ASHRAE HVAC APPLICATIONS SI CH 30-2015 INDUSTRIAL DRYING.pdf

1、30.1CHAPTER 30INDUSTRIAL DRYINGMechanism of Drying 30.1Applying Hygrometry to Drying 30.1Determining Drying Time . 30.2Drying System Selection 30.3Types of Drying Systems . 30.3RYING removes water and other liquids from gases, liquids,Dand solids. The term is most commonly used, however, todescribe

2、removing water or solvent from solids by thermal means.Dehumidification refers to the drying of a gas, usually by conden-sation or by absorption with a drying agent (see Chapter 32 of the2013 ASHRAE HandbookFundamentals). Distillation, particu-larly fractional distillation, is used to dry liquids.It

3、 is cost-effective to separate as much water as possible from asolid using mechanical methods before drying using thermal meth-ods. Mechanical methods such as filtration, screening, pressing, cen-trifuging, or settling require less power and less capital outlay perunit mass of water removed.This cha

4、pter describes industrial drying systems and their advan-tages, disadvantages, relative energy consumption, and applications.Special Warning: Certain industrial spaces may contain flam-mable, combustible, and/or toxic concentrations of vapors or dustsunder either normal or abnormal conditions. In sp

5、aces such as these,there are life-safety issues that this chapter may not completelyaddress. Special precautions must be taken in accordance withrequirements of recognized authorities such as the National Fire Pro-tection Association (NFPA), the Occupational Safety and HealthAdministration (OSHA), a

6、nd the American National StandardsInstitute (ANSI). In all situations, engineers, designers, and install-ers who encounter conflicting codes and standards must defer to thecode or standard that best addresses and safeguards life safety. 1. MECHANISM OF DRYINGWhen a solid dries, two processes occur s

7、imultaneously: (1) thetransfer of heat to evaporate the liquid and (2) the transfer of mass asvapor and internal liquid. Factors governing the rate of each processdetermine the drying rate.The principal objective in commercial drying is to supply therequired heat efficiently. Heat transfer can occur

8、 by convection, con-duction, radiation, or a combination of these. Industrial dryers differin their methods of transferring heat to the solid. In general, heatmust flow first to the outer surface of the solid and then into the inte-rior. An exception is drying with high-frequency electrical currents

9、,where heat is generated within the solid, producing a higher temper-ature at the interior than at the surface and causing heat to flow frominside the solid to the outer surfaces.2. APPLYING HYGROMETRY TO DRYINGIn many applications, recirculating the drying medium improvesthermal efficiency. The opt

10、imum proportion of recycled air balancesthe lower heat loss associated with more recirculation against thehigher drying rate associated with less recirculation.Because the humidity of drying air is affected by the recycle ratio,the air humidity throughout the dryer must be analyzed to determinewheth

11、er the predicted moisture pickup of the air is physicallyattainable. The maximum ability of air to absorb moisture corre-sponds to the difference between saturation moisture content at wet-bulb (or adiabatic cooling) temperature and moisture content at sup-ply air dew point. The actual moisture pick

12、up of air is determined byheat and mass transfer rates and is always less than the maximumattainable.ASHRAE psychrometric charts for normal and high tempera-tures (No. 1 and No. 3) can be used for most drying calculations. Theprocess does not exactly follow the adiabatic cooling lines becausesome he

13、at is transferred to the material by direct radiation or by con-duction from the metal tray or conveyor.Example 1. A dryer has a capacity of 41 kg of bone-dry gelatin per hour.Initial moisture content is 228% bone-dry basis, and final moisture con-tent is 32% bone-dry basis. For optimum drying, supp

14、ly air is at 50Cdb and 30C wb in sufficient quantity that the exhaust air is 40C db and29.5C wb. Makeup air is available at 27C db and 18.6C wb.Find (1) the required amount of makeup and exhaust air and (2) thepercentage of recirculated air.Solution: In this example, the humidity in each of the thre

15、e airstreamsis fixed; hence, the recycle ratio is also determined. Refer to ASHRAEPsychrometric Chart No. 1 to obtain the humidity ratio of makeup airand exhaust air. To maintain a steady-state condition in the dryer, waterevaporated from the material must be carried away by exhaust air.Therefore, t

16、he pickup (the difference in humidity ratio between exhaustair and makeup air) is equal to the rate at which water is evaporatedfrom the material divided by the mass of dry air exhausted per hour.Step 1. From ASHRAE Psychrometric Chart No. 1, the humidityratios are as follows:Moisture pickup is 22 1

17、0 = 12 g/kg (dry air). The rate of evapora-tion in the dryer is41(228 32)/100 = 80.36 kg/h = 22.3 g/sThe dry air required to remove the evaporated water is 22.3/12 =1.86 kg/s.Step 2. Assume x = percentage of recirculated air and (100 x) =percentage of makeup air. ThenHumidity ratio of supply air =(H

18、umidity ratio of exhaust and recirculated air)(x/100)+ (Humidity ratio of makeup air)(100 x)/100Hence,18.7 = 22(x/100) + 10(100 x)/100x = 72.5% recirculated air100 x = 27.5% makeup airThe preparation of this chapter is assigned to TC 9.2, Industrial AirConditioning.Dry bulb,CWet bulb,CHumidity ratio

19、,g/kg dry airSupply air 50 30 18.7Exhaust air 40 29.5 22Makeup air 27 18.6 1030.2 2015 ASHRAE HandbookHVAC Applications (SI)3. DETERMINING DRYING TIMEThe following three methods of finding drying time are listed inorder of preference:Conduct tests in a laboratory dryer simulating conditions for thec

20、ommercial machine, or obtain performance data using the com-mercial machine.If the specific material is not available, obtain drying data on sim-ilar material by either of the above methods. This is subject to theinvestigators experience and judgment.Estimate drying time from theoretical equations (

21、see the Bibliog-raphy). Care should be taken in using the approximate valuesobtained by this method.When designing commercial equipment, tests are conducted in alaboratory dryer that simulates commercial operating conditions.Samples used in the laboratory tests should be identical to the mate-rial f

22、ound in the commercial operation. Results from several testedsamples should be compared for consistency. Otherwise, test resultsmay not reflect the drying characteristics of the commercial mate-rial accurately.When laboratory testing is impractical, commercial drying datacan be based on the equipmen

23、t manufacturers experience.Commercial Drying TimeWhen selecting a commercial dryer, the estimated drying timedetermines what size machine is needed for a given capacity. If thedrying time has been derived from laboratory tests, the followingshould be considered:In a laboratory dryer, considerable dr

24、ying may result from radia-tion and heat conduction. In a commercial dryer, these factors areusually negligible.In a commercial dryer, humidity may be higher than in a labora-tory dryer. In drying operations with controlled humidity, this fac-tor can be eliminated by duplicating the commercial humid

25、itycondition in the laboratory dryer.Operating conditions are not as uniform in a commercial dryer asin a laboratory dryer.Because of the small sample used, the test material may not berepresentative of the commercial material.Thus, the designer must use experience and judgment to modify thetest dry

26、ing time to suit the commercial conditions.Dryer CalculationsTo estimate preliminary cost for a commercial dryer, the circu-lating airflow rate, makeup and exhaust airflow rate, and heat bal-ance must be determined.Circulating Air. The required circulating or supply airflow rateis established by the

27、 optimum air velocity relative to the material.This can be obtained from laboratory tests or previous experience,keeping in mind that the air also has an optimum moisture pickup.(See the section on Applying Hygrometry to Drying.)Makeup and Exhaust Air. The makeup and exhaust airflow raterequired for

28、 steady-state conditions within the dryer is also dis-cussed in the section on Applying Hygrometry to Drying. In a con-tinuously operating dryer, the relationship between moisturecontent of the material and quantity of makeup air is given byGT(W2 W1) = M(w1 w2)(1)whereGT= dry air supplied as makeup

29、air to the dryer, kg/sM = stock dried in a continuous dryer, kg/sW1= humidity ratio of entering air, kg water vapor per kg dry airW2= humidity ratio of leaving air, kg water vapor per kg dry air (in a continuously operating dryer, W2is constant; in a batch dryer, W2varies during part of the cycle)w1

30、= dry basis moisture content of entering material, kg of water/kgw2= dry basis moisture content of leaving material, kg of water/kgIn batch dryers, the drying operation is given asGT (W2 W1) = (M1) (2)whereM1= mass of material charged in a discontinuous dryer, kg per batchdw/d = instantaneous time r

31、ate of evaporation corresponding to w The makeup air quantity is constant and is based on the averageevaporation rate. Equation (2) then becomes identical to Equation(1), where M = M1/. Under this condition, humidity in the batchdryer decreases during the drying cycle, whereas in the continuousdryer

32、, humidity is constant with constant load.Heat Balance. To estimate the fuel requirements of a dryer, aheat balance consisting of the following is needed:Radiation and convection losses from the dryerHeating of the commercial dry material to the leaving temperature(usually estimated)Vaporization of

33、the water being removed from the material (usu-ally considered to take place at the wet-bulb temperature)Heating of the vapor from the wet-bulb temperature in the dryer tothe exhaust temperatureHeating of the total water in the material from the entering tem-perature to the wet-bulb temperature in t

34、he dryerHeating of the makeup air from its initial temperature to theexhaust temperatureThe energy absorbed must be supplied by the fuel. The selectionand design of the heating equipment is an essential part of the over-all design of the dryer.Example 2. Magnesium hydroxide is dried from 82% to 4% m

35、oisture con-tent (wet basis) in a continuous conveyor dryer with a fin-drum feed(see Figure 7). The desired production rate is 0.4 kg/s. The optimumcirculating air temperature for drying is 71C, which is not limited bythe existing steam pressure of the dryer.Step 1. Laboratory tests indicate the fol

36、lowing:Specific heatsair (ca) = 1.00 kJ/(kgK)material (cm) = 1.25 kJ/(kgK)water (cw) = 4.18 kJ/(kgK)water vapor (cv) = 1.84 kJ/(kgK)Temperature of material entering dryer = 15CTemperature of makeup airdry bulb = 21Cwet bulb = 15.5CTemperature of circulating airdry bulb = 71Cwet bulb = 38CAir velocit

37、y through drying bed = 1.3 m/sDryer bed loading = 33.3 kg/m2Test drying time = 25 minStep 2. Previous experience indicates that the commercial dryingtime is 70% greater than the time obtained in the laboratory test. Thus,the commercial drying time is estimated to be 1.7 25 = 42.5 min.Step 3. The hol

38、ding capacity of the dryer bed is0.4(42.5 60) = 1020 kg at 4% (wet basis)The required conveyor area is 1020/33.3 = 30.6 m2. Assuming the con-veyor is 2.4 m wide, the length of the drying zone is 30.6/2.4 = 12.8 m.Step 4. The amount of water in the material entering the dryer is0.482/(100 + 4) = 0.31

39、5 kg/sThe amount of water in the material leaving isdwd-Industrial Drying 30.30.44/(100 + 4) = 0.015 kg/sThus, the moisture removal rate is 0.315 0.015 = 0.300 kg/s.Step 5. The air circulates perpendicular to the perforated plate con-veyor, so the air volume is the face velocity times the conveyor a

40、rea:Air volume = 1.3 30.6 = 39.8 m3/sASHRAE Psychrometric Charts 1 and 3 show these air properties:Supply air (71C db, 38C wb)Humidity ratio = 29.0 g/kg (dry air)Specific volume = 1.02 m3/kg (dry air)Makeup air (21C db, 15.5C wb)Humidity ratio W1= 8.7 g/kg (dry air)The mass flow rate of dry air is39

41、.8/1.02 = 39.0 kg/sStep 6. The amount of moisture pickup is0.300/39.0 = 7.7 g/kg (dry air)The humidity ratio of the exhaust air isW2= 29.0 + 7.7 = 36.7 g/kg (dry air)Substitute in Equation (1) and calculate GTas follows:GT (36.7 8.7)(1 kg/1000 g) = (0.4/1.04)(82 4)/100GT= 10.7 kg (dry air)/sTherefor

42、e,Makeup air = 100 10.7/39.0 = 27.4%Recirculated air = 72.6%Step 7. Heat BalanceSensible heat of material = M(tm2 tm1)cm= (0.4/1.04)(38 15)1.25= 11.1 kWSensible heat of water = Mw1(tw tm1)cw= 0.315(38 15)4.18= 30.2 kWLatent heat of evaporation = M(w1 w2)H = 0.300 kg/s 2411 kJ/(kgK)= 723.3 kWSensible

43、 heat of vapor = M(t2 tw)cv= 0.300(71 38)1.84= 18.2 kWRequired heat for material = 782.8 kWThe temperature drop (t2 t3) through the bed is= 20 KTherefore, the exhaust air temperature is 71 20 = 51C.Required heat for makeup air = GT (t3 t1)ca= 10.7(51 21)1.00=321 kWThe total heat required for materia

44、l and makeup air is782.8 + 321 = 1100 kWAdditional heat that must be provided to compensate for radiation andconvection losses can be calculated from the known construction of thedryer surfaces.4. DRYING SYSTEM SELECTIONA general procedure for selecting a drying system is as follows:1. Survey of sui

45、table dryers.2. Preliminary cost estimates of various types.(a) Initial investment(b) Operating cost3. Drying tests conducted in prototype or laboratory units, prefera-bly using the most promising equipment available. Sometimes apilot plant is justified.4. Summary of tests evaluating quality of samp

46、les of the driedproducts.Factors that can overshadow the operating or investment costinclude the following:Product quality, which should not be sacrificedDusting, solvent, or other product lossesSpace limitationBulk density of the product, which can affect packaging costFriedman (1951) and Parker (1

47、963) discuss additional aids todryer selection.5. TYPES OF DRYING SYSTEMSRadiant Infrared DryingThermal radiation may be applied by infrared lamps, gas-heatedincandescent refractories, steam-heated sources, and, most often,electrically heated surfaces. Infrared heats only near the surface ofa materi

48、al, so it is best used to dry thin sheets.Using infrared heating to dry webs such as uncoated materialshas been relatively unsuccessful because of process control prob-lems. Thermal efficiency can be low; heat transfer depends on theemitters characteristics and configuration, and on the properties o

49、fthe material to be dried.Radiant heating is used for drying ink and other coatings onpaper, textile fabrics, paint films, and lacquers. Inks have been spe-cifically formulated for curing with tuned or narrow wavelengthinfrared radiation.Ultraviolet Radiation DryingUltraviolet (UV) drying uses electromagnetic radiation. Inks andother coatings based on monomers are cure-dried when exposed toUV radiation. This method has superior properties (Chatterjee andRamaswamy 1975): the print resists scuff, scratch,