ASHRAE Journal July

Transcription

1 Copyright 2002, Aerican ociety of Heating, Refrigerating and Air-Conditioning Engineer, Inc. ( Reprinted by periion fro AHRAE Journal, July Thi article ay not be copied nor ditributed in either paper or digital for without AHRAE periion. By.A. herif, Ph.D., ellow AHRAE P ychroetric i the cience of oit air propertie and procee, which i ued to illutrate and analyze air-conditioning cycle. It tranlate the knowledge of heating or cooling load (which are in kw or ton) into volue flow rate (in 3 / or cf) for the air to be circulated into the duct yte. The approxiate copoition of dry air by volue i: 79.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.03% carbon dioxide and 0.01% other gae. Water vapor i lighter than dry air. The aount of water vapor that the air can carry increae with it teperature. Any aount of oiture that i preent beyond what the air can carry at the prevailing teperature can only exit in the liquid phae a upended liquid droplet (if the air teperature i above the freezing point of water), or in the olid tate a upended ice crytal (if the teperature i below the freezing point). The ot exact forulation of therodynaic propertie of oit air in the teperature range of 100 C to 200 C ( 148 to 392 ) are baed on the tudy perfored by Hyland and Wexler. 1,2 More recent tudie by auer et al. 3 and Nelon et al. 4 provide pychroetric data for oit air in the teperature range of 200 C to 320 C (392 to 608 ) and huidity ratio fro 0 to 1 kg/kg air at preure of (correponding to an altitude of 2250 [7,382 ft]), , 0.2, 1.0, and 5 MPa. Both tudie developed the pychroetric data uing the ot current value of the virial coefficient, enthalpy, and entropy of both air and water vapor. Other pychroetric data were generated by tewart et al., 5 who created pychroetric chart in I unit at low preure. More pychroetric chart and table are available in the AHRAE brochure on pychroetry. 6 The ot coonly ued pychroetric quantitie include the dry- and wetbulb teperature, dew point, huidity ratio, relative huidity, and degree of aturation. Thee will be briefly defined and dicued. Dry-Bulb Teperature. Thi i the teperature eaured by a dry-bulb theroeter. everal teperature cale coonly are ued in eauring the teperature. In the inch-pound (I-P) yte of unit, at tandard atophere, the ahrenheit cale ha a water freezing point of 32 and a boiling point of 212. In the International yte (I) of unit, the Celiu cale ha a water freezing point of 0 C and a boiling point of 100 C. Wet-Bulb Teperature and Therodynaic Wet-Bulb Teperature. Wetbulb teperature i the teperature eaured with a wet-bulb theroeter after the reading ha tabilized in the airtrea. Becaue of the evaporative cooling effect, the teperature eaured with a wet-bulb theroeter i lower than the dry-bulb teperature, except when the air i aturated. Then, the wet-bulb and dry-bulb teperature are the ae. The therodynaic wet-bulb teperature i the aturation teperature of oit air at the end of an ideal adiabatic aturation proce. The latter proce i defined a one of aturating an airtrea by paing it over a water urface of infinite length in a well-inulated chaber. Dew-Point Teperature. Thi i the teperature at which oiture will begin to condene out of the air. Huidity Ratio and Relative Huidity. Huidity ratio i the ratio of the a of water vapor to the a of dry air contained in the ixture of oit air. The relative huidity i the ratio of the ole fraction of water vapor in a oit air aple to the ole fraction of water vapor in a aturated oit air aple at the ae teperature and preure. Degree of aturation. Thi i the ratio of the huidity ratio of oit air to the huidity ratio of aturated oit air at the ae teperature and preure. Pychroetric Procee enible Heating or Cooling. Thi i the proce of heating or cooling the air without changing it oiture content. It i repreented by line of contant huidity ratio on the pychroetric chart. enible heating i accoplihed when the air pae over a heating coil. en- About the Author.A. herif, Ph.D., i a profeor in the Departent of Mechanical Engineering at the Univerity of lorida, Gaineville, la. AHRAE Journal July

2 Dry-Bulb Teperature C igure 1: Pychroetric depiction how coil bypa factor, enible heat ratio. ible cooling i accoplihed when the air pae over a cooling coil with a urface teperature above the dew-point teperature of the air. Huidification (with Heating or Cooling). Thi i the proce of introducing oiture into the airtrea. In winter, huidification frequently i required becaue the cold outide air, infiltrating into a heated pace or intentionally brought in to atify the pace ventilation requireent, i too dry. In uer, huidification i uually done a part of an evaporative cooling yte. Huidification i achieved in variou way that range fro uing pray waher to paing the air over a pool of water to injecting tea. The proce i repreented on the pychroetric chart a a line of contant wet-bulb teperature when the prayed water i not externally heated or cooled. When there i external heating or cooling, the proce i repreented by a line to the right or to the left of the wet-bulb teperature line, repectively. Depending on the agnitude of water heating, the huidification proce line can be oriented in uch a way that reult in an increae in the dry-bulb teperature of the airtrea at the exit of the dehuidifying device. In the extree cae of praying cold water in the airtrea, uch that the water teperature i le than the dew-point teperature of water vapor in air, the praying proce will actually reult in air dehuidification. Cooling and Dehuidification. Thi proce i ued in airconditioning yte operating in hot, huid cliate. It i accoplihed by uing a cooling coil with a urface teperature below the dew-point teperature of water vapor in air. On the pychroetric chart, the proce i repreented by a line in which both the dry-bulb teperature and the huidity ratio decreae. Becaue not all the air olecule going through the cooling coil phyically contact the coil urface, the air condition at the exit of the coil uually i not aturated (but cloe to the aturation curve). Thi i reflected by the ue of the ocalled coil bypa factor, which i defined a the ratio of the teperature difference between the leaving coil air condition and coil apparatu dew point and that between the entering air 34 July 2002 AHRAE Journal

3 Pychroetric condition and coil apparatu dew point (ee igure 1). Thi i expreed a: T cc T cadp B = T T cadp Coil bypa factor range fro 0 (when no dehuidification i achieved by the coil) to 1 (when axiu dehuidification i achieved by the coil). igure 1 how the apparatu dew point and the coil bypa factor. The figure alo how the line repreenting the roo enible heat ratio (RHR), the coil (or grand) enible heat ratio (GHR), and the effective roo enible heat ratio (ERHR). Thee are defined according to the following equation: RHR = + ERHR = GHR = + [ + B ] + [ + B& ] + B where Q & repreent the load. ubcript and G repreent roo, outide, and grand, repectively, while and repreent enible and latent (load), repectively. The effective enible heat ratio i interwoven with both the coil apparatu dew-point teperature and the coil bypa factor, with the ole intention of iplifying pychroetric calculation. The a flow rate uing the effective roo enible heat ratio can be coputed fro the following equation: ( + B ) & = ρ C p T r T cadp ( )( 1 B ) where T r i the indoor (roo) deign teperature and T cadp i the coil apparatu dew-point teperature. The a flow rate ter repreent the quantity of air per unit tie (kg/ or lb/h) upplied to the conditioned pace. The previou equation i not exact ince the product of the pecific heat (C p ) and the teperature difference wa ued in lieu of the enthalpy difference. However, the pecific heat of air doe not change uch with teperature in the air-conditioning teperature range, therefore, the error introduced by the earlier approxiation i negligible. Heating and Dehuidification. Thi i alo referred to a deiccant (or cheical) dehuidification, which take place when air i expoed to either olid or liquid deiccant aterial. The echani of dehuidification in thi cae i either aborption (when phyical or cheical change occur) or adorption (when there are no phyical or cheical change). During the orption proce, heat i releaed. Thi heat i the u of the latent heat of condenation of the aborbed water Mixing Chaber Outdoor Air Exhaut Air upply an cc ch f Heating Coil Cooling Coil Return Air Duct Return an vapor into liquid plu the heat of wetting. The latter quantity refer to either wetting of the urface of the olid deiccant by the water olecule, or the heat of olution in the cae of liquid deiccant. Dehuidification by olid deiccant i repreented on the pychroetric chart by a proce of increaing dry-bulb teperature and a decreaing huidity ratio. Dehuidification by liquid deiccant i alo repreented by a iilar line, but when internal cooling i ued in the apparatu, the proce air line can go fro war and oit to cool and dry on the chart. Mixing of Airtrea. Thi uually refer to either adiabatic ixing of two or ore airtrea or to bypa ixing. In the forer proce, two or ore trea are ixed together adiabatically foring a unifor ixture in a ixing chaber. In thi cae, a, energy, and water vapor balance yield the following equation: & 1 + & & j & 1h1 + & 2h2 + & j h j h & 1W 1 + & 2W2 + + & j W j W Bypa ixing uually happen in air-handling unit where the airflow i divided into upper hot-deck and lower cold-deck trea. In a uer air-conditioning operation, the upper hot deck act a a bypa airtrea to oderate the teperature of the otherwie overcooled air leaving the cooling and dehuidifying coil. In winter, the lower cold deck act a the bypa trea, by ixing with the war air after it ha paed over the heating coil. Air-Conditioning Cycle and yte An air-conditioning cycle i a cobination of everal airconditioning procee. Different yte are characterized AHRAE Journal July o r Conditioned pace rf =ixing condition cc=leaving cooling coil condition ch=leaving heating coil condition hh=leaving huidifier condition f=leaving upply fan condition upply Air Duct upply Air igure 2: Baic air-conditioning yte. Return Air Huidifier rf=leaving return fan condition o=outide deign condition r=inide deign condition =upply condition

4 Dry-Bulb Teperature C igure 3: Pychroetric depiction of hot, huid uer air-conditioning yte. by the type of air-conditioning cycle they ue. The ain function of the pychroetric analyi of an air-conditioning yte i to deterine the volue flow rate of air to be puhed into the ducting yte and the izing of the ajor yte coponent. There are generally four extree cliatic condition that ay involve an air-conditioning yte. In uer operation, for exaple, the dry-bulb teperature of the outdoor air i alway high, but the huidity ratio ay be either high or low. In hot, huid cliate (e.g., Miai), the air-conditioning yte i typically copoed of a cooling coil with a urface teperature below the dew-point teperature (igure 2). That way, the yte can achieve cooling and dehuidification. In hot, dry cliate (e.g., Phoenix), evaporative cooler typically are ued. Winter condition ay have iilar extree, with the drybulb and dew-point teperature both being low. In extreely cold condition (e.g., Minneapoli), the environent typically i very dry. In thi cae, the air-conditioning yte uually i copoed of a heating coil and a huidifying device, with the forer being located uptrea of the latter. The huidifying device ay be a pray waher copoed of a pray chaber in which a nuber of pray nozzle and rier are intalled. pray waher are typically ued in indutrial application where the device perfor the dual function of air huidification and cleaning. The waher ay have one or ore bank of pray nozzle that have the capacity of injecting 1 to 2 gp (0.6 to 0.13 /) of atoized water per nozzle into the airtrea. Adequate atoization of the water can be achieved by operating at puping preure ranging fro 20 to 40 pi (138 to 276 kpa). 7 Waher typically ue baffle at the inlet air ection to ditribute the air uniforly throughout the chaber. At the exit ection, oiture eliinator prevent carryover water fro exiting the chaber into the 36 July 2002 AHRAE Journal

5 Pychroetric Dry-Bulb Teperature C igure 4: Pychroetric depiction of a hot, dry uer air-conditioning yte. conditioned pace. In cae where the outdoor air i cool but huid (e.g., eattle), there ay not be a need to huidify the air, and the airconditioning yte typically i copoed of a heating coil only. However, becaue the huidity ratio at low teperature i low even though the relative huidity ay be high, oe huidification ay be required when the dew point i low during the winter. In addition to thee four extree cliatic condition, the air-conditioning yte ay need to operate in the fall and pring. Thi ixed-ode operation uually require witching between heating and cooling baed on the value of the outdoor teperature. However, becaue of the potential energy wate aociated with thi ode of operation, the yte uually i et to operate only if the outdoor teperature goe outide of a pre-pecified wide band (to iniize the frequency of cycling between the heating and cooling ode). To provide the reader with an idea of the pychroetric analye that need to be perfored on an air-conditioning yte, only three of the previou yte (hot, huid; hot, dry; and cold, dry) will be decribed in ore detail. The cool, huid ode of operation i identical to the cold, dry ode (except in the abence of the huidifying device). uer Hot, Huid Mode. igure 2 how a baic airconditioning yte capable of both uer and winter ode operation. igure 3 how the pychroetric repreentation of a hot, huid uer ode operation applicable to Miai. In thi ode, outdoor air at tate o i adiabatically ixed with recirculated air fro the roo after paing through the ceiling plenu, the return duct, and the return fan (tate rf ) to for the ixed air condition, which i alo the entering tate to the cooling coil. Air i then cooled and dehuidified until it exit the coil at tate cc. After that, air i reheated, due to paing over the upply fan and through the upply duct, to tate, the upply condition. The pace enible, latent, and total load can, repectively, be expreed a: AHRAE Journal July

6 igure 5: Pychroetric depiction of a cold, dry winter air-conditioning yte. RT ( h r h ) ( h r h r ) ( ) where, Q & repreent the load quantitie and the enthalpie have ubcript correponding to the repective point identified on the pychroetric chart. Any one of the thee equation can be ued to copute the upply a flow rate. The upply volue flow rate, however, i coputed with the knowledge of the pecific volue at the upply condition v. Thi i expreed a: V & v The pace (roo) enible heat ratio (RHR) i graphically repreented by line r on the pychroetric chart. A for the cooling coil (or grand) load, the following equation apply: h r G, h h ( ) h cc Dry-Bulb Teperature C ( h ) ( ) 38 July 2002 AHRAE Journal T h h h cc Again, the ubcript of the enthalpy quantitie correpond to tate point identified on the chart. The above equation can be ued to ize the cooling coil. The grand (or cooling coil) enible heat ratio (GHR) i graphically repreented by line cc. The relative huidity of the air exiting the cooling coil i a function of the coil deign, fin pacing, coil urface area, and coil face velocity, aong other factor. or a coil with 10 or ore fin per inch and four row of coil, the relative huidity of the leaving air i approxiately 93%. or ix- and eight-row coil with fin pacing of 10 or ore per inch, the exiting relative huiditie are 96% and 98%, repectively. 8 uer Hot, Dry Mode. A pychroetric depiction of the procee involved in thi ode of operation i hown in igure 4. A indicated earlier, the air-conditioning equipent in

7 Pychroetric thi cae i iply copoed of a huidifying device where atoized water i introduced into the airtrea. Outdoor air at tate o i typically ixed with return air fro the conditioned pace after paing through the return duct and over the return fan (tate rf ) to for the ixed tate. The evaporative cooling proce that reult fro the pray equipent uually follow a contant wet-bulb teperature line, with the air exiting at tate hh. After that, the air pae over the upply fan and through the upply duct until it enter the conditioned pace at tate. The huidifying effectivene, η H, can be expreed either in ter of a teperature deficit ratio or a huidity deficit ratio a: T T η hh H = T T at W hh W η H = W at W pecial care ha to be exercied in analyzing roo and grand load in thi ode a the enible and latent coponent have oppoite ign. or exaple, the evaporative cooling proce ued reult in a decreae in the air teperature (enible cooling), wherea it increae the huidity ratio (latent heating). The proce reult in a very all change in the air enthalpy (alot zero) a the adiabatic aturation proce through the huidifying device follow a contant wet-bulb teperature line. Winter Cold, Dry Mode. Two cenario are poible in a winter yte. One involve the upply of heated air, and the other involve the upply of unheated air to the conditioned pace. The latter ode i applicable in cae when the outdoor teperature i not o cold that the fan and duct heating ay be enough to provide for cofort level teperature. Becaue of the iilaritie between thee cenario, we will only conider the forer cenario for further analyi. A entioned earlier, for the cold, dry ode there i a need to huidify the airtrea. igure 5 how the baic cycle repreented on the pychroetric chart for thi operational ode. Outide air at tate o i ixed with recirculated air fro the roo after paing over the return fan (tate rf ) producing the ixed tate. Air at thi tate pae over a heating coil and leave at tate ch. After the enible heating proce ch, air enter the huidifying device where both the dry-bulb teperature and huidity ratio of the airtrea are increaed, producing tate hh at the exit of the device. In the huidifying device, either tea or atoized hot water i injected. Air then pae over the upply fan and through the upply duct to the conditioned pace at the upply condition. The aount of heating required by the heating coil typically need to be coordinated with the heating perfored by the huidifying device due to tea or hot water injection. The optiu aount of heating perfored by both device ay be hard to pinpoint in an exact way, but a careful choice of the tate of air leaving the heating coil ay go a long way toward iniizing unneceary ue of heating energy. The enible heat ratio for the huidifying device i repreented by line ch hh. The grand enible heat ratio (GHR) of the overall yte i repreented by line hh. Concluion Thi article preent an overview of pychroetric procee and yte a applied to different operational ode. Proper execution of thi tage i crucial in accurately coputing the volue flow rate of air through the air-conditioning duct. Thi phae, in an air-conditioning deign proce, follow the load calculation phae. While the latter phae produce quantitie that repreent the enible and latent load ipoed on the conditioned pace, the forer phae (pychroetric analyi) i capable of incorporating the effect of introducing freh outide air into the pace for ventilation purpoe. Cooling or heating equipent izing ha to take into account not only the load ipoed on the conditioned pace, but alo the outide load. By pecifying the aount of outide air to be introduced for a pecified et of outdoor and indoor condition, pychroetric analyi enable the HVAC deigner to copute the load ipoed on the conditioning equipent (grand load). The analyi i inherently capable of ditinguihing between the enible and latent load quantitie of outide and conditioned pace (roo) air, thu providing an inightful picture of how to handle the exiting oiture. Pychroetric analyi alo enable the deigner to account for other aller load that ay be ipoed on the yte uch a duct and fan in equipent izing. By identifying the different tate point of the air a it pae through the duct yte and over the upply and return fan, the volue flow rate of air coputed by the analyi becoe necearily incluive of the effect of the duct and fan in equipent izing. Reference 1. Hyland, R.W. and Wexler, A. 1983a. orulation for the therodynaic propertie of the aturated phae of H 2 O fro K to K. AHRAE Tranaction 89(2A): Hyland, R.W. and Wexler, A. 1983b. orulation of the therodynaic propertie of dry air fro K to K, and of aturated oit air fro K to K, at preure to 5 MPa. AHRAE Tranaction 89(2A): auer, H.J., H.. Nelon, and X. Huang The earch for high teperature experiental pychroetric data. AHRAE Tranaction 107(2): Nelon, H.., H.J. auer and X. Huang High teperature propertie of oit air. AHRAE Tranaction 107(2): tewart, R.B., R.T. Jacoben, and J.H. Becker orulation for therodynaic propertie of oit air at low preure a ued for contruction of new AHRAE I unit pychroetric chart. AHRAE Tranaction 89(2A): AHRAE Pychroetric: Theory and Practice. 7. Clifford, G Modern Heating, Ventilating, and Air-conditioning. Englewood Cliff, N.J., Prentice Hall. 8. Wang,.K Handbook of Air-conditioning and Refrigeration. New York: McGraw-Hill Book Copany. AHRAE Journal July