Page 197 - Boiler_Operators_Handbook,_Second_Edition

182 Boiler Operator’s Handbook

and water mixture but you will seldom use them as well chrometric chart. Comparing design and existing condi-
as the scale on the chart for enthalpy. The chart will also tions helps you locate the source of problems. Learning
indicate that it’s based on a barometric pressure of 29.92 about psychrometrics is best done by using the design
inches of mercury but deviations associated with chang- values. You can do it before there’s a problem and then
es in barometric pressure will not alter your understand- have the design situation plotted out ready to use when a
ing and use of the chart. Changes in barometric pressure problem occurs. Now, follow the design data to produce
are not significant unless you’re maintaining the air con- one as described so you’re familiar with the process and
ditioning of the visitor’s shelter on Pike’s Peak. what you need to do to plot a system on the chart.
Your first task when you get your laminated psy-
chrometric chart is to mark it up for the comfort zones, The Air Conditioned Space (Room)
copy it and keep the copy handy. Because people typical- Engineers use the generic word “room” to describe
ly change their clothes in response to outdoor conditions the air conditioned space. Room conditions are indicated
two comfort zones have been established by ASHRAE, on the Contract Drawings and are normally expressed
the American Society of Heating, Refrigeration, and Air in dry bulb and wet bulb temperature or dry bulb and
conditioning Engineers. A significant number of samples humidity values. You’ll learn that you only need two
for both men and women were used to determine those values for a given condition of air to locate its point on
comfort zones for situations where people are sedentary a psychrometric chart. Locate the intersection of the dry
or slightly active and 80% indicated they were comfort- bulb and wet bulb or humidity values, mark it as a point
able so you will not always be able to make everyone with your trusty number 2 pencil then circle the point so
truly happy. When people are active lower temperatures it’s easy to find again. Noting where the point is within
and humidity are required. Keep it close to the origi- the comfort zone may be a key to the design engineer’s
nal design conditions (found on system drawings) and understanding of the use of the spaces served by the
you should keep a lot more than 80% happy. The winter equipment. A normal design point for office buildings is
comfort zone is outlined by dry bulb temperatures of 75°F and 50% RH (relative humidity). A home for senior
69° and 76°F at a moisture level of 31 grains per pound citizens may have higher values because seniors move
of dry air at the bottom and 68° and 74°F along a wet around less and tend to prefer warmer temperatures. If
bulb of 64°F on top. Connect the left and right corners the space is a gym you might see cooler and dryer design
of the top and bottom lines to enclose the area and that’s conditions because the occupants are not sedentary, the
called the winter comfort zone. For the summer comfort same might be said for a plant’s production line where
zone stick with the 31 grains and dry bulb temperatures workers are always busy. Indoor swimming pools pro-
of 74° and 81°F at the bottom and plot the upper corners vide some unique considerations. Sometimes the room
along the 68°F wet bulb line at 73° and 79°F dry bulb conditions are specified for the manufactured product
temperatures. Now, just because the wet bulb and dry and not for the comfort of personnel; a good example
bulb temperatures in that space indicate the room is in would be a refrigerated space for food storage. On rare
the comfort zone that’s not an excuse to tell someone to occasions a room’s pressure, relative to atmospheric, is
get a sweater or take off their jacket, there are other fac- specified as well; we’ll cover why in a bit.
tors that affect comfort and I’ll cover them later.
You will also need a set of prints or drawing files Outdoor Conditions
on the computer for the air conditioning systems you’re Also on the drawings will be the design outdoor
responsible for. They contain information that you can temperature conditions. That will be the condition of
use to isolate problems when they come up. Plotting the the air pulled in to provide ventilation. Plot that point
values from the drawings for the equipment on your and circle it then draw a straight line between the room
psychrometric chart during those rare periods when conditions and the outdoor air conditions. The room air
you have nothing to do and copying them can help you and the outside air are usually mixed in the mixing box
prepare to tackle a problem. When an addition to your of the apparatus before conditioning of the air is contin-
facility is under consideration or construction it would ued. The equipment inlet conditions listed on the sched-
help to find the design engineer during one of his vis- ule on the drawings should fall on that line when you
its and ask for copies of his charts for the systems and plot and circle it. Why? Because when you mix two air
equipment to save you time. streams the conditions for the mixture should always
In order to understand the function of a piece of air fall on a straight line drawn between the two points for
conditioning equipment you plot the values on a psy- conditions of the air streams on a psychrometric chart.