Load Profiles; Big Changes, Big Challenges

In my last post, I began to look at the challenges faced by a
fairly typical
VAV system
(Figure 1) operating in a Midwestern climate (Figure
2) on a day where the conditions were no-where near the summer or
winter design conditions or extreme conditions (Figure 3).


Figure 1 – System
Diagram
 

Figure
2 – The St. Louis Climate on January 25, 2005


Figure 3 – St. Louis Design Conditions

(Screen shot from the Climate Tool in Akton Psych
Chart
)

As some of you may have already deduced from the weather
pattern, in general terms, the day in question could be
particularly challenging because the system will start out dealing
with subfreezing air and then transition to operating using
preheat, then an economizer process only, then an integrated
economizer cycle, then back again as the day progresses. In the
morning, the changes are more gradual than they are in the
afternoon. All of this is in response to a frontal passage (note
the shift in wind direction and dew point over the course of the
day) and the interactions of the sun with the increasing cloud
cover that occurred as the day progressed.

As a side note, if you are interested in taking a closer look at
what happens when a front goes through, micro climates, and the
general difficulties associated with trying to measure what is
actually going on outside, then you might find the documents at the
following links to be of interest. They are the pages of a little
memo I wrote after a lab session at the PEC.

Micro
climates, Fronts, and Measuring Outdoor Conditions, Page
1

Micro
climates, Fronts, and Measuring Outdoor Conditions, Page
2

Micro
climates, Fronts, and Measuring Outdoor Conditions,
Page
 3

Micro
climates, Fronts, and Measuring Outdoor Conditions, Page
4

Micro
climates, Fronts, and Measuring Outdoor Conditions, Page
5

Micro
climates, Fronts, and Measuring Outdoor Conditions, Page
6

Micro
climates, Fronts, and Measuring Outdoor Conditions, Page
7

(I’m just figuring out how to use
Google Documents 
and, to get the file size down to
something I could up load, I had to post one page at a time. The
graphics look a bit grainy on a screen but if you print the page,
its pretty legible.)

In just one day, the system we are considering will need to deal
with ambient temperatures that swing over 30°F. In contrast,
there are places in the country like San Francisco where the
temperature hardly swing that much in the course of a
year
let alone a day. Reacting to this wide range of
environmental variables in the course of one day requires a system
that is well designed incorporating robust control sequences and
precise, well adjusted control components which have been
commissioned and tuned to work over the entire range of conditions
that will be encountered.

Given this perspective, what subtle details of the system design
and operating sequence do you think will come into play over the
course of the day? In subsequent posts, I will take a look at some
of the issues that have come up on systems of this type in the
course of my career when they were challenged with some of the
conditions that occurred on January 25, 2005.

This entry was posted in Weather and Climate Interactions with Buildings and Systems. Bookmark the permalink.

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