rates for hot water the properties of hot water should be used.
Water Mass Flow Rate Water mas s flow can be express ed as:
m = h / ((1.2 Btu/lb- 0F) x ▲T)
Where
• m = mass flow (lb/hr)
