Page 105 - Boiler_Operators_Handbook,_Second_Edition

90 Boiler Operator’s Handbook

reduces the amount of blowdown and the amount of don’t have blowdown heat recovery system then you are
energy removed from the boiler in the blowdown water. wasting a valuable resource. With the exception of low
Maintaining a continuous blowdown also makes temperature hot water heating boilers boiler blowdown
it possible to use the heat in the water removed from is discharged to a flash tank where some of the energy
the boiler to preheat the makeup water that has to be in the high temperature water is used to convert some
added to replace water removed by blowdown. That’s of the water to steam that is vented to atmosphere. The
accomplished with blowdown heat recovery systems. remaining water, now at 212°F is conveyed to a drain.
There are two types of blowdown heat recovery systems The Plumbing Code clearly restricts the temperature
depending primarily on boiler operating pressure. One entering a drain to 140°F so we use high quality drink-
simply includes a heat exchanger arranged for counter- ing water to cool it by dumping more water down the
flow with the continuous blowdown on one side and drain. We refer to it as quench water and it’s normally
makeup water on the other. These are only employed high quality drinking water that goes down the drain to
with low-pressure steam systems and HTHW. For high- sewer along with the blowdown. The typical blowdown
pressure steam systems the blowdown heat recovery heat recovery system cools the blowdown to about 90°F
equipment consists of a flash tank and a heat exchanger. so quenching with fresh water isn’t required. I can’t
The high-pressure system blowdown heat recovery oc- tell you how much water costs in your particular area
curs in two stages. The high temperature boiler water because the cost differs substantially. Where I live in the
enters the flash tank which operates at a much lower Baltimore Metropolitan area we’re blessed with a rather
pressure than the boiler so some of the blowdown water low cost of around four dollars per thousand gallons of
flashes into steam. That steam is used in the deaerator to water. That charge is a combination of water and sewer
help preheat the boiler feed water. The remaining water charges and a large proportion of it recovers the expense
then passes through a heat exchanger to preheat the of processing the sewage. I am aware of jurisdictions
makeup water. where the combined water and sewer cost is over $25
Automatic continuous blowdown control systems per thousand gallons. Because the engineers don’t think
come in two types. The least expensive and more com- about the fact that blowdown heat recovery eliminates
monly used has a sensor to detect the TDS in the boiler that water waste they fail to provide a suitable blow-
water and opens the blowdown control valve at set time down heat recovery system in many boiler plants. When
intervals for short period of time to allow the water in wasted drinking water costs are considered I have yet
the boiler to reach the sensor. Then the valve is held to fail to justify a blowdown heat recovery system. And,
open until the TDS in the boiler drops below the control- because potable water is such a valuable and rapidly
ler setting. This is basically an intermittent controller. depleting resource I insist on having one anyway.
The other type modulates the blowdown control valve So, don’t disparage that system; make sure it’s
to maintain the TDS at set point. When the plant is fit- operating properly and, if it isn’t, point out to the pow-
ted with a continuous blowdown heat recovery system ers that be how important it is. The amount of blow-
intermittent operation of the blowdown is undesirable. down is determined by dividing the average TDS of the
With the intermittent type control the solution is to in- boiler feedwater by the average TDS level maintained
stall a small control valve bypassed by a manually set in the boiler. The result multiplied by 100 is percent
valve so the Operator can adjust the manual valve to blowdown. Determine how much is flashed into steam
handle the bulk of the blowdown and allow the auto- by dividing the enthalpy of the boiler water less the
matic control to cycle the automatically controlled valve enthalpy of saturated water at the pressure in the flash
to maintain the TDS level. That produces a constant tank by the enthalpy of steam at the pressure in the flash
flow of blowdown with small intermittent increases in tank. Multiply the result of that calculation by 100 to de-
the flow to better match a constant flow of makeup. You termine percent of flash steam then subtract that value
increase or decrease the constant flow to keep the auto- from 100 to determine the percent going down the drain
matic valve open about 50% of the time. or through a heat exchanger. The heat lost to blowdown
Normally the operator doesn’t have to be con- is determined by dividing the difference between the
cerned with the economics of blowdown heat recovery boiler water enthalpy and the feedwater enthalpy by
but this is one system that engineers fail to understand. the difference between steam enthalpy and feedwater
Several engineers have told me that a blowdown heat re- enthalpy. That result times 100 is percent heat in the
covery system can’t be justified economically. The prob- blowdown. Finally, to determine the quantity of quench
lem is they’re only looking at the heat recovery. If you water divide the difference between the temperature of