HEAT EXCHANGER FORMULAS & EXAMPLE Heat Exchanger Example: Heating Water with Steam using a Modulating Control Valve
STEAM TRAP SIZING J) What type and size of steam trap should be chosen for this application?
Initially we will assume that the condensate from the HX is being discharged to a condensate return line at atmospheric pressure (0 PSIG). At full-load, the steam pressure at the inlet of the trap is 50 PSIG. We have already calculated the maximum condensate load that would be generated at this pressure to be around 5,000 lbs/hr (see Part E). If this was the only set of conditions, selection of the steam trap would be fairly simple. We simply look at the capacity chart and choose a steam trap that will pass at least 5,000 lbs/hr at 50 PSI differential pressure. (See the capacity chart below for the FTE Series Float & Thermostatic steam traps.) However, at different flow rates of water, the HX will have very different pressures. At a flow rate of ~58 GPM, the pressure in the HX drops to 0 PSIG and the HX is still producing condensate at the rate of ~2,700 lbs/hr. This condensate still needs to be drained from the HX but how can this happen with NO differential pressure? The purpose of having an extended drip leg under the HX is to give the trap a certain amount of head pressure. If the trap is mounted 14 inches below the HX, this will correspond to ½ PSI of head pressure. The trap must then be able to pass at least 2,700 lbs/hr of condensate at ½ PSI Δ P .
CAPACITIES – Condensate (lbs/hr) PMO Pipe Orifice
Differential Pressure (PSI)
Model Code
(PSIG) Size Size 1/4
1/2
1
2
5
10
15
20
30
50
75
100
125
200 250 300
FTE-20-17-N* FTE-50-17-N
20 50
2” .937” 6100 7800 9300 11800 15900 19500 22500 26000
2” 2.125” 12800 16900 20100 25300 33000 40200 43500 46000 47800 52500
FTE-50-18-N 50 2 1 / 2 ” 2.125” 20400 25700 31000 37000 46300 55100 60300 65100 72000 82100 FTE-125-18-N 125 2 1 / 2 ” 2.125” 20400 25700 31000 37000 46300 55100 60300 65100 72000 82100 90400 97700 105000 FTE-200-16-N 200 1 1 / 2 ” .375” 950 1350 1900 2200 2700 3300 3900 4400 5300 6400 7600 8500 9400 11900 FTE-200-17-N 200 2” .75” 2700 4100 5700 7400 9900 11800 13400 14400 16400 19000 21500 23000 24500 29200 FTE-200-18-N 200 2 1 / 2 ” 1.5” 7200 12300 17400 21500 27600 32600 36000 39300 43100 49200 54700 58800 61900 74000 FTES-50-18-N 50 2 1 / 2 ” 2.125” 20400 25700 31000 37000 46300 55100 60300 65100 72000 82100 FTES-125-18-N 125 2 1 / 2 ” 2.125” 20400 25700 31000 37000 46300 55100 60300 65100 72000 82100 90400 97700 105000 FTES-300-18-N 300 2 1 / 2 ”
1.5” 7200 12300 17400 21500 27600 32600 36000 39300 43100 49200 54700 58800 61900 74000 86000100550
* Single seat orifice. All others are double seated.
As a general rule, for HX applications using steam pressures over 30 PSIG, the steam trap should be sized for 2.5X maximum condensate load at full differential pressure. Therefore, to provide an appropriate safety margin, we must select a trap that can pass 2.5 x 5,000 = 12,500 lbs/hr of condensate at 50 PSI Δ P . In addition, the steam trap must be able to handle the maximum possible inlet pressure which is 100 PSIG (the steam supply pressure). Referring to the FTE capacity chart above; the best trap to select is the FTE-200-17-N . This trap can pass 19,000 lbs/hr at 50 PSI Δ P and 4,100 lbs/hr at ½ PSI Δ P , which meets the above criteria.
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www.watsonmcdaniel.com •• Pottstown PA • USA • Tel: 610-495-5131
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