1、4699 (RP-1121) Dehumidification Characteristics of Com me rcia I Bu i Id i n g Ap p I i cat i on s Michael J. Brandemuehl, Ph.D., P.E. Member ASHRAE ABSTRACT ASHRAE Research Project 11 21 -Rp: Evaluating the Abil- ity of Unitary Equipment to Maintain Adequate Space Humid- ity Levels: Phase 1, was es
2、tablished to develop aplan to guide the evaluation of humidity control by unitary HVAC equip- ment. Fart of the development of the plan involved the evalu- ation of dehumidjcation requirements in different building applications and different climates. This paper describes the analysis of building lo
3、ad characteristics to identi internal gains always exceed envelope heat loss, even in winter. Figure 5 shows an analogous plot of hourly zone latent load when the indoor conditions are maintained at 50% relative humidity. Unlike the sensible load, there is relatively little influence of outdoor cond
4、itions on latent load since there is assumed to be no diffusion of moisture through the envelope and the building is assumed to be sufficiently pressurized that infiltration is limited to 0.1 cfm/ft2 (0.5 L/s m2). Figures 4 and 5 also show the impact of the ventilation on HVAC system loads for a ven
5、tilation rate of 0.3 cfm/ft2 (1.5 L/s m2), which corresponds to the minimum requirement of the ventilation rate procedure of ANSI/ ASHRAE Standard 62-2001. Ventilation increases the sensible loads when the outside air temperature is warmer than the zone temperature and decreases the sensible loads w
6、hen it is lower. The figure shows that, with ventilation, heating is required when the outdoor air temperature is below about 58F. Under cooling design conditions, the HVAC system loads are almost doubled due to ventilation. The results of Figure 5 dramatically illustrate the role of ventilation on
7、latent loads when outside air humidity level is higher than the room humidity setpoint. The slope of the data line in the figure is directly proportional to the ventilation airflow rate. Figures 4 and 5 clearly show the impact of ventilation on the sensible and latent loads on an HVAC system. Howeve
8、r, ASHRAE Transactions: Research 71 Outdoor Temperature, G -17 -7 3 13 23 33 - 10 O 10 20 30 40 50 60 ?O 80 90 I00 Outdoor Temperature, F Figure 4 Sensible cooling loads, large retail store, Atlanta. 0.3 cfm/f? (1 5 Us mz) - U- ri 8 / -40 4 30 20 10 -I 10 5 O O -0.m O 0.002 0.004 0.006 0.008 0.01 0.
9、012 Difference Between Outdoor end Indoor Humidity Ratio Figure 5 Latent cooling loads, large retail store, Atlanta. the individual loads are not necessarily the most important indicator of the ability of HVAC equipment to maintain adequate indoor humidity. The relative comparison between the sensib
10、le and latent loads is a more valuabIe indicator of the dehumidification performance of HVAC equipment. Figure 6 illustrates this comparison by showing the frequency of occurrence of the HVAC load sensible heat ratio, defined as the ratio of the sensible load to the total load. (In the figures, an S
11、HRof-1 indicates neither sensible nor latent loads during the hour.) 72 Figure 6 gives SHR histograms for three different ventilation scenarios. The first graph represents the case where there is no ventilation. The sensible load dominates the latent load, and the SHR of the loads occurs mostly with
12、in the range of 9.9 to 1 .O. When adding ventilation at a rate of O. 1 cfm/ft2 (0.5 L/s m2), latent loads are increased and the occurrence profile is shifted down to SHRs in the range of0.7 to 0.9. When the ventilation rate is increased to 0.3 cfm/ft2 (1.5 L/s m2), many hours are shifted down to SHR
13、s of 0.4 to 0.7, which typically cannot be met by conventional packaged HVAC ASHRAE Transactions: Research systems. A similar trend was also found in all other applications and cities. The building load characteristics presented above indi- cate a strong dependence on outdoor conditions, especially
14、in the presence of ventilation loads. As shown in Figures 4 and 5, sensible loads are a relatively strong function of outdoor temperature, and latent loads are dnven by outdoor humidity. Other building types exhibit similar load dependencies. Figure 7 shows the sensible load characteristics for the
15、four building types. The dashed lines represent the zone loads (without ventilation) and the solid lines represent the system loads, including ventilation. The amount of ventilation varies “-7 , W+ht%75F.SG%RH Figure 6 SHR distribution, large retail store, Atlanta. with each building type according
16、to the requirements of Standard 62, with values ranging from 0.15 to 1.0 cfm/ft2 (0.75 to 5.0 L/s m2). Note that eachpair of lines crosses at the zone setpoint of 75“F, since ventilation air at the zone temper- ature does not increase the sensible load. Similar sets of curve can be generated for the
17、 latent loads in the building. In virtually all cases, the zone loads without ventilation are very small compared to the loads associated with the ventilation air, as illustrated in Figure 5. That is, the slope of the latent load line, analogous to sensible load lines in Figure 7, is simply proporti
18、onal to the ventilation rate. The ability of unitary HVAC equipment to maintain adequate control of indoor humidity levels is most signifi- cantly affected by the sensible heat ratio of the loads. That is, the magnitudes of the sensible or latent loads are less impor- tant than their coincidence. Th
19、e different curves in Figure 7 indicate that different buildings have different occurrences of sensible and latent load levels. The SHR frequency distributions ofFigure 6 captured this coincidence for the retail building in Atlanta. Similar SHR distributions can be generated for the different buildi
20、ng types. Figure 8 shows these distributions for the four building types in Atlanta. The figure shows the hours of occurrence of SHR for the case of no ventilation (the zone loads) and with an outdoor ventilation airflow that depends on the building type. As noted above, the retail store SHR values
21、fall between 0.5 and 0.8 with the recommended ventilation rate, which is often below that available from conventional unitary HVAC equipment. If the SHR of the HVAC equipment is greater than that required by the loads, indoor humidity levels will rise above the setpoint of 50% RH. Outdoor Temperatur
22、e, C -1 7 -7 3 13 23 33 30 25 20 s f 8 15 u- L J 6 s 10 5 O 90 80 70 6o N 50 $ u- 40 4 30 20 10 O m O 10 20 30 40 50 60 70 80 90 100 Outdoor Temperature, F Figure 7 Sensible load characteristics for four prototypical applications. ASHRAE Transactions: Research 73 Resiaurani lmm Room No Venhlahon Sen
23、sibie Heat Ratio -i Figure 8 SHR distributions, various applications, Atlanta. With lower internal gains, the office has fewer hours during the year with cooling loads. However, ventilation reduces the SHR of the system loads to the 0.7-0.9 range, which can generally be met by conventional unitary e
24、quip- ment. There are many hours when the addition of ventilation air eliminates all loads, suggesting that economizer cooling However, the net sensible heat ratio of the annual cooling loads is reducedin eveiy location. As expected, Houston and Atlanta show the most dramatic impact of ventilation o
25、n the annual SHR. CONCLUSIONS can sometimes be of value in Atlanta High occupant density in the restaurant causes the zone loads to have a significant latent component, and the zone SHRs are often in the 0.6-0.8 range. With the addition ofventi- lation, these SHRs drop to the 0.2-0.7 range. Note tha
26、t the ventilation rate for the restaurant dining room is very large due to the high occupant density. The large number of hours with- out any load (SHR = -1) again indicates that there are many hours when economizer cooling is beneficial. School zone load SHRvalues are higher due to higher internal
27、gains, but the high ventilation rates drive the SHRs to low values. The results presented above are all for a single location: Atlanta. Since the loads depend on outdoor conditions, the effects of climate or location can be significant. Most impor- tant to our discussion, the combination of outdoor
28、temperature and humidity can dictate relative magnitudes of the sensible and latent loads and the SHRs imposed on the equipment. Figure 9 shows the comparison of the SHR distributions for the large retail store with and without ventilation for five locations. Ventilation reduces the total annual loa
29、d for three of the five locations. In these milder cooling locations, the introduction of additional outdoor air often reduces the load, especially in an application such as the restaurant with high internal gains. Houston and Atlanta show a net total increase. The ability of unitary HVAC equipment
30、to maintain adequate indoor humidity levels depends heavily on the build- ing load characteristics and the climate characteristics of the building location. There is a large amount of anecdotal evidence of unacceptable humidity control in many building types, mostly throughout the Southeastern US. T
31、he analysis described in this paper identifies some of the key characteris- tics of buildings that affect humidity control. An analysis of the sensible and latent loads imposed on HVAC equipment has been performed for a set of prototypical buildings in different US. cities. These buildings represent
32、 some of the largest markets of unitary HVAC equipment and cover a range of building load characteristics. The following conclusions can be drawn from the analy- sis: The load sensible heat ratio (SHR) is the most influen- tial load characteristic in the evaluation of dehumidifica- tion requirements
33、. Conventional equipment could easily meet the load requirements of all building types in all locations if ven- tilation loads were not present. These zone loads typi- cally have SHRs above 0.7. 74 ASHRAE Transactions: Research Figure 9 SHR distributions, large retail store, various locutions. Venti
34、lation loads are the single most important factor in determining the SHR imposed on the HVAC equipment. Ventilation requirements vary widely among different building types. Minimum values range from 0.1-2.0 cfm/ft2 (0.5-10 L/s m2). At high ventilation rates, the loads from ventilation air can domina
35、te other load sources. Even at modest ventilation rates of 0.3 cfm/ft2 (1.5 L/s m2), HVAC equipment is expected to meet SHR require- ments of 0.5-0.7. As the ventilation rate increases, the range of SHRs increases, imposing broad and challeng- ing requirements on the HVAC equipment. Ventilation has
36、a greater impact on load applications with high ratios of internal to envelope heat gains, espe- cially if the internal gains are largely due to high occu- pant density. Many, if not most, commercial buildings operate at lower occupancy than the estimated maximum value used in ASHRAE Standard 62. Ef
37、fective control of ven- tilation airflow or heat recovery from ventilation could significantly affect requirements on HVAC equipment. ACKNOWLEDGMENTS This work was performed under ASHRAE Research Project 1121-RF with the guidance of Technical Committee 7.6, Unitary Air Conditioners and Heat Pumps, a
38、nd the co- sponsorship of Technical Committee 3.5, Sorption. This project also enjoyed the Co-sponsorship and financial support of the Air-conditioning and Refrigeration Technology Insti- tute (ARTI) through their 2 1CR program. Over the course of the project, the following individuals served on the
39、 Project Monitoring Subcommittee and were instrumental in directing the research: Charles Bullock, Eckhard Groll, Doug Kosar, Paul Brilhard, and Siva Gopalnarayanan. REFERENCES ASHRAE. 200 1. ANSI/ASHRAE Standard 62-2001, Ventilu- tion for Acceptable Indoor Air Quality. Atlanta: Ameri- can Society o
40、f Heating, Refrigerating and Air- conditioning Engineers, Inc. ASHRAE. 2001. 2001 ASHRAE Handbook-Fundamea- tals. Atlanta: American Society of Heating, Refrigerat- ing and Air-conditioning Engineers, Inc. EIA (Energy Information Agency). 1992. Commercial Building Energy Consumption und Expenditures.
41、 U.S. Department of Energy, Washington, D.C. Huang, Y.J., and E. Franconi. 1999. Commercial heating and cooling loads component analysis. LBNL Report 37208, Lawrence Berkeley National Laboratory, Berkeley, Calif. Winkelman, F.C., et al. 1994. DOE-2 Supplement Version 2. lE, LBNL Report 34947, Lawren
42、ce Berkeley National Laboratory and Hirsch & Associates, Berkeley, Calif. ASHRAE Transactions: Research 75 DISCUSSION Bede W. Wellford, Vice President Marketing, Airxchange, Inc., Rockland, Mass.: (1) Conclusions of your study demonstrate synergy and support of the TIAX building load SHR evolution a
43、s it relates to varitory/DX equipment and preconditioning of ventilation air. (2) Perhaps we should also consider a description, LHR or latent heat ratio. (3) ARI stud- ies by Godwin and research by Amrane have demonstrated that SHRs of DX packaged equipment have not changed in 30 years as efficienc
44、ies have increased. Michael Brandemuehl: (1) The results do indicate that venti- lation loads are a dominant factor in determining dehumidifi- cation requirements. The results of the Phase II project, ASHRAE Research Project 1254-RP should address the value of preconditioning the ventilation air. (3
45、) I agree that the DX equipment manufacturers have often been unfairly blamed for contributing to humidity control problems by designing systems with larger coils to increase SEER. In fact, there have also been evolutions in compressor technologies that have improved system performance. I note, thou
46、gh, that the high- efficiency two-speed systems often have poorer dehumidifi- cation performance at low speed. David Springer, President, Davis Energy Group, Davis, Calif.: Excellent graphical illustration of SHRs under vary- ing conditions. Are relatively high SHRs for small offices with ventilatio
47、n a result of greater internal heat gains from lighting and equipment? Brandemuehl: Table 4 of the paper shows that the lighting and equipment heat gains for the small ofice are not very large. Rather, the small office has the largest ratio of envelope to floor area ratio, which allows a greater inf
48、luence of outdoor conditions on building loads. Jim Coogan, Principal Engineer, Siemens Bldg. Tech.: The bulk of the information presented pertains to loads. In what ways is this paper particularly relevant to unitary equipment? Brandemuehl: This paper addresses loads in buildings that are typically
49、 served by unitary equipment. Phase II of the project (1254-RP) will address the performance of specific unitary equipment options in meeting these loads. Dennis Stanke, Trane, La Crosse, Wis.: (i) Ventilation rates in 62.1-2001 are superceded by 62n. (2) Chilled water coils and DX coils have similar performance characteristics. (3) Equipment SHR is a result, not a selection parameter. Holding spare conditions constant may be necessary for the purposes of this research, but that is not how systems work. This leads to the incorrect conclusion that increased ventilation loads to inc
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