Page 120 - Boiler_Operators_Handbook,_Second_Edition

Chapter 3

What the Wise Operator Knows

the temperature in the facility and the outdoor air tem-
To know is to perceive or understand clearly and perature. For more than half a century we have used
with certainty. Knowledge is based on training, experi- Degree Days as a measure of the heating load, normally
ence, and the ability to use that training and experience on a month to month basis. Degree Days are, as the units
to develop perceptions of outcomes that haven’t oc- imply, degrees multiplied by days. They are calculated
curred. When you are in control of a facility that has the for a particular day by subtracting the average outdoor
potential to level a city block under the worst of circum- temperature during the day from 65°F. A typical ex-
stances that certainty becomes very important. ample would be a day with a high of 50°F and a low of
40°F where the average is 45°F and the Degree Days are
20 (65-45). Why use 65°F? If you think about it you never
KNOW YOUR LOAD really need to turn the heat on until the temperature
drops below 65°F so it’s reasonable to say that the heat-
The product generated by a boiler plant is steam, ing requirement for a 65° day is zero. The numbers for
hot water, or similar products that deliver the heat to the each day are combined to provide the number of Degree
facility served by the boiler plant. The load is the rate at Days for a period of time.
which heat must be delivered to the facility served by The numbers for all the days in a heating season
th
th
the boiler plant. Your normal concern (remember the (normally October 15 to March 15 ) are added up to
priorities) is to maintain steam pressure or supply (re- provide the number of Degree Days in a season. We
turn) water temperature. Do you know your load? engineers talk of a geographical region in terms of their
When I ask that question I seldom get an answer. seasonal degree days. We’ll also compare degree days
When I’m more specific by asking for a peak load, low for one heating season to an average that’s based on a
load, weekend load, winter load, or summer load the collection of data over more than a century.
result is usually the same. Most of the time the operator You may still find reports of the number of degree
moves to a recorder or log book to try to derive an an- days in the newspaper and on your fuel and electric
swer from there. I’ve never understood why operators bills. Some utilities now list the average temperature for
didn’t know how much heat the facility required at a the month which may also be converted to degree days.
particular time because they have to know it to operate The number of degree days is about equal to the number
the plant properly. You have to know your load. of days in the month multiplied by the difference be-
Let’s face it, when it’s late Friday evening near the tween the average temperature and 65.
middle of October and the weather forecast calls for a Today we will typically preface Degree Days with
stiff cold front coming through before the end of your the word “heating” because there is an effort to establish
shift you better know whether or not you will have to a comparable value for Cooling Degree Days. In Sep-
start another boiler. You can’t always count on the chief tember and May you have to read the paper carefully to
leaving instructions either. You have to know your load. ensure you’re reading heating degree days. It could be
Your heating load is one of the first things you a hot month that produced more cooling degree days so
need to know because the weather is fickle and changes that’s what they report.
without notice. Maybe your plant is simply a heating Problem is, Degree Days are reported after the
plant so it’s the most important load for you to know fact so they’re not available for predicting a boiler load.
about. On the other hand you could be in a production However, the same logic can be used to predict load.
facility where the weather has a minimal effect on your Whether your plant is strictly for heating, or provides
total load. Regardless, it’s a load you should be aware of heat for other purposes as well, you can determine a
and be able to quantify. heating load based on outdoor air temperature. We have
The amount of heat needed to maintain tempera- the 65°F value for zero load and there are published ex-
tures in a facility is a function of the difference between treme temperatures, data are provided in the appendix
105