Engineering Analysis

   AirBoss Smart Design (US Patent No. 10,473,348)

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 Engineering Analysis

 

Simulations with Solidworks® 2016, psychrometric analyses, and cooling performance data from conventional packaged rooftop units were used to compare AirBoss systems to “conventional” systems in cooling and heating modes.

  

System Design: 

A conventional system design relies on a single RTU (Rooftop unit) to condition a zone with its own thermostat.  When there is a call for cooling or heating by the thermostat, the RTU will enable cooling or heating. As shown in Figure 1, the supply air is dispersed through the space by a 4-way diffuser.

Figure 1. Typical system design.

For an AirBoss system, an RTU feeds supply air into the return end of the AirBoss unit. The AirBoss unit then circulates the air through the entire space as shown in Figure 2.

Figure 2. AirBoss Design

 

Cooling Analysis

Theoretical Analysis: 

 

Using a typical design, assuming a space setpoint of 75°, the average temperature of the space is 77.5°.  This occurs due to the heat gain rising from the lights, people, and other sources when the RTU cycles off.  When there is a call for cooling, the RTU has to cool the entire space by 2.5°. It should also be noted; the cold supply air is cooling off the warmest air first and then migrating downward to the floor as shown in Figure 3.  

Figure 3.  Space temperature profile of a typical system. 

Looking at Figure 4, the return air temperature for the RTU is 80° and the unit must deliver 55° air to overcome the heat gain from the space.  

Figure 4. Entering and leaving air temperatures of an RTU.

Using a simple sensible heat calculation, Equation 1, the required tonnage for the RTU can be calculated. 

MBHs=CFM×∆t×1.0812,000

Equation 1. Sensible heat.

For this example, we will assume the supply airflow is 10,000CFM.  

MBHs=10,000×25℉×1.0812,000=22.5 tons

Knowing the calculated load equals 22.5 tons, a 25-ton unit is selected for cooling the space.  However, if we look at the actual performance of a 25-ton unit under these conditions, the unit has a decreased unit capacity of 10% to 22.2 tons.  This is due to the additional motor heat produced by the required supply ductwork with a 4-way diffuser and the 80° return air temperature. Not only is the unit capacity reduced, but also the compressor power draw increases as the return air temperature increases.  

For an AirBoss design, the average temperature of the space is a constant ~75° as well as the return air temperature due to the air changes produced by the AirBoss as shown in Figure 5 and Figure 6.

 

Figure 5. Space temperature profile of an AirBoss system.

Figure 6. Entering and leaving air temperatures of an RTU.

When there is a call for cooling or heating, the RTU only has to cool/heat the entire volume by 1°. Using Equation 1 and assuming the return air temperature is 75°, the calculated cooling requirement is shown below.

MBHs=10,000×20℉×1.0812,000=18 tons

Since the calculated requirement is 18 tons, a 20-ton unit is selected to handle the cooling requirement of the space.  Under these conditions, the unit capacity is only reduced by 3% to 19.4 tons. This reduction in capacity is caused by the airflow of the system being larger that the nominal airflow of the unit (8000 CFM). Even though there is a slight reduction in capacity, the unit is more efficient than a unit in a conventional design.  

 

Simulation Analysis:

Simulations with Solidworks® 2016 were used to compare the AirBoss design to a conventional design.  The design parameters are shown below in Figure 7.

Figure 7. Design Parameters.

Both systems were modeled with 66,000 CFM and 58° off the evap.  The conventional system had the following equipment shown along with the performance in Table 1.  The performance of each unit was interpolated from the performance tables in Appendix A from the AirBoss presentation. 

 

Conventional

System

95 Ambient
Tonnage # of Units Airflow MBh SHC EAT LAT
25 4 7500 301.45 214.60 85/70 58/57.5
20 6 6400 261.85 191.85 85/70 58/57
     
Total 220 10 68400 2776.90 2009.50

Table 1. Conventional system equipment list and performance.

 

From the Table 1, it should be noted, the 25 ton unit is running at 300CFM/ton and the 20 ton unit is running at 320 CFM/ton to achieve proper performance with the entering air conditions.   Using the simulation software, the conventional system was able to cool the space to 75° in 14 minutes. The entering air temperature at the return of the unit remained between 83°-84° and is shown in Figure 8.  The simulation video is labeled as 4way84.avi.

  

Figure 8. Space thermal profile with a conventional System.

 

    Knowing the amount of time to cool the space and the unit(s) FLA, the kW/hr for the conventional system on design day was 48.60.  The supply fan esp. was assumed at 1.5” w.c. to account for the 4-way diffuser. The energy analysis is shown in Table 2. 

Conventional System Design Day
Tonnage # of Units Compressor 1 Compressor 2 Condenser Fan Supply Fan Unit FLA AirBoss FLA Unit Total Amps kW kw/hr 
25 4 18.60 18.60 5.00 6.30 48.50 N/A 194.00 89.24 20.82
20 6 20.00 13.20 5.00 4.93 43.13 N/A 258.78 119.04 27.78
 
Total  220 10   452.78 208.28 48.60

Table 2. Conventional system energy usage.

 

 

Using the same design parameters, the AirBoss System was modeled with the equipment list shown in Table 3.

AirBoss

System

95 Ambient
Tonnage # of Units Airflow MBh SHC EAT LAT
25 5 9000 283.70 185.16 77/67 58/57
20 3 7200 248.20 162.50 77/67 57/57
   
Total  185 8 66600 2163.10 1413.30

Table 3. AirBoss System equipment list and performance.

 

From the table, the 25 and 20-ton units are running 360 CFM/ton.  This allows the AirBoss system to supply the required 66,000 CFM with a 58° evaporator leaving temperature.  Since the AirBoss units are constantly de-stratifying the air, the return temperature back to the RTU’s is less.  This allows the design engineer to reduce the amount of compressor capacity required. An airflow simulation showing the space profile is shown in Figure 9.  The simulation video is labeled A84.avi.  

Figure 9. Space thermal profile with AirBoss System.

 

During design day, the simulation calculated that the RTU’s must run for 16 min to satisfy the space.  This increased runtime in addition with the AirBoss units running continuously; the kW/hr is calculated to be 51.80.  This is a ~6% increase in energy usage compared to the conventional system. The energy analysis is shown in Table 4

 

 

AirBoss System Design Day
Tonnage # of Units Compressor 1 Compressor 2 condenser fan supply fan Unit FLA AirBoss FLA Unit Total Amps kW kw/hr 
25 5 18.60 18.60 5.00 6.89 49.09 8.30 245.45 112.91 30.11
20 3 20.00 13.20 5.00 4.14 42.34 4.98 127.02 58.43 15.58
 
Total  185 8   13.28 372.47 171.34 51.80

Table 4. AirBoss System energy usage.

 

Even though the AirBoss system uses more energy during design days, those days make up about 1% of the cooling season.  In addition, the amount of compressor capacity required for a design day is reduced by ~20%, which decreases the upfront capital equipment cost. To get a more accurate energy reduction for the entire cooling season, the simulations were modeled for part load conditions at 150 tons as shown in Figure 10.  The RTU’s performance for a conventional system is shown in Table 5 and Table 6 shows the AirBoss system.

Figure 10. Part load design parameters at 150 tons.

 

Conventional

System

85 Ambient
Tonnage # of Units Airflow MBh SHC EAT LAT
25 4 7500 294.20 204.04 78/67 54/53.9
20 6 6400 261.22 170.34 78/67 54.5/53.5
     
Total  220 10 68400 2744.12 1838.20

Table 5. Conventional system equipment list and performance.

 

AirBoss 

System

85 Ambient
Tonnage # of Units Airflow MBH SHC EAT LAT
25 5 9000 283.70 185.16 75/65 55.5/54.5
20 3 7200 258.43 169.00 75/65 53.5/52.9
   
Total  185 8 66600 2193.79 1432.80

Table 6. Conventional system equipment list and performance.

 

The simulation for both systems show the space is satisfied at 72.5° in 11 min.  The space profile is shown in Figure 11 with the conventional system on the left and AirBoss system on the right.  The simulation video for the conventional system and AirBoss system are labeled 4way78.avi and A78.avi, respectively.   

Figure 11. Space thermal profile for a conventional system and AirBoss system.

 

As you can see from the simulations, the return temperature to AirBoss system is ~3° less than a conventional system.  Using equation 1, the AirBoss systems decreases the required compressor capacity by ~20 tons. This in turn reduces the amount of energy used by 5% even with the AirBoss units running continuously.  The energy analysis is shown in Tables 7 and 8.  

Typical

Design

85 Ambient
Tonnage # of Units Compressor 1 Compressor 2 Condenser Fan Supply Fan Unit FLA AirBoss FLA Unit Total Amps kW kw/hr 
25 4 18.60 18.60 5.00 6.30 48.50 N/A 194.00 89.24 16.36
20 6 20.00 13.20 5.00 4.93 43.13 N/A 258.78 119.04 21.82
 
Total  220 10   452.78 208.28 38.18

Table 7. Conventional system energy usage.

 

 

AirBoss

Design

85 Ambient
Tonnage # of Units Compressor 1 Compressor 2 Condenser Fan Supply Fan Unit FLA AirBoss FLA Unit Total Amps kW kw/hr 
25 5 18.60 18.60 5.00 6.89 49.09 6.65 245.45 112.91 20.70
20 3 20.00 13.20 5.00 3.10 41.30 3.99 123.90 56.99 10.45
 
Total  185.0 8   10.64 369.35 169.90 36.04

Table 8. Conventional system energy usage. 

Another simulation was run for a 115 ton part load day and the energy saving was calculated to be 5% less for the AirBoss system.  Please refer to the AirBoss presentation for the results and energy analysis for this simulation.  

 

Heating Analysis

For high-ceiling open interior facilities, the roof surface is the largest source of heat loss.  The amount of heat loss is calculated using Equation 2.

qr=U×A×∆tr

where,

     qr = Heat loss through the roof (Btu/hr)

     U = Average heat transfer coefficient for the roof (Btu/hr·ft2·°F)

     A = Roof area (ft2)

  tr= Difference of the indoor roof temperature and outdoor temperature (°F)

 

Based on the equation, the heat loss through the roof can be reduced by decreasing the indoor temperature.   Using AirBoss, this can be accomplished by destratification. A conventional system allows heat from the space to rise and increases the indoor temperature at the roof.   To determine the indoor temperature at the roof for both systems, simulations with Solidworks® 2016 were performed. The space setpoint was set at 75° with the design parameters in Table 9.  

Zone Load Details Sensible (Btu/hr)
Wall Transmission 39060 (ft2) -163,622
Roof Transmission 102300 (ft2) -573,800
Window Transmission 4000 (ft2) -112,000
Overhead Lighting 112530 w 268,762
Infiltration 12450 CFM -996,000
Total Zone Load -1,915,911
RTU’s Heat Input 1,920,000

Table 9. Zone Loads

 

For the AirBoss simulation, the average temperature was ~76° when destratification has occurred as shown in Figure 12.    

Figure 12. Space thermal profile with AirBoss System

 

The result seems reasonable since the average of the indoor air temperature should be greater than the space setpoint.  This is also depicted in Figure 13 as profile B.  

Figure 13. Characteristic vertical profiles of indoor air temperature determined by location of warm air supply.1

 

The conventional system simulation’s results were more inconsistent and displayed wide temperature swings as shown in Figure 14.  The calculated average temperature at the ceiling was ~84°.

Figure 14. Space thermal profile with Conventional System

 

To verify the results as suggested by Pignet and Saxena2 from their case study, Equation 3 was used to solve for the indoor temperature.

 

tiad=(tahArHah)+tsArHbhAr×(Hah+Hbh)

Equation 3. Vertical profile of indoor air temperature

 where,

tiad = Average indoor air temperature (AirBoss) (°F)

tah = Indoor air temperature at roof (°F)

  Ar = Roof area (ft2)

Hah = Height above 4-way diffuser to the roof (ft)

  ts  = Space setpoint (°F)

Hbh = Height below 4-way diffuser to the floor (ft)

76°=(tah×102,300×5)+75×102,300×25102,300×(5+25)

tah=85°

After verifying the results, the heat loss at varying outdoor air conditions can be calculated using Equation 2.  For example, the outdoor air temp is 28°, space temperature setpoint is 75°, and the roof heat transfer coefficient is 0.79, the heat loss for each system is calculated below.

qAirBoss=0.079×102,300×(76°-28°)

qAirBoss=387.9 MBh

qConventional=0.079×102,300×(85°-28°)

qConventional=460.6 MBh

The heat loss through the roof is reduce by 16% using the AirBoss system.  Demonstrating destratification reduces the heat loss, a comparison of the monthly heat load for each system is calculated in Table 10 with the assumptions used in Table 9.

 

Average OAT 28°
Occ  Setpoint 75°
Unocc Setpoint 65°
Avg # People 250
# Occ Hours 16
Days 31
Therm ($/100 Mbh) $0.8

Table 9. Assumptions for estimating energy savings.

 

  Occupied Unoccupied
AirBoss Conventional AirBoss Conventional
Hourly  Load (Btu) 821,377 1,160,570 450,876 523,612
Monthly Load (Btu) 407,403,249 575,642,858 111,817,491 129,855,845
Total 519,220,740 705,498,704

Table 10. Total monthly heat load

The monthly energy savings is calculated using Equation 4.

FS=QCQA100,000×Ƞ($0.8)

Equation 4. Energy savings

 

where,

FS = Fuel Savings

QC = Heat load for conventional system (Btu)

QA = Heat load for AirBoss system (Btu)

Ƞ = Heating system efficiency

FS=705,498,704-519,220,740100,000×0.8 ($0.8)

FS= $2,328.47 or 26%

The same calculations were performed for varying average outside air temperatures and are shown in Appendix B.

 

Summary Conclusion: 

This engineering analysis concludes that the AirBoss – AirFlow design reduces the required compressor capacity needed by ~20% compared to a conventional system.  Using AirBoss air distribution units, the system is supplying the design cfm at the given off coil temperature. This is achieved by lowering the return temperature to the RTU’s due to de-stratification and increasing the cfm/ton of each RTU.  The energy saving for an AirBoss system is ~5% less for most cooling days compared to the conventional system. During heating mode, the AirBoss system reduces the heat loss through the roof and mixes the internal warmer air. During this mode of operation, the energy savings can range between 20% – 40% with the AirBoss system.

The direct economic benefits when utilizing AirBoss technology include a capital and an operational cost reduction.  The approximate capital and maintenance cost reduction can be determined by an engineering analysis based on geographic and application considerations. The energy savings presented in this analysis utilizes data extracted from the 2017 ASHRAE Handbook – “Fundamentals, Climatic Design Information Manual”, Chapter 14 and is based on a central Midwest location (Columbus, Ohio).  Expressed in temperature degree days, each area of the country will vary based on number of cooling degree days and number of heating degree days for heat/cool applications and the number of heat days for heat only applications. In Columbus, Ohio, for example, the number of temperature cooling days is 1137 and the number of temperature heating days is 5025, which would result in an approximate annual energy cost savings of 23.76%.  In Atlanta, GA, for example, the number of temperature cooling days is 1901 and the number of temperature heating days is 2640, which would result in an approximate annual energy savings of 18.37%.