Page 123 - Boiler_Operators_Handbook,_Second_Edition

108 Boiler Operator’s Handbook

thermostats are reset in the morning. When it’s very cold how many people are in the building or a certain area of
out the temperature of walls and other surfaces exposed the building. The new technical name for that is demand
to the outdoors will drop quickly and may be cooler controlled ventilation. If you don’t have the advantage
than the 55°F. Because partitions, floors and ceilings, of one of those specialized controls you’ll probably
furniture, etc. cooled slower, they might still be warmer have systems like time clocks that set the ventilation at
and help offset the effect of the colder walls. Warm-up a minimum when people aren’t supposed to be in the
loads can be higher than heating loads if ventilation is building and adjust them to a value for full occupancy
not controlled. Unless the thermostat settings are timed the rest of the time.
to compensate for the variation in storage temperatures Any of those controls should be set for minimum
you may get some complaints in cold weather or waste ventilation air during the period when the building is
heat in milder weather. warming up in preparation for occupancy. That way
Ventilation loads are primarily people loads. For you avoid the ventilation load while handling the warm
all practical purposes a facility has to introduce 20 cfm up load to limit the load on your boilers. It also makes
(cubic feet per minute) of fresh outside air for every per- no sense to heat up cold outside air to warm up walls.
son in the facility. There are more specific requirements The ventilation should increase for a short period before
that vary with the Jurisdiction but that is a good rule of people start entering the building to flush out the stale air.
thumb. Many older facilities may still be set for ventila- Except for some process requirements the hot wa-
tion rates as low as 5 cfm per person so it pays to check ter heating load is largely a function of people activities.
the actual values before trying to determine the heating People have a direct relationship with hot water needs
load they create. The amount of heat required for ventila- for cooking, showers, and washing. Each of those hot
tion air is easy to determine, it’s the total of ventilation air water uses is sporadic, occurring at specific (sometimes
in cfm multiplied by a constant of 1.08 and the difference inconsistent) times so they’re more on and off than a
between the outdoor air temperature and room tempera- constant load. There are several means of producing
ture. As an example, for 100 people you need 2,000 cfm of hot water and satisfying the irregular loads so there’s
0°F outside air which requires 162,000 Btuh (2,000 × 1.08 × a section in this book devoted specifically to hot water
(75 – 0). If you recall our earlier discussion that’s equiva- heating. When the hot water is heated by many heat ex-
lent to about 162 pounds per hour of steam. Note that we changers throughout the facility you have little control
used 75° not 65° because we can’t count on the heat from of those loads and you’ll have to monitor plant loads to
people, etc. to cover that portion of the load. determine their effect.
In areas containing a high concentration of people An unusual load that I encountered at one chemi-
(movie theaters, stadiums, office buildings) the ventila- cal production facility a few years ago is a rain load. I
tion load can be the largest single load of the facility. The was collecting nameplate data at one of the boilers and
core of a building, in the middle where there are no out- found myself almost run over by the operator who was
side walls, and floors and ceilings separate them from suddenly rushing around trying to get that boiler operat-
other occupied spaces, the ventilation air can produce ing. Once he had it on line I asked what the rush was all
a heating load that would not exist without it. If your about. “It’s about to rain” was his simple reply. That plant
facility has large changes in the number of people from experienced a 30,000 pph increase in boiler load every
day to night or over weekends you should see swings in time it rained! Many district heating plants experience
load due to changes in the ventilation air. a delayed rain load which is due to rain leaking into the
Of course many older buildings don’t adjust venti- manholes and tunnels containing the steam lines. It’s a
lation air depending on building occupancy. Yours may load that indicates inadequate or ineffective maintenance
be one that continues full flow ventilation at night when and shouldn’t be as significant as that one plant. You may
There are only a few people, if any, in the building. If have one and it shouldn’t be difficult to identify it.
you have a way of closing that off at night (you’ll never Finally, there are production loads. These are
be able to get zero ventilation) you’ll save a lot on heat- requirements for heat to warm raw materials for pro-
ing all that air unnecessarily. duction, to convert the product to another form (like
Modern facilities are using a combination of se- melting it) or steam actually injected into the product to
curity and air conditioning controls to determine how alter it. They can include tank heating and heat tracing
many people are in the building and adjusting ventila- where heat is used to keep the product in tanks and pip-
tion loads accordingly. Another method is measuring ing hot enough that it will flow or remain a liquid. Those
the carbon dioxide content of return air which indicates heating requirements are independent of actual produc-