ARE346P: HVAC Design Tyler BolingerFinal Design Report Priscilla
Williams
Project Statement
Our project was to size components for a VAV system. A
variable-air-volume (VAV) system controls the dry-bulb temperature
in a zone by varying the supply-airflow rate rather than the
supply-air temperature. This is done through a space thermostat at
a set point that sends a control signal to a VAV box which then
controls the flow with dampers. If the room temperature is greater
than the set point temperature, the dampers are opened to increase
the supply-airflow rate. If the room temperature is less than the
set point temperature, the dampers are closed to decrease the
supply-airflow rate. If there is no difference in the room
temperature and set point temperature, the dampers do not change.
At times, different zones of a building will experience peak loads
while other zones do not. When this happens, the zones experiencing
peak loads will borrow the extra air from the other zones. VAV
systems typically cool buildings and do not provide simultaneous
heating and cooling in different zones. When simultaneous heating
and cooling is needed, secondary systems provide the required
heat.
The ASHRAE Handbook describes several advantages and design
precautions for VAV systems. For example, VAV can offer inexpensive
temperature control when combined with perimeter heating systems.
Also, the variation in loads can be made useful VAV systems. In
addition, these systems are self balancing and easy and inexpensive
to change zones for new loads or users. But when designing these
systems, special care must be taken to ensure acceptable operation
at off-peak supply-airflow rates. Also, fan controls should be
considered to reduce power consumption and noise, but only for
systems where this option is economical. Finally, in zones with
fluctuating loads, heating or reheat should be considered to
prevent excessive space humidity.
Many HVAC systems use too much energy. Built-up VAV systems can
improve system performance, energy efficiency, cost effectiveness,
and occupant comfort of these systems by following design
recommendations made based on findings of a three-year study of
five California office buildings. Careful sizing of VAV boxes,
minimizing VAV box minimum airflow setpoints, and controlling VAV
boxes using a “dual maximum” logic can significantly save fan and
reheat energy because typical systems operate at a higher than
necessary airflow.
Project Outcomes
· Determine heating and cooling loads for all zones
· Size VAV boxes for all zones
· Size Reheaters for all zones
Building Description
The building in question consists of two levels with five zones
each (North, East, South, West, Core). A diagram is shown in Figure
1.
Figure 1: A two story office building with five zones will have
a VAV system.
A sample VAV schematic of one floor is shown in Figure 2.
Figure 2: VAV system schematic for one level.
Methodology
Based on the heating and cooling loads of the entire building,
shown in the attached spreadsheet, the conditions of each zone are
established. The VAV boxes are sized first given that the outside
air is cooled to 55 degrees Fahrenheit before passing through the
VAV box and reheater. Based on this, and the 75 degree set point of
the room, the necessary maximum volumetric airflow can be assessed.
This is the size of the VAV box. If we assume that the whole
building will be run be one ACU, for the sake of simplicity, we can
also size the ACU components for heating and cooling loads.
With the VAV boxes sized, we can sum all the airflow rates to
calculate the total airflow rate through the ACU. We will assume
50/50 mixing of the outdoor and return air to reduce the load on
the reheating units while keeping minimum fresh air requirements.
For heating loads in the winter, after mixing, the supply air will
be at 50 degrees. A heating coil will heat the air up to about 72
degrees. The air will then pass through an evaporative cooler,
until saturation, which happens at 55 degrees. This air will pass
through the ducts to the VAV boxes and reheaters where it will be
heated to the room set point of 75 degrees and 50% relative
humidity. Heating loads will be read off of a psychrometric
chart.
For cooling loads, it’s a little simpler. Outdoor and return air
mix at 85 degrees, then pass through a cooling coil until
saturation at 55 degrees. After this temperature is reached, the
reheater will heat the air to the set temperature of 75 degrees.
Again, the cooling loads will be read off the psychrometric chart.
These charts will be given in the data.
Results
Our first step was to size the VAV boxes. As stated before, we
did this via the peak cooling loads for each zone. The cooling load
for the plenum on each floor was distributed throughout the 5 zones
on each floor for the sake of convenience. The required maximum
flowrate for each zone was calculated using equation (1):
Q = macp(t2 – t1) (1)
The mass flowrate was found, then converted to a volumetric
flowrate, assuming a density of 0.078 lbm/cf for air. Delta T was
taken as 20 degrees difference between the duct and the zone. Table
1 lists the peak cooling load and resulting volumetric flowrate for
each zone’s VAV box.
Table 1: Peak cooling Loads for each zone and the respective
volumetric flowrates.
Zone
Flowrate
Q cool
South Level 1
2381 cfm
53 KBTU/Hr
East Level 1
2337 cfm
52 KBTU/Hr
North Level 1
1356 cfm
30 KBTU/Hr
West Level 1
2228 cfm
50 KBTU/Hr
Core Level 1
1584 cfm
35 KBTU/Hr
South Level 2
2854 cfm
63 KBTU/Hr
East Level 2
2520 cfm
56 KBTU/Hr
North Level 2
1634 cfm
36 KBTU/Hr
West Level 2
2483 cfm
55 KBTU/Hr
Core Level 2
2944 cfm
65 KBTU/Hr
With these values, we can calculate the total CFM passing
through the ACU. The sum is roughly 22,300 CFM. With the flowrate,
we can size the heating and cooling components of the ACU. From a
psychromentric chart, we determine that the difference in enthalpy
for the heating coil is about 5btu/lbm Using equation (2):
SQs + SQL = ma(h2-h1)(2)
We can conclude that the max load for the heating coil is about
520 Kbtu/hr.
Passing through the evaporative cooler is an adiabatic process,
so there is no load to calculate.
The cooling load is calculated in the exact same way. The
enthalpy difference over the cooling coil from the psych chart is
about 10 or 11 btu/lbm, we will assume 10. Given the same
volumetric flowrate, this will require a cooling load twice that of
the heating. The calculated load on the cooling coil is about 1400
Kbtu/hr.
The reheater boxes only activate once the VAV dampers have
reached the minimum fresh air flowrate required for each zone. The
enthalpy difference is always the same (5 btu/lbm), therefore, the
heating load is dependent on the flowrate through the reheater.
With 50% mixing, the actual flowrate will be twice the required
minimum for fresh air. Table 2 shows the loads calculated for each
zone.
Table 2: Reheat loads for each zone and the respective minimum
flowrates.
Zone
Minimum CFM
Q reheat (kBtu)
South Level 1
328
7.675
East Level 1
328
7.675
North Level 1
328
7.675
West Level 1
328
7.675
Core Level 1
1692
39.59
South Level 2
296
6.926
East Level 2
296
6.926
North Level 2
296
6.926
West Level 2
296
6.926
Core Level 2
1870
43.758
Conclusion
We can conclude that there are endless possibilities for this
system to heat and cool. Levels of cooling and heating as well as
mixing levels of the return and supply air can be adjusted based on
design parameters in order to reduce required loads in this system.
With more extensive analyzation, the total cooling and heating load
using a different combination could be minimized. To make design
less complicated, we have assumed many variables and simply
calculated the loads based on these conditions. But based on this
model of design, the system could be sized for any desired indoor
environment.
Appendix:
Charts and Data
Figure 1: Cooling Loads
Figure 2: Heating Loads
Table 3: Zone Loads
Table 4: Fresh air Reqs.