PPLIED INDURIL ENERGY ND ENIRONMENL MNGEMEN Z. K. Moray, D. D. Gozdenac Part III: FUNDMENL FOR NLYI ND CLCULION OF ENERGY ND ENIRONMENL PERFORMNCE lied Industrial Energy and Enironental Manageent Zoran K. Moray and Dusan D. Gozdenac John Wiley & ons, Ltd oolbo 6 HERMODYNMIC ND RNPOR PROPERIE OF MOI IR. Definitions. Moist air is a iture of dry air (which is itself a iture of a nuber of gases and water aor. he water aor ay eist in a saturated or suerheated state. Dry air and water aor for a binary iture. iture of two substances requires three therodynaic roerties to define its therodynaic state coletely. Both dry and water aor can be considered as erfect gases since both eist in the atoshere at relatiely low ressures. Water aor is resent in the atoshere at a absolutely low artial ressure. he roerties of oist air are called sychroetric roerties and the toic which deals with the behaior of oist air is nown as sychroetry. For defining and calculating the releant sychroetric roerties, one can consider a certain olue of oist air at iture ressure P and teerature, containing a [g] of dry air and [g] of water aor as shown in Fig. 6.. DRY IR: Mass a [g] ecific olue a [ 3 /g] olue Mass Pressure P eerature WER POR: Mass [g] ecific olue [ 3 /g] Figure 6.: Miture of Dry ir and Water aor (Moist ir One can recognize the following regions in Mollier's diagra (Fig. :. Unsaturated region B. Foggy region
Part III oolbo 6: HERMODYNMIC ND RNPOR PROPERIE OF MOI IR C. Ice-cold fog region D. Ice and fog iture region For ractical industrial alications the ost iortant region is. he ajority of industrial rocesses occur in this region and, because of that, only unsaturated oist air will be analyzed. Enthaly, h [J/g] t = const. UNURED REGION aturation Line h = const. FOGGY REGION t = 0 o C bsolute Huidity, [g/g] ICE and FOG MIXURE REGION t = const. Ice-Cold Fog at 0 o C Fog at 0 o C ICE-COLD FOG REGION Figure 6.: Mollier's Diagra for Moist ir at Constant Pressure. Dry Bulb eerature is the teerature of oist air easured by an ordinary theroeter. his ter is used only to distinguish it fro wet bulb teerature. 3. bsolute Huidity is defined as the ratio of the ass of water aor to the ass of dry air in a gien olue of the iture. hus (6. a where the subscrits a and refer to dry air and water aor, resectiely. Considering both dry air and water aor as erfect gases it can be obtained that the absolute huidity is as follows: 0.6 0.6 (6. a P s the absolute huidity is defined as ass of water aor er ass of dry air, the total ass of oist air will be as follows: ( (6.3 a
Part III oolbo 6: HERMODYNMIC ND RNPOR PROPERIE OF MOI IR 3 4. he Relatie Huidity of the oist air is the ratio of the actual ass of the water aor in a gien olue to that which it would hae if it were saturated at the sae dry bulb teerature and the sae total ressure of the air: RH (6.4,sat or, it can be defined as the ratio of artial ressure of water aor in unsaturated air and the artial ressure of aor if it is saturated at the sae teerature: RH (6.5,sat Relatie Huidity is frequently eressed as a ercentage. 5. Dew Point eerature. If unsaturated oist air is cooled at constant ressure, the iture will eentually reach the saturation teerature of water aor corresonding to its artial ressure at the oint at which the first dro of dew will be fored, i.e. the water aor in the iture will be saturated. his teerature is called the dew oint teerature t d [ o C] and it is, therefore, the teerature to which unsaturated oist air ust be cooled at constant ressure before condensation of oisture taes lace. 6. Wet Bulb eerature. herodynaic wet bulb teerature is the teerature at which liquid or solid water, by eaorating into air, ay bring the air to saturation adiabatically at the sae teerature. Wet bulb teerature is the teerature indicated by a wet bulb sychroeter constructed and used according to secifications. 7. he enthaly of unsaturated oist air or the oist air on saturation line is obtained by suation of the enthalies of the constituents. hus the enthaly of oist air is equal to the su of the enthalies of dry air and associated water aor: h h (6.6 a h where: h = enthaly of oist air, [J/g dry air ] h a = enthaly of dry air, [J/g dry air ] h = enthaly of water aor, [J/g aor ] = absolute huidity, [g aor /g dry air ] aing the reference state enthaly as zero for saturated liquid at 0 o C, and that the latent heat of aorization at this teerature is 500.84 J/g, and taing that the secific heat at constant ressure of dry air and water aor are: c, a.009 5.4 0 t [J/g K] (6.7 c,.856.0 0 t [J/g K] (6.8 the enthaly of unsaturated oist air is as follows: h (.009 5.4 0 t t 500.84 (.856.0 0 t t [J / g da ] (6.9
Part III oolbo 6: HERMODYNMIC ND RNPOR PROPERIE OF MOI IR 4 his equation could be used in a range fro 0 to 300 o C. he total ressure of the air could be changed fro 0. to 5 bar. In those ranges of teerature and ressures this equation gies good results for any technical calculations. 8. he secific heat of oist air is as follows: c (.009 5.4 0 t (.856.0 0 t [J/g da K] (6.0 9. he density of oist air is defined as follows: 46.56 (0.698 P [Pa] [ / g] o (6. t [ C] 73.5 3 0. he dynaic iscosity of oist air can be calculated by using following equation: Where: (6. and.6 = 9 = 8 Molecular ass of dry air, [g/ol] Molecular ass of dry air, [g/ol] 6 5 8 3 4 a 0 0.4040 0.07458 5.77 0.998 0 6.54 0 [Pa s] (6.3 6 647.7 0 [Pa s] 3 (6.4 647.7 647.7 647.7 0.08583 0.07764 0.00587 0.0036744 are the dynaic iscosity of dry air and water aor under low ressure, resectiely. In the last two equations [K] = t [ o C] + 73.5.. he theral conductiity of oist air is:
Part III oolbo 6: HERMODYNMIC ND RNPOR PROPERIE OF MOI IR 5 (6.5 Where: 0.75 0.75 and resectiely. 0.733. = [K]; = 96 [K] utherland constants for air and water aor, he theral conductiity of dry air in the range fro 0 to +500 o C and for ressure.03 bar is as follows: - -8-3 4.4374 0 7.83035 0 t -.940 0 t.85943 0 t -.640 0 t (6.6 and for saturated water aor in the range fro 0 to +0 o C is as follows: - -7-9 3-4.748 0 7.697 0 t - 3.3464 0 t.5954 0 t - 3.7650 0 t (6.7 ll those araeters of unsaturated oist air and oist air on the saturated line can be calculated by using oftware 6.. Prandtl Nuber. his nuber is calculated by using the isobaric secific heat of oist air. he influence of the total ressure of oist air on transort roerties is relatiely sall and is not considered in the resented forulae his has to be et in ind when the software is used. 3. oftware. he deterination of the therodynaic and transort roerties of oist air can be done by using oftware 6: herodynaic Proerties of Moist ir. References Bosnjaoic, F. (976 Naua o tolini, II dio, I izdanje, ehniča njiga, Zagreb, Croatia. Chysy, J. (977 lahý zduch, NL, Praha. Häussler, W. (960 D MOLLIER i DIGRMM für feuchte Luft und seine technischen nwendungen, erlag on heodor teinoff, Dresden und Leizig.