Page 35 - Boiler Operator’s Handbook

20 Boiler Operator’s Handbook

the system will produce a differential pressure of 15 psi as long as water is present. At the saturated condition
which can be enough to crush pressure vessels in the sys- any addition of heat will convert water to steam and
tem. To prevent that happening low temperature steam any removal of heat will convert steam to condensate.
systems usually have vacuum breakers to allow air into The temperature cannot change as long as steam and
the system. Check valves make good vacuum breakers water are both present. When the heat is only added to
because they can let air in but not let the steam out. Ther- the steam then the steam temperature will rise because
mostatic steam traps and air vents are required to let the there’s no water to convert to steam. Whenever the
air out when steam is admitted to the system. If installed steam temperature is above the saturation temperature
and operated properly low pressure steam systems can it is called superheated.
work well because the metal in the system will be hot and Superheated steam doesn’t just require addition of
dry when the air contacts it so corrosion is minimal. heat. If you have an insulated vessel containing nothing
To know how much heat is delivered per hour but saturated steam and lower the pressure then the
you determine the difference in enthalpy of the water saturation temperature drops. The energy in the steam
or steam going to the facility and what’s returning then doesn’t change so the temperature cannot drop and the
multiply that difference by the rate of water or steam steam is superheated. In applications where high pres-
flowing to the process. The basic formula is (enthalpy sure steam is delivered through a control valve to a much
in less enthalpy out times pounds per hour of steam or lower pressure in a process heater the superheat has to
water). In the case of water there’s a little problem with be removed before the steam can start to condense. The
that formula because you normally determine flow in heat transfer is from gas to the metal, without all the tur-
water systems in gallons per minute. Well, just like the bulence associated with steam condensing to a liquid.
others, there’s a simple rule of thumb; gpm times 500 It isn’t as efficient as the heat transfer for condensing
equals pounds per hour. One gallon of water weighs steam. Process heaters can be choked by superheated
about 8.33 pounds and one gpm would be 60 gallons per steam where the poor gas to metal heat transfer leaves
hour so 8.33 × 60 equals 499.8 and that’s close enough. much of the surface of the heat exchanger unavailable
Since the difference in enthalpy is about the same as the for the higher rates of condensing heat transfer. That’s
difference in temperature for water, heat transferred in right, your concept that superheated steam would be
a hot water system can be calculated as temperature in better just went out the window.
minus temperature out multiplied by gpm times 500. So why superheat the steam? We superheat steam
For steam systems it’s simply 1,000 times the steam so it will stay dry as it flows through a steam turbine or
flow in pounds per hour if the condensate is returned. engine. Without superheat some water would form as
There are times when the condensate isn’t returned be- soon as energy is extracted. The water droplets would
cause a condensate line or pump broke or the condensate impinge on the moving parts of the turbine (a familiar
is contaminated. That’s common in a lot of industrial concept would be spraying water into the spinning
plants because it’s too easy for the condensate to be con- wheel of a windmill) damaging the turbine blades. In
taminated so it’s wasted intentionally. In those circum- an engine it would collect in the bottom of the cylinder.
stances you have to toss in the heat lost in the condensate In electric power generating plants it’s common to pipe
that would have been returned. What you’re really de- the steam out of the turbine, raise its temperature again
livering to the plant under those conditions is the heat (reheating it) then returning it to the turbine just to
to convert the water to steam plus the energy required to maintain the superheat.
heat it from makeup temperatures to steam temperature. When we’re generating superheated steam some of
There are also applications where the steam is it is needed for uses other than the turbine so we don’t
mixed with the process, becoming part of the production want it superheated. In that case we desuperheat it. Heat
output. An example is heating water by injecting steam is removed or water is added to the superheated steam
into it. The amount of heat you have to add to make the for desuperheating. When water is added, it absorbs the
steam is the same as the previous example but the heat heat required to cool the steam by boiling into steam.
delivered to the process is all the energy in the steam. In most applications superheat cannot be eliminated
The one problem many boiler operators have is entirely because we need some small amount of super-
grasping the concept of saturation. Steam can’t be gen- heat to detect the difference between that condition and
erated until the water is heated to the temperature cor- saturation. As long as we have a little superheat, we
responding to the saturation pressure. Once the water is know it’s all steam. When it is at saturation conditions,
at that temperature, the temperature can’t go any higher we can’t tell how much water is in the steam.