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.