Page 122 - Boiler_Operators_Handbook,_Second_Edition

What the Wise Operator Knows 107

sheet program then determining the formula is rather generate and that’s the people load. The use of the facil-
easy. If you aren’t capable of doing that, try to get help ity will determine most of the people load. A nursing
from a friend that is. Should those options fail, get a home or prison will have a relatively constant people
cheap calculator and go at it. Create a table of values load because the people are always there and doing
using your recorded data. In the first column put all the same thing. Apartment buildings will have a more
the degree day readings. You can precede that one with variable people load, one of the more difficult to de-
such values as average outdoor temperature or the low termine. College dormitories are another story because
and the high if those are the values you recorded then all the students are on the same schedule; if you know
use them to calculate the degree days. In the second the schedule the loads are predictable despite the fact
column record the steam generation, Btus or fuel use that they will vary considerably. Simply picture all the
for that day. For the third column, calculate the heating students rising at the same time to get ready for class,
load by subtracting the base load value from the value taking showers and washing then vacating the build-
in the second column. If any of the results are negative, ing; they will create a short-term peak load during that
substitute a zero for that result. For the fourth column, time. If the building was equipped with night set-back
calculate the heating ratio by dividing the heating load thermostats the load swing will begin with the warm-up
value of the third column by the number of degree days and end with the students leaving for class.
in the first column. When people are present your loads will be higher
The values in that fourth column should all be and when they’re absent they’ll be lower. In an office
close to each other. If you run into one, or some, that building, for example, everyone but the cleaning staff
seems to be significantly different and you can’t resolve goes home in the evening so you don’t have to heat the
it, cross out that row of data. After eliminating several building to a comfortable 75°F at night. In that case you
rows from one set of plant data I finally realized that may have all the thermostats set back to 55°F. Under
they were every seventh one and I was looking at data those circumstances your peak heating load isn’t based
taken on weekdays where the fuel use covered the on 65°F, it’s based on 55°F. The difference between the
weekend. Simply dividing the odd result by three made thermostat set point of 75 during the day and the 65°F
the data useful. Count the number of rows of good data base we use for calculating degree days is covered by
(each daily set of readings) and write the number down the people themselves (an office worker puts out about
at the bottom of the page. 550 Btuh of heat); then there’s the equipment they’re us-
Add up all the values in the fourth column and ing (computers, etc.), and the lights.
divide by the count of good data rows to get an average People have other effects on heat load depending
of the values in the fourth column. Your load formula on what they’re doing. When everyone is arriving for
can now be determined as equal to the base value plus work in the morning they manage to pump a lot of the
a factor times degree days and the factor is that average building heat out and the cold in when passing through
value. To get an idea of how accurate it is you can use doors. I know one building where they set the lobby
it to calculate another value (put it in the fifth column) thermostat for 85°F about an hour before starting time
then compare that to the steam generation, Btus or fuel so they store some heat in the area to offset all the cold
use in the second column. When using monthly data I air that comes in with the arriving workers.
find I’m normally within 5%, daily data are within 10% Store heat? Yes, everything can store heat to one
and hourly data are within 25% of the actual values. degree or another. You have to raise the thermostat set-
Continuing to record data and adjust the base and factor ting to 75°F in that office building well before the work-
values improves the accuracy. ers start arriving or it will still be 55°F when they arrive
I use those formulas to compare the performance and you won’t hear the end of it. It takes time for the
of a building at different times. Adjusting for the num- temperature to return to 75°F because the air in the room
ber of degree days corrects for variations in outdoor air has to warm up the walls, floors, ceilings, furniture,
temperature. It helps me detect when something went etc., from 55° to 75°. How fast it warms up depends on
wrong in a boiler plant or the degree of improvement in the weight of the materials and their specific heat, the
efficiency a particular installation provided. You can use amount of heat required to raise the temperature of the
the formula to predict loads and to detect problems with substance one degree Fahrenheit. The appendix has a
the plant. table of specific heats for various materials.
There’s also another factor that changes your When the outdoor temperature is mild the mate-
heating load and influences other uses of the heat you rials in the building may never get to 55°F before the