Design of Roll Bond Evaporator for Room Air Conditioner

Design of Roll Bond Evaporator for Room Air Conditioner

P. S. Ravi

Research Scholar, Osmania University, Hyderabad, Lect in Mech Engg, SS Govt Polytechnic, Zaheerabad, Telangana, India

Dr. Arkanti Krishnaiah Professor & Head of Mechanical Engineering, College of Engineering, Osmania University,

Hyderabad, India

Dr. Suresh Akella

Professor and Principal,

Sreyas Institute of Engg & Technology, Hyderabad, India

Dr. Md. Azizuddin

Professor,

Deccan College of Engineering and Technology, Hyderabad, India

AbstractThe Evaporator is a heat exchanger which gives the desired refrigeration effect in all refrigeration and air conditioning appliances. The evaporators used in air conditioners are predominantly tube and fin type. Domestic refrigerators use roll bond evaporators. This paper presents design of a roll bond evaporator for a room air conditioner. The design is carried out for a 1.5 TR room air conditioner which uses R-22 ( Mono Chloro Di Fluro Methane) as refrigerant. The refrigerant enters evaporator in liquid phase and leaves in vapor phase. The design takes into consideration Two phase flow of the refrigerant.

KeywordsEvaporator; Roll bond ; Room air conditioner; R-22 ; Two phase heat transfer

1. INTRODUCTION

   In modern world use of refrigeration and air conditioning systems is on rise. Room air conditioners are very common in comfort air conditioning system. Room air conditioners may be of window type or of split type. In India, it is estimated that approximately 70% of air conditioners are of window type. Window air conditioners of capacities 0.5 TR to 2 TR are very common in India. These contribute to steadily increasing energy consumption. There is need to develop higher efficiency products, which consume less energy while delivering the desired cooling effect. In this pursuit, it is found that the evaporator made from roll bond process gives more uniform distribution of cooling effect when compared to tube and fin evaporator. Roll bond evaporators can be easily manufactured and cost of manufacturing is also low.

   Heat exchangers, including evaporators, are designed and employed according to two criteria: heat transfer and pressure drop.

2. LITERATURE SURVEY

   The following literature gives ample evidence that roll bond evaporator can be conveniently used as a heat exchanger in many refrigerating and air conditioning devices. It has been proved experimentally that the roll bond evaporator is an alternative to traditional heat exchangers, if proper care is taken in designing it. P.S. Ravi, Dr Arkanti Krishnaiah et al.

   [1] have studied the suitability of using roll bond evaporator

   in room air conditioner. A procedure for finding inside heat transfer coefficient has been discussed. Two phase correlations were used to find the heat transfer coefficient. Using experimental works, Fieramonte, Luigi [2], found out that the refrigerators performance increases drastically with the use of roll bond evaporator. They experimented with different materials of different thickness for manufacture of roll bond evaporator and found an optimum solution to increase the performance of refrigerator container. Chandrakant patel [3] developed a cooling apparatus for a computer sub system. He used a roll bond panel. He found that maximum cooling occurs when it was configured by a roll bond panel. Validating their experimental data, Christian

   J.L Hermes et al. [4] have developed a numerical simulation model for plate type roll bond evaporators. Anderson SW et al. [5] have studied the flow of refrigerant R-22 through a horizontal tube and gave a correlation to find the heat transfer coefficient. Bio Pierre [6] in their publication studied the impact of Flow resistances of refrigerants which are in boiling conditions. Chawla J M [7] gave correlations of convective heat transfer coefficient for two- phase liquid- vapour flow. The flow through evaporator is predominantly two-phase; this correlation gives an insight of heat transfer coefficient taking viscosity of refrigerant into consideration. Lavin JG and EH Young [8] studied heat transfer characteristics of evaporating refrigerants in two-phase flow. Dembi NJ et al. [9] in their paper presented the effect of use of wire screens in D-X evaporators. Dhar P L [10], in his PhD Thesis, gave the procedure to Optimise a Refrigerating system. The procedure to design different types of heat exchangers of different geometry and for different conditions has been elaborated by Kern D Q [11]. Piert E L and H S Isbin [12] have given correlations for two phase flow in natural circulation evaporators. Heat transfer data books, Manuals from Heat transfer Research Institute, The Tubular Exchanger Manufacturers Association, Inc. (TEMA) etc have given practical approach to design a heat exchanger depending upon consumer needs.

3. DESIGN PROCEDURE OF ROLL BOND EVAPORATOR

The following procedure is established to design a roll bond evaporator which can be used for a room air conditioner.

• Step 1: Calculate Log Mean Temperature Difference (LMTD) for the evaporator.

• Step 2: Assume suitable Overall heat transfer coefficient for the evaporator. This can be approximated with the use of hand books on heat exchangers.

• Step 3: Take heat duty of the evaporator and convert into consistent system of units.

• Step 4: From the relation Q = U A (LMTD), find Area, A of the evaporator.

• Step 5: Find total length, L of the evaporator tube from the relation A = d 0 L. For the further subsequent calculations, this value of length L will be considered.

• Step 6: Depending on the selected gauge of the evaporator material find the inside diameter. Generally for roll bond evaporator, BWG 14 (Birmingham wire gauge) is used.

• Step 7: Find inside heat transfer coefficient hi and outside heat transfer coefficient h0 from different correlations.

• Step 8: Find overall heat transfer coefficient for the design using the correlation

  of 18°C . From the manufacturers catalogue, outside diameter of evaporator tubes is taken as 1/2//. The roll bond evaporator the plate is made of BWG 14.

  V. DESIGN CALCULATIONS

• Step 1; LMTD Calculation :- With R 22 as refrigerant, surface temperature of evaporator is 5°C for a air conditioner. From LMTD calculations, LMTD = 17.527

  ° C.

• Step 2:From Chemical Engineering hand book, assumed value of Overall heat transfer coefficient is 100 W/m2K, for refrigerant air , as heat transfer fluids.

• Step 3: Heat load , Q = 1.5 TR = 5250 W

• Step 4: From the relation Q = U A (LMTD), the area of evaporator is A = 2.995 m2. This is assumed value of Area

• Step 5: From the relation of Area, with the given outside diameter of the evaporator tube, the Length of evaporator tube is 75 m.

• Step 6. The inside diameter is calculated to be di= 8.484 mm.

• Step 7:

  CALCULATION OF INSIDE HEAT TRANSFER COEFFICIENT

  Basic calculations are done with reference to figure 1

  1

  0 0

  = 1

  + 1

  +

  + {

  }frost + 1

  0 0

  here U represents Overall Heat transfer coefficient, A is Area, h is convective heat transfer coefficient, x is thickness of plate ( length of heat transfer), K is thermal conductivity, subscripts o and i represents outside and inside geometries. As contact resistance is very small, it is neglected. The value ofU calculated from above equation is termed as U design.

• Step 9: If

  ~

  30 %, go to step 10. If the

  Figure 1: Basic Pressure- Enthalpy diagram

  above condition is not met, use the value of U obtained in step 8 and repeat the calculations from step 4. Repeat the iterations till the criteria are met.

• Step 10: If the above criteria are met then the Area A obtained in step 4 will be the design Area and hence it is actual area of evaporator. In practice, the required area is taken more (at the maximum of extra 10 %) to compensate for fouling.

  IV. DESIGN PARAMETERS

  For the present design, a room air conditioner with a capacity of 1 ½ TR (heat load of 18000 BUTH) is considered. The refrigerant which is used in this air conditioner is R-22 (CHClF2 – mono chloro di fluro methane) A high ambient outside conditions for air is considered. Hence a temperature of 50° C is taken as condensing temperature. Air enters evaporator at 28 °C and after cooling, leaves at temperature

  From Refrigerant R-22 properties, enthalpies at state1 and state 4 are 407.45 kJ/kg and 262.27 kJ/kg respectively.

  The mass flow rate is calculated from the relation m (p-h4) = heat load. The estimated mass flow rate of refrigerant is m = 0.03616 kg s-1

  Initially, inside heat transfer coefficient of refrigerant is calculated by considering the flow to be in single phase. The following correlations are used

  1. Butterworth correlation

     Stanton number, St= E Re-0.205 Pr -0.505 Where E = 0.0225 exp { -0.0225 ( ln Pr)2} Stanton number = h/( u Cp)

     Reynolds number Re = (4m)/( di µ) Prandtl number, Pr = (µCp)/k

     is density of refrigerant, u is velocity of refrigerant, Cp is specific heat at constant pressure, k is thermal conductivity of refrigerant, µ is dynamic viscosity of refrigerant.

     The heat transfer coefficient so calculated for R-22 is 1244.38 W/m2K

  2. Dittus Boelter equation:

  Nusselts number Nu = 0.023 Re0.8 Pr0.4 Nuseelts number is given by Nu = (h d)/k

  The heat transfer coefficient so calculated for R-22 is 1236.46 W/m2K

  (ii) Correlation from heat transfer data book

  Nusselts number Nu = 0.026 Re0.8 Pr(1/3)

  The heat transfer coefficient so calculated for R-22 is 1299.31 W/m2K

  (iv) Gnielinski equation:

  Nud = { (f/8) x (Re 1000) Pr }/{ 1 + 12.7 (f/8) 0.5 (Pr2/3 -1)}

  friction factor, f = (0.79 ln Re 1.164) -2

  The heat transfer coefficient so calculated for R-22 is 1175.25 W/m2K

  In all the above calculations, the length of evaporator is not considered. In roll bond evaporator, the refrigerant is in two phase. So two phase correlations are considered for further calculations. The following calculations show the estimation of inside heat transfer coefficient of refrigerant in two phase.

  1. Spitter, Parker, Quinston Correlation.

     (h D /k) = C1{ [GD/µL]2 [ J x hfg gc /(L g)] }0.4

     where D is inside diameter of the tube, C1 = 8.2 x 10-3 , G = m/A , x = difference in quality of refrigerant, hfg is enthalpy of evaporation, L is length of evaporator, gc for SI system is 1 and g is acceleration due to gravity

     The heat transfer coefficient so calculated for R-22, in two- phase flow is 2322.123 W/m2K

  2. Bio-Pierre correlation for complete evaporation:

     This correlation considers complete evaporation of refrigerant with 5 to 7K of super heat.

     Num = 0.0082 ( Ref 2 kf )0.4

     kf is load factor = (x hfg)/L

     The heat transfer coefficient so calculated for R-22, in two- phase flow is 5788.198 W/m2K

  3. Two phase Chato- Wattelet correlation

     h t p = p[ 4.3 + 0.4 (Bo. 104)1.3]

     p is from single phase correlation, Dittus Boelter correlation is considered.

     Bo= q///(G hfg) , q// = q/A

     The heat transfer coefficient so calculated for R-22, in two- phase flow is 5325.116 W/m2K

  4. Chaddock correlation

     htp= 1.85 h [ Bo x 104 + ( 1/ Htt)0.67] 0.6

     Where, Htt = [(1-x)/x]0.9 x ( g/f) 0.5 (µf/µg)0.1

     here the subscripts g and f refers to gaseous phase and liquid phase respectively.

     The heat transfer coefficient so calculated for R-22, in two- phase flow is 3383.92 W/m2K

  5. Bogdanov correlation

     h = [ Z2 G0.4 T]/d

     The value of Z is given from the data book and is 1.46.

     The heat transfer coefficient so calculated for R-22, in two- phase flow is 4995.465 W/m2K.

  6. Chato/Dobson correlation

     l

     htp = f( tt) { [ L (L- v)g hfg k 3]/[d T µL] } 0.25

     where f( tt) = 3.75 /( tt) 0.23

     tt = [ (1-x)/x]0.9 x ( g/f) 0.5 (µf/µg)0.1

     The heat transfer coefficient so calculated for R-22, in two- phase flow is 3998.98 W/m2K

  7. Chaddock Brunemanns correlation

  htp = 1.91 h [ Bo 104 + 1.5 ( 1/ tt) 0.67] 0.6

  The heat transfer coefficient so calculated for R-22, in two- phase flow is 4362.16 W/m2K

  To consider the effects of all the parameters in two- phase flow the average value of all the above heat transfer coefficients is taken for further calculations. The average inside heat transfer coefficient for the refrigerant R-22 is found to be 3796.566 W /2 K

  CALCULATION OF OUTSIDE HEAT TRANSFER COEFFICIENT

  The following correlations were used to find outside heat transfer confident

  1. Mc Quiston, Spitter & Parker – First correlation

     St Pr2/3 = J factor

     Where St is Stanton number, J factor is obtained from the graph given by Spitter. It is a function of Jp, where Jp is given by Jp = ReD-0.4( A/At)-0.15, QA is Total heat transfer area ( outside area of roll bond evaporator), At is Bare tube area of roll bond evaporator ( without fins).

     For this geometry (A/At) = 2.927, hence Jp is calculated to be 0.0056.

     The outside heat transfer coefficient (air side) so calculated is

     25.1 W/m2K

  2. Mc Quiston, Spitter & Parker -Second correlation J factor = 0.0014 + 0.2618 Re-0.4(A/At)-0.15

     J factor, from the above relation is 5.472 x 10-3.

     The outside heat transfer coefficient (air side) so calculated is

     24.52 W/m2K

  3. Correlation given in heat transfer data book

  The outside heat transfer coefficient (air side) so calculated is

  20.29 W/m2K

  (vii) Correlation from heat transfer data book This correlation takes into consideration convection, diffusion and radiation phenomenon.

  h total = hconvection+ hdiffusion+ hradiation

  Calculation of hconvection is based on Nu = 0.193 Re0.618 Pr1/3

  The convective heat transfer coefficient (air side) so obtained is 19.09 W/m2K

  calculation of hdiffusion: hdiffusion/ hconvection = 0.1 , so hdiffusion = 1.909 W/m2K

  hradiation is fixed as 2.5 W/m2K So htotal = 23.499 W/m2K

  To consider the effects of all the parameters, the average value of all the above heat transfer coefficients is taken for further calculations. The average outside heat transfer coefficient (air side) is found to be 22.61 W /m2 K

• Step 8: Overall heat transfer coefficient for the design using the correlation

Nu = 0.117 Re0.65 Pr (1/3)

1

0 0

= 1

+ 1

+

+ {

}frost + 1

0 0

Nusselts number is based on hydraulic diameter, Dh = (4A/P) For the geometry of roll bond evaporator, Dh = 0.1167.

The outside heat transfer coefficient (air side) so calculated is

15.82 W/m2K

1. Correlation given in Engineering science data unit

   Nu = a Rem Pr0.34

   In this expression

   A0 = 8.76 m2, Ai = 1.998 m2, hi = 3796.566 W /m2 K, 1/hfi = 0.00009, x = 2.1332 x 10-3 m, k = 202 W/m K

   x of frost = 3 x 10-3 m , k of frost = 0.25 W/m K, Afrost = A0

   Calculated value of overall heat transfer coefficient is U0 =

   1. W/m2K. This is Udesign., So Udesign = 22 W/m2K

      • Step 9:

   a= 0.211 and m = 0.651

   ~

   = 78 %. This is greater than 30 %. So the

   The outside heat transfer coefficient (air side) so calculated is

   28.74 W/m2K

2. Briggs & Young correlation

   Nu = 0.134 Re0.681 Pr(1/3)

   calculation should be repeated from step 4. Now the value of U should be taken as 22 W/m2 K in further calculations.

   1. iteration:

      The steps are repeated for second iteration and at the end the

      value of overall heat transfer coefficient U was found to be 17.768 W/m2 K and if the design criteria is checked

      The outside heat transfer coefficient (air side) so calculated is

      24.73 W/m2K

3. Correlation given in heat transfer data book, based on film

~

met.

= 19.23 %.is less than 30 %. The criteria are

temperature and hydraulic diameter

Nu = 0.25 Re0.6 Pr0.38

To have more closeness, the iteration is repeated.

1. iteration:

The value of overall heat transfer coefficient was found to be

1. W/ m2 K.

   Check of design criteria

   VI . GRAPHS AND CONCLUSIONS

   The results obtained from the design of the roll bond evaporator are tabulated in Table 1. These values are plotted in a graph for comparison.

   ~

   = 2.63%. This is very much less than 30 %.

   Temp of ref in condenser: 50 deg C

   Hence this value of overall heat transfer coefficient is accepted.

   • Step 10. The area of heat transfer i.e. the area of roll bond evaporator is found to be 17.3 m2.

Air at 45 deg C ( High ambient conditions )

Overall Heat transfer Coeff in SI Units

20

19

The total length of evaporator tube is found to be 148 m. The number of passes (bends) of roll bond evaporator depends on the geometry of the room at customer end.

Conclusion: For high ambient conditions i.e. Condenser temperature of 50 °C and moderate evaporator temperature

18 17.3

17.315 17.29

17

16

15

17.273

5 °C, The overall heat transfer coefficient is found out to be

17.78 W/ m2 K, Area of roll bond evaporator is 17.3 m2 and length of roll bond evaporator is 148 m.

The design has been performed for other parameters and the results are tabulated as shown below in Table 1.

Table 1: All design parameters, for fixed condenser temperature and varying evaporator temperature

0 5 10 15

Temp of Refrigerant in Evaporator in deg C

Fig 2: Variation of Overall heat transfer coefficient with temp of refrigerant in evaporator

Temp of ref in condenser: 50 deg C

Air at 45 deg C ( High ambient conditions )

Area of Evaporator in Sq. meters

50

45

40

35

30

25

20 13.398 17.3

15

10

5

0

24.63

44.56

0 5 10 15

Temp of refrigerant in Evaporator in deg C

Fig 3: Variation of Area of evaporator with temp of refrigerant in evaporator

Temp of ref in condenser: 50 deg C

Air at 45 deg C ( High ambient conditions )

Total Length of evaporator tube in meters

450

400

ACKNOWLEDGMENT

The authors wish to thank Osmania University and Sreyas Institute of Engg & Technology, Hyderabad for allowing us to utilise their Library and Laboratory facilities in preparation of

this paper.

350

300

250

200

150

100

50

0

114.61

148

210.7

381

REFERENCES

1. P.S.Ravi, Dr Arkanti Krishnaiah , Dr Suresh Akella , Dr Md. Azizuddin, Evaluation Of Inside Heat Transfer Coefficient of Roll Bond Evaporator for Room Air Conditioner, International Journal of Innovative Research in Science, Engineering and Technology, Vol. 4, Issue 5, May 2015, pp. 3378-3384.

   0 5 10 15

   Temperature of refrigerant in Evaporator in deg C

   Fig 4: Variation of total length of evaporator with temp of refrigerant in evaporator

   Conclusions from the graphs:

   Fig 2 shows variation of overall heat transfer coefficient, when the temperature of refrigerant in evaporator is varying, for a fixed temperature of refrigerant in condenser. It can be concluded from graphs that the overall heat transfer coefficient is almost constant, even if the inside temperature changes.

   Fig 3 & Fig 4 variation of area and length of roll bond evaporator, when the temperature of refrigerant in evaporator is varying, for a fixed temperature of refrigerant in condenser. It can be concluded that if the temperature of refrigerant in evaporator increases, then the area hence the length of roll bond evaporator increases. To have a optimum value, the

   Temperature of refrigerant in evaporator is generally maintained at around 5°C. So for this design the area of evaporator will be 17.3 m2

2. Fieramonte, Luigi, An improved refrigerating container and a process for manufacturing the same, European Patent Specification :EP No 1016838A1, 1998

3. Chandrakant patel, Cooling apparatus for computer sub system, United States Patent No 6,115,251, year 2000.

4. Christian J.L Hermes, Claudio Melo, Cezar O.R. Negrao , A numerical simulation model for plate-type, roll-bond evaporators, Science Direct, Elsevier, International Journal of Refrigeration Vol 31, pp 335-347, 2008.

5. Anderson SW, DG Rich and DF Geary, Evaporation of Refrigerant 22 in a Horizontal ¾ // OD Tube, ASHRAE Trans., Vol 72, Part I, 1966, pp.28-42

6. Bio Pierre, Flow Resistance with Boiling Refrigerants, ASHRAE J, Vol 6, Nos 9 and 10, 1964.

7. Chawla J M, Correlations of Convective Heat Transfer Coefficient for Two-phase Liquid-Vapour Flow,Paper B 5.7, Heat Transfer, Vol V, 1970, Proceedings of the IV International Conference on Heat Transfer, Paris.

8. Lavin JG and EH Young, Heat transfer of evaporating refrigerants in two-phase flow,AIChEJ Vol 11, Nov 1965, pp 1124-1132.

9. Dembi NJ, Dhar PL and CP Arora, An investigation into the use of wire screens in D-X evaporators, Paper No B 1 25, XV International Congress of Refrigeration, Venice, 1979.

10. Dhar PL, Optimization of Refrigeration systgem,Ph.D Thesis, Indian Institute of Technology, Delhi, 1974.

11. Kern DQ, Process Heat Transfer, Mc Graw-Hill, pp.136-139, 1950.

12. Piert EL and HS Isbin, Two-phase heat transfer in natural circulation evaporators, AIChE, Heat Transfer Symposium, St. Louis, Dec 1953.