Page 121 - Boiler_Operators_Handbook,_Second_Edition

106 Boiler Operator’s Handbook

for locations throughout the United States and Canada, things that aren’t related to outside air temperature.
that will allow you to determine what temperature In most systems used just for heating you’ll find
matches full load or 100% heating load. the loads are rather consistent in the summer and you
Your local air conditioning equipment salesman can call that value a base load or summer load to which
can tell you what the design low is in your area. You can you can add the heating load. I’ve been able to gener-
also select your own number because your site could be ate formulas for steam loads that are very consistent for
as much as 5 degrees warmer or cooler than the nearest apartment buildings, nursing homes, and similar loads.
reporting station. If you have several years of logs to The formula becomes the base load plus a factor times the
check back through you should be able to find the typi- number of degree days. Each base load and degree days
cal coldest temperature. Don’t use one or even four ex- should be for a specific period of time and degree days
tremes, they’re so uncommon that nobody expects you should be for a specific time frame (hour, day, month).
to satisfy heating requirements for such temperatures. When generating a formula for heating load it’s
It’s also unlikely that those temperatures will produce important to realize that the actual steam load at any
the predicted load because they’re normally of short du- one time will seldom match the formula due to every-
ration, only that cold for an hour or two, and the mass of thing from people opening and closing doors to the
the building will limit the effect on your load. kitchen starting up in the morning while everyone’s get-
Using my home town of Joppa, Maryland, I can ting up and taking a hot shower. My experience is that
calculate my instantaneous heating load readily using the actual load will swing 25% of the maximum heating
the outdoor temperature. The extreme low for Joppa load in a typical heating plant. If you generate a formula
is 5°F, one degree cooler than the Baltimore airport, so to use, the actual load should be equal to the formula
the range of temperatures for heating at my home is value plus or minus 25%.
between 5 and 65°F where the load is zero at 65°F and Why produce a formula? Because boiler operators
100% at 5°F. To determine the percentage of load for a have to deal with us engineers and you can’t convince
given outdoor temperature all I have to do is divide the an engineer of much without some supported docu-
difference between 65° and the current outside air tem- mentation. So, by having a formula that represents your
perature by 60. My heating load is 50% at an outside air plant load conditions you can convince an engineer that
temperature of 35°F. you do know what you’re talking about.
All you need do for your location is determine the Here’s how you do it. Keep track of your load
range by subtracting the extreme low from 65. You get using steam flow or Btu meter readings, fuel meter
the current Degree Day value by subtracting the outside readings or tank soundings, preferably recorded each
air temperature from 65. Your percent load is the Degree day. Also record the average temperature or number of
Day value divided by the range times 100. Remember degree days each day. You can use a properly installed
that you convert a number to a percentage by multiply- (in the shade and away from sources of heat) high/low
ing the result by 100. For an outdoor temperature of 42°F thermometer and average those two readings to have
in Joppa my load is calculated as 65 less 42 divided by an accurate value for your site if the nearest airport isn’t
60 to get 0.3833 which times 100 gives me 38.33%. That’s consistent with you. Eventually you’ll have to convert
how you determine a common heating load. Simply average temperatures to degree days by subtracting
checking the weather forecast in the paper or from the them from 65. Any negative values should be converted
radio or television will let you know what the load will to zero. Once you have some data you can start deter-
be. I do hope you understand that I’m not implying you mining the value of the formula. If you haven’t been
should only listen to a radio or watch television during collecting data it will take you a year to collect enough
your shift, you need those ears on the plant. data to produce a reasonably accurate formula.
Of course the truth is that very few plants have Once you have data you begin by determining your
a simple heating load. Boiler plant output is usually base load. During the months of July and August, when
used for other purposes, a common one being hot water it’s never cold, you can correctly assume that there’s no
heating. Hospitals have sterilizers that run year round. heating requirement and the average steam generation,
Kitchens or cafeterias in the building can introduce sub- Btus, or fuel consumption is representative of the base
stantial loads independent of outdoor temperatures too. load. For the few of you that live in the far north, you’ll
However, they also require considerable ventilation so have to take the average of those readings on days when
much of that load is outdoor temperature related. The the outdoor temperature never got below 65°F.
heat in your steam or hot water can be used for many If you’re computer literate and can use a spread-