Pump-Trap Combinations Sizing & Selection
Summary of conditions for a Heat Exchanger (HX) using Steam to heat Water
Set of conditions used to size the Heat Exchanger:
What is the Heat Transfer Rate (E) to heat 60 GPM of water from 45˚to 140˚F? E Btu = Water Flow Rate ( GPM ) x 500 x Temp Rise (˚F) hr
1) Maximum Flow of Hot Water
=
60 GPM
T o
2) Water temperature required
=
140˚F
=
60 x 500 x [140˚- 45˚F]
3) Steam Pressure in Heat Exchanger
=
50 PSIG
= 2,850,000 Btu/hr
T s
4) Temperature of 50 PSIG Steam
=
298˚F
How much Steam Flow is required?
T i
5) Inlet Water Temperature
=
45˚F
E LH
Q s (steam)
(For 50 psi steam, the LH is 912 Btu/lb)
=
6) Temperature Rise of Water
=
95˚F
(140˚ F - 45˚F = 95˚ F)
2,850,000 Btu/hr 912 Btu/lb
=
= 3,125 lbs/hr
Fundamental formula for heat transfer and the basic formula for HX sizing
E = U x A x Δ T
The formula shows that the heat transfer rate ( E ) between the hot steam and cold water is directly proportional to the Surface contact area ( A ) inside the HX and the difference in temperature between the steam and water ( Δ T). The more surface area (larger HX) the more heat will get transferred or the hotter the steam temperature (higher pressure) the more heat will get transferred. E = Heat Transfer Rate in Btu/hr of the energy in the steam to the water. The flow of steam ( Q s ) required in lbs/hr is determined by dividing E by the Latent Heat of Steam (LH) in Btu/lb. U = is referred to as the Overall Heat Transfer Coefficient . This depends on the HX type and the materials involved. Typical U values are 120 for Stainless Steel and 200 for Copper. We will use 120 for Stainless Steel HX. A = The internal Surface Area (size) of the HX in Sq. Ft. The size of a HX is determined by the surface contact area between the Steam and Water. Δ T = Average Temperature Difference between Steam & Water. Since the water temperature changes as it flows thru the HX, we need to use the average temperature difference between the steam temperature and the water temperature. See formula below:
Average Temperature Difference
Heat Exchanger Size
E = U x A x Δ T
(T s – T i ) + (T S – T o ) 2
Δ T =
Above formula is rearranged to solve for A :
E U x Δ T
A =
(298 – 45) + (298 – 140) 2
=
2,850,000 120 x 205
=
Δ T = 205 F = Avg Temp. Difference
A = 116 (sq ft.)
The actual size of a Heat Exchanger depends on many factors; however, based on the criteria given, 116 sq. ft of surface area is required to heat 60 GPM of water from 45˚F to 140˚F, based on a steam pressure of 50 PSIG.
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