BUILDING ENVIRONMENTAL SYSTEMS
BUILDING ENVIRONMENTAL SYSTEMS ARE 346N
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This 25 page Class Notes was uploaded by Rowena Barton DVM on Sunday September 6, 2015. The Class Notes belongs to ARE 346N at University of Texas at Austin taught by Jeffrey Siegel in Fall. Since its upload, it has received 39 views. For similar materials see /class/181537/are-346n-university-of-texas-at-austin in Architectural Engineering at University of Texas at Austin.
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Date Created: 09/06/15
Objectives Review psychrometric variables Draw psychrometric processes Relate psychometric variables to human comfort Differences What is the difference between dry and wet bulb temperature 0 Where are they on psychrometric chart What is the difference between relative and absolute humidity Where are they on psychrometric chart How do you use the psychrometric chart to get dewpoint temperature Examples Is it more moist When it is 70 RH at 30 0F or 30 RH at 70 OF Air comes off a chilledwater coil at 90 RH and 55 OF How cold does the duct surface need to be for condensation to form 5536quot m nnMMmmwumm E a i 2 mm m tmmrrmnmmm mm m y Sensible vs Latent Sensible due to drybulb temperature change 39 Heating and cooling Latent due to absolute humidity change 39 Humidi cation and dehumidi cation Both are extremely important 39 Sensible is o en the focus of control latent is often the source of problems Total Sensible Latent Psychrometric Processes Going from one state point to another Typically drawn as a line on a psychrometric chart Hint for problems involving psychrometric processes 39 Think about What parameter is staying constant Saturation Heating curve Cooling Ahsnlule nummw DIV bum temperature Sinsiblo hemlnn Ind cunlinn Sauiraiinn cum Ahsolme humidity Dry mun emperatur Dlhum union by coollny Saturation curve Absolute humidity Dry bulb lemperltuis Humidi miou with stem Suturauon curve Absolule humidity Dry mm tamperalurl Dahumidl cl on hv hmmion Salum on curve Absolute humidity Dry bulb tamperalure Humidi ulinn by super v coming Which of the following statements is not true A Adding steam to a room increases its relative humidity B Placing a cup ofice water in a room decreases the humidity ratio of the room s air C Lowering the temperature of a room increases its humidity ratio D Lowering the temperature while increasing the humidity ratio in a room will eventually cause the air in the room to become saturated Example 1 cup of room temperature water evaporating ina20 ft X 20 ftX 20 ftroom S oluti on What is a reasonable humidity ratio for an indoor environment What are the units on humidity ratio in the IP system How do we go from volume of water to mass of water For air What is the humidity ratio in the room after evaporation What are some important assumptions that we made Example Two ducts coming together Duct l l lbmin 100 F RH 20 Duct 2 4 lbmin 40 F RH 95 What is mixed stream dewpoint temperature Other Temperatures 39 Not on psychrometric chart because they are environment speci c Im Typically ca1cu1atednotmeasur O en combine many factors into one parameter Comfort temperatures Mean Radiant Temperature t 0 Temperature of an imaginary enclosure that would cause the same radiant heat loss as the present environment Operative Temperature to 0 Temperature that is weighted average of dry bulb and mean radiant temperature 0 Effective Temperature ET 0 Temperature at 50 RH that yields the same heat loss as for the actual environment 0 Environments at the same ET should have the same comfort response 1989 ASHRAE Comfort Zone ozw Pow rzmwuas m m lines valm Wt 0 2 Welbutb mm valid Duly 5 m 7 Vapor Pressure mm Hg 1993 ASHRAE Comfort Zone 6 k HMDITYRATIOIbmvapa por1000bdryair 39 8 I Q 5 DEWPOI NT TEMPERATURE quotF 21 45 407 g 5 Xvk E 3539 so 2 Z Em 2 1 139 Figure 22m 6 yourtext 60 85 70 75 BO 55 90 OPERATIVE TEMPERATURE F 19972001 ASHRAE Comfort Zone a 15 y 5 5 ah ah g g E 10 E 55 g a 2 so q E Wlnter Summer 5 lt z 2 45 quot d n E 40 5 E 35 39 g 30 g 25 I 20 quot 10 391 60 65 70 75 80 85 90 OPERATIVE TEMPERATURE F What is the maximum relative humidity that is still in a 1997 comfort zone A Relative humidity isn t on the comfort zone B 60 C 70 D 80 E 90 7o W y gtlt or Summ O O 0 0 0 0 HUMIDITY RATIO lb water vapor per 1000 lb dry Ii DEW POINT TEMPERATURE F 3 8 d Alll 005008 5 70 75 80 85 OPERATIVE TEMPERATURE F 90 33 lb 39er Rut Temperature Ifan Hurridt39 Ratio Panda 1 montlm pct mind at dry air Objectives Calculate heat transfer by all three modes 0 Phase change Next class 0 Apply Bernoulli equation to ow in a duct Heat Transfer Conduction Convection Radiation Definitions Conduction 1D steady conduction Qx kA QX heat transfer rate W Btuhr k thermal conductivity WmK BtuhrftK A area m2 ftz t temperature C F Q internal heat generation Wm3 Btuhr K k thermal conductivity WmK Btuhr K t temperature C F Conduction 2 mus cp specific heat kJkgdegCBtulbm F p density kgm3 lbmftK 3D trans1ent Cartes1an i k Li k i k lQ 90p 6x 6x 6y By 62 62 613 3D transient cylindrical 1 hitti k ti k Lercp rar 6quot 7 2 6 64 62 62 at Important Result for Pipes Assumptions Steady state 0 Heat conducts radially only g 0 Thermal conductivity is constant 0 No internal heat generation 2 2nkti to Q heat transfer rate W Btuhr L r k thermal conductivity W mK BtuhrftK 1n 1 L length m h 39 t temperature C F r0 7 subscript i for inner and o for outer Convection and Radiation Similarity Both are surface phenomena 0 Therefore can often be combined Difference Convection requires a uid radiation does not 0 Radiation tends to be very important for large temperature differences Convection tends to be important for uid ow Forced Convection 1 Transfer of energy by means of large scale uid motion h A At V velocity ms min Q heat transfer rate W Btuhr v kinematic viscosity up mZs Zmin A area m2 ftz D tube diameter m ft t temperature C F u dynamic viscosity kgms lbm min a thermal dif isivity mZs Zmin cp specific heat Jkg C Btulbm F k thermal conductivity WmK Btuhr K h hE convection heat transfer coefficient WmZK BtuhrftZF Dimensionless Parameters Reynolds number Re VDv Prandtl number Pr ucpk va Nusselt number Nu hDk What is the difference between thermal conductivity and thermal diffusivity Thermal conductivity k is the constant of proportionality between temperature difference and conduction heat transfer per unit area Thermal diffusivity a is the ratio of how much heat is conducted in a material to how much heat is stored k thermal conductivity W mK BtuhrftK a 7 v kinematic viscosity up mZs Zmin i i a thermal diffusivity mZs Zmin I Pr Va u dynamic viscosity kgms lbmftmin cp speci c heat Jkg C Btulbm F k thermal conductivity W mK BtuhrftK a thermal diffusivity mZs Forced Convection External turbulent flow over a flat plate Nu thk 0036 PUD43 RefL 9200 pm My External turbulent flow 40 lt ReD lt105 around a single cylinder Nu hmDk 04 ReD 0 06 Rey3 Fido401 41W 0 Better than nothing but use with care ReL Reynolds number based on length Q heat transfer rate W Btuhr ReD Reynolds number based on tube diameter A area m2 ftz L tube length m h t temperature C F k thermal conductivity WmK Btuhr K Pr Prandtl number um dynamic viscosity in free strearn kgms lbm min um dynamic viscosity at Wall temperature kgms lbmftmin h mPnn rnnx Pr Hnn hpqt tram FM 3 39 Btuhr RUIN Natural Convection Common regime when buoyancy is dominant Dimensionless parameter Rayleigh number Ra 2 VG TV Pr Ratio of d1ffus1ve to advective t1me sca es 0 Book has empirical relations for For an ideal gas Vertical at plates eqns 255 256 Horizontal cylinder eqns 257 258 H Plate height mi ft t temperature C F 39 Spheres qu lS Q heat transfer rate W Btuhr g acceleration due to gravity Ins2 ftminz caVItles eqns39 T absolute temperature K R Pr Prandtl number v kinematic viscosity up mZs ftZmin 0t thermal diffusivity mZs gBH3At gH3At Phase Change Pool Boiling What temperature does water boil under ideal conditions I urc mum Iransferrcd Log 4m a Boiling cum l 71 T Figure 27 Characteristic boiling curvc rm pun boiling Reprinted by pCI39 mission from ASHRAE Fundmlwnlalx 1093 11 a s1 7 41 Forced Convection Boiling Example refrigerant in a tube 0 Heat transfer is function of Surface roughness Nu hmDIkl00082Re2KU4 Tube diameter Re GDM Flmd VCIOClty G mass velocity Vp kgsmz lbmmin ali z erma con uctlvrt m Btu r K gudty kth l dquotyWK h m prope les D1 inner diameter of tube m h Heat ux rate K chh L f C 0255 kgmkl 778 lbmBtu hm for halocarbon refrigerants is 100800 BtuhrOFft2 5004500 WmZOC Condensation Film condensation On refrigerant tube surfaces 0 Water vapor on cooling coils Correlations Eqn 262 on the outside of horizontal tubes Eqn 263 on the inside of horizontal tubes Radiation Transfer of energy by electromagnetic radiation Does not require matter only requires that the bodies can see each other 100 7 10000 nm mostly IR 0 Issues Surface properties are spectral f0 Assume integrated properties Surface properties are directional f6 Usually assume diffuse Assume total properties Blackbody Idealized surface that Absorbs all incident radiation Emits maximum possible energy Equation 266 0 Radiation emitted is independent of direction Figure 210 7 Imcn39on iransmitred Figure 210 Schematic diagram illustrating incident rc ccled absorbed and transmitted radiation 0 0rpr1 a8forgray surfaces Radiation TABLi 22 Emiliance and Absorplance Values for Various Surfaces Emirrance or Absorplance Absorplance for Solar Surface 507 100 LF 1000 F Radiation A small hole in a large box sphere urnace or enc osure Black nonmetallic surfaces such as a ban slate palnl paper 09010 098 0 90 lo 098 085 to 098 097 lo 099 097 lo 099 097 lo 099 e e a s e d Iron dark painis red brown green eic 08510 095 07510 090 005 lo 080 Yellow and bu brick and s lone rebrick iire clay 085 70 095 070 lo 085 050 to 070 White or llghi cream brick tile paint or paper plaster whiiewcsh 085 lo 095 060 to 075 03010 050 Window glass 09010 095 Brlghi aluminum paint gilt or bronze palm 040 to 060 v 030 lo 050 Dull brass copper or aluminum galvanized sleel polished iron 020 to 030 030 lo 050 040 to 065 Polished brass copper monel melal 002 to 005 005 lo 015 03010 050 Highly polished aluminum lln plate m nickel chrom u 002 lo 004 005 lo 010 010 to 040 A A W u dba mg Relrlgerallng and Air Condiilonlng Engineers 1993 p 3 a Radiation Equations A10 01 T24 Ql Z I 1 51 1 A1 1 52 Qrad hrAAt 7 E 81 F1 2 A2 62 4 4 a T1 2 3 h T2 0439ng r 1 81 1 IPA 1 82E l El 1 1 82E 1 Fi z A2 82 81 F1 2 A2 82 Q 172 de heat transferred by radiation W BTUhr FL2 shape factor hK radiation heat transfer coefficient WmZK BtuhrftZF A area ftz m2 Tt absolute temperature R K temperature F C s emissivity surface property 0 StephanBoltzman constant 567 X 10398 VVmZK4 01713 X 10398 BTUhrftz R4 Combining Convection and Radiation Both happen simultaneously on a surface 33355231 39 Slightly different temperatures Figure 211 Combined convection and radiation heal lransfcr from a surface Often can use h hC hr Z R 8 Z 333 s 555 5 2 parallel IlkaAr RlAl lak3 RKAK 1 Add resistances for series k A2 2 Add UValues for akaAZ RZAZ 1 thickness k thermal conductivity Rthennal resistance A area RlA1 R2A2 A1A2 113m3 1 RlA1 R2A2 R1 R2 A1 R12A11U12A1 2 R3 A31U3A3 3 U3A3 U12A1 4 q U3A3Jr U12A1AT Combining all modes of heat transfer Enlarged wall Cllull mm 1391qu 1 Surrounding surmces A Figure 212 Schematic illustration of overall heal nansl cr Summary Use relationships in text to solve conduction convection radiation phase change and mixedmode heat transfer problems Next class Analyze heat exchangers Apply Bernoulli equation to ow in a duct 0 Answer all of your questions on review material