Design and analysis of household direct cool refrigerator with vertical evaporator #1 Ashish Devidas Matkar, #2 R.D.Shelke, #3 H.N.Deshpande #123 Mechanical Engineering Heat Power, PES Modern College of Engineering, Pune 05, India. ABSTRACT The aim of this paper is to study design and analysis of household direct cool refrigerator by improving usable cabinet space by 25~30 % with roll bond vertical evaporator by instead of conventional c or o type roll bond evaporator and crisper on top instead of bottom which makes life easy for valuable customers through its ergonomically innovative product design. This improvement in usable space is based on volume of existing evaporator ie freezer section of house hold direct cool refrigerator.based on 43 C pull down test, freezer section becomes warmer by approximately 9 C~-12 C so it can be considered as refrigerator without freezer compartment. Keywords vertical evaporator, roll bond evaporator, direct cool refrigerator, crisper on top I. INTRODUCTION A refrigerator is commonly referred as fridge is a the device used to reduce rate of spoilage of foodstuff by decreasing the rate of reproduction of bacteria with its cooling technology. It consist of thermally insulated cabinet enclosed with doors and sealed system components. This thermally insulated cabinet enclosed with doors gives idea of its gross & storage ie usable volume. The sealed system components consist of evaporator, compressor, condenser & heat exchanger with expansion device which is shown in figure 1.. Figure 1 : Refrigeration cycle Household refrigerator works on simple vapor compression cycle where the refrigerant R134a or R600a may be used as per manufactures choice. The evaporators is important devise used in low pressure side of refrigeration system and its function is to absorb heat from surrounding medium which is to be cooled by means of boiling the refrigerant. The evaporators used in household refrigerators are Tube & fin, roll bond evaporator, tube on plate and coil evaporators. This paper discuss regarding roll bond evaporators which are commonly popular in direct cool refrigerators. Roll bond evaporators are commonly used in L,O or C type shape to make & differentiate as freezer compartment of refrigerator. 1. Nomenclature Symbol Parameter Unit A Area m2 Ac Cross-sectional Area m2 b Width m D Diameter m R radius m h Heat transfer coefficient W/m2K k Thermal conductivity W/mK ln logarithm to base e 2015, IERJ All Rights Reserved Page 1
m mass flow rate kg/ sec q Rate of heat flow W/m2 Q Rate of heat transfer W R Thermal resistance K/W T Temperature C U Overall heat transfer coe. W/m2K ν Specific volume m3/kg Pr Prandtl number Re Reynolds number Nu Nusselt no Thi Temp. of hot fluid at entry of heat ex. C The Temp. of hot fluid at exit of heat ex. C Tci Temp. of cold fluid at entry of heat ex. C Tce Temp. of cold fluid at entry of heat ex. C hi Inside heat transfer coefficient W/m2K ho Outside heat transfer coefficient W/m2K. II.LITERATURE REVIEW In international journal paper A simulation model for plate type & roll bond evaporator, design & simulation is discussed to understand the thermal behavior of roll bond evaporator. In international journal paper Flow boiling heat transfer at low flux conditions in a domestic refrigerator evaporator. In this paper Flow boiling heat transfer at low flux conditions, frequent bends, non circular cross sections is discussed. Effect of vertical evaporator on performance in household refrigerator instead of conventional L,O or c type evaporator is not discussed in these paper. 2.Therotical calculations Theoretical calculations are done to calculate the length of passage, which gives idea regarding design consideration of channel on roll bond panel and panel dimensions The total heat load to be handled in refrigerator is the sum of the heat load of following sources :- 1] Thermal load from walls of refrigerator. 2] Air change load. 3] Commodity load. The evaporator is an important device used in low pressure side of a refrigeration system. The liquid refrigerant from the capillary tube enters into evaporator, where it boils and changes into vapor. The function of an evaporator to absorb heat from the surrounding location or medium which is to be cooled by means of a refrigerant. The temperature of the boiling refrigerant must always be less than that of the surrounding medium so that the heat flows to the refrigerant. Heat gained in freezer compartment ie in evaporator should be equal to heat load of freezer compartment. For calculating length of evaporator, LMTD method is used. By LMTD method Q = U A LMTD (1) Q = Heat load for FC ( Watts)=Qty. of heat transferred (Watts) LMTD = Logarithmic mean temperature difference. Dimensions of evaporator tube ie passage are as follows Outside diameter = 0.008 M Thickness (t) = 0.00061 M Inside diameter = 0.00678 M First outside & inside convective heat transfer coefficient are calculated. A) Outside convective heat transfer coefficient : Umax = U *(ST / (2* SD-D)) (2) Umax = max. velocity based on the min flow area available. Umax = 0.9966667 Re d = (ρ * Umax *D ) / µ (3) For ST/ SL < 2 Nu = 0.40 *(Re d)^0.6 *(Pr)^0.36* (Pr / Prw)^0.25 (4) Nu = 16.2436 Nu = ho / k (5) 2015, IERJ All Rights Reserved Page 2
B) Inside convective heat transfer coefficient : The flow of refrigerant in evaporator tube is two phase flow, so we can use heat transfer co-relation for two phase flow. htp =hl {(1-x)0.8+[3.8 x0.78 (1-x)0.04]/(Pr0.38)} (6) htp = Two phase flow convective heat transfer coefficient x= quality hl =Liquid phase convective heat transfer coeficent. R = (1/hi*Ai)+[1/(ho*ηs*AT))+[ln(r2/r1)/2π*L*k] (7 ) Where, r1 = Inside radius of evaporator tube. Thi=1 C The=5 C r2 = Outside radius of evaporator tube. Tci=-23 C Tce=-20 C L = Length of evaporator tube. 1=Thi-Tce=21 C 2=Tci=21 C (8) LMTD= ( 1-2)/ln( 1/ 2) (9) LMTD = 9.053 Q = U A LMTD F ( 10) F = Correction factor = 0.94 Q = 38.93 Watts (as calculated earlier for heat load of given 180L refrigerator) UA = Q/(LMTD F) = 4.57 = 1/ R R =0.218 R=1/(hi 3.141 Di L +[1/(ho ηs AT)]+[ln(r2/r1)/2πLk] (10) R =R1+R2+R3 (11) R1*L = 1(hi*3.14*Di) = 0.876528343 R2 = 1/ho*ns*AT=0.021378834 R3*L = ln(r2/r1) / 2*3.14*k =0.011904837 (12) L = 3.416m Total length of evaporator passage = 1.2 Calculated length = 1.2 3.416 (13) Total length of evaporator passage = 4.101 m. Based on above calculated length roll bond panel is designed. 2015, IERJ All Rights Reserved Page 3
III. Experimental set up Existing and proposed cabinet set showing different freezer and evaporator arrangement shown in figure 2. In proposed scenario, freezer section is removed and it is converted into usable refrigerator compartment with 2 options ie crisper on top and crisper in bottom. This paper disused regarding design analysis & performance evaluation of option 1only. Figure 2 : Block diagram of vertical evaporator The temperature is measured with the help of thermocouple, the sensitive part of which are inserted in the centre of a tined copper cylinder, weighing 25 g nd of minimum external area (diameter = height = about 15.2 MM). Temperature measuring instruments shall be accurate ±0.3 C.K types thermocouples are used to measure temperature inside cabinets. Figure 3 : Block diagram of Temperature measurement scheme 3.Test matrix : To understand the performance, need to conduct various tests and which are to be done in chamber as per IS std 1476 Performance of household appliances. In this paper, 43 C no load pull down test is considered for performance comparison between existing vs proposed vertical evaporator scenario. Table 1:- 43 C Performance data comparison for existing vs proposed scenario 180L Regular O type evapora tor 180L with Proposed Vertical evaporator Option Option Option1 Option2 3 4 vertical 3 2 with evaporator vertica Vertica perforated without l strip l strip Cover Frz.avg -14.7-3 -5.3-3.5-2.27 Cabine t avg -1.7-3.48-1.1-1.15-1.83 cab1-3.3-1.7 0.7 0.1 0.2 cab2-0.7-3.4-0.9-1.8-1.6 cab3-1.1-5.4-3.1-3 -4.1 Delta Rc 2.6 3.7 3.8 3.1 4.3 Freezer average becomes warmer from -14.7 C to -3 C in case of vertical evaporator without. 2015, IERJ All Rights Reserved Page 4
Refrigerator compartment s average becomes colder from -1.7 C to -3.48 C in case of vertical evaporator without but delta temperature increased from 2.6 C to 3.7 C. Table 2:- 43 C Performance data comparison for existing vs proposed scenario Parameters 180L 180L with Proposed Vertical evaporator comments Regular O type evaporator Option1 Option2 Option3 Option4 vertical evaporator without with perforated 3 vertical strip 2 strip Crisper 6.4 1.1 3.6 3.4 3.1 Evap. in -22.6-22.9-22.4-21.3-22.9 Evap.out -22.6-18.1-13.7-19.5-16.1 Plate -22.9-23.8-24.9-22.6-23.7 Wattage 95 99.7 96 101.9 96.4 Current 0.8 0.8 0.8 0.8 0.8 Figure 4 : 43 C Performance data comparison for Freezer & Cabinet average existing vs proposed scenario IV. CONCLUSION 1 Based on 43 C pull down test as freezer section becomes warmer by 9~12 C so it can be considered as refrigerator without freezer compartment. 2) Freezer to refrigerator compartment volume ration is 30::70 %,so it gives improvement in refrigerator usable space by 25~30%. ie larger food storage area. 3) Based on 43 C pull down test need to work on improvement of crisper temperature to make it warmer by 3~5 c to avoid the vegetable spoilage. 4) crisper on top instead of bottom which makes life easy for valuable customers through its ergonomically innovative product design. 5)Crisper is on top so customer need not to bend frequently for its usage point of view. REFERENCES 1) Technical paper A numerical simulation model for plate type, roll bond evaporators by Christian JL Hermes, Claudio Melo, Cezer O R Negrao. 2) Technical paper Flow boiling heat transfer at low flux conditions in a domestic refrigerator. By Eric bijork, Bijorn Plam. 3) Technical paper Transient response of dry expansion evaporator in household refrigerator by S P Porkhial, B khastoo, M saffar Avval. 4) Technical paper Air side heat transfer enhancement of a refrigerator evaporator using vortex generation by A D Sommers, A M Jacobi. 5) Technical paper A study of the air side heat transfer and pressure drop characteristic of tube fin no frost evaporators by Jader R Barbosa Jr, Claudia Melo, Christian JL, Hermes, Paulo J Waltrich. 6) Technical paper Modelling of fin and tube evaporators considering non uniform in tube heat transfer by C oiltet, CD Perez Segarra,J Castro, A Oliva. 2015, IERJ All Rights Reserved Page 5
7) Technical paper Performance investigation of a finned tube evaporator under the oblique frontal air velocity distribution by Nan chen,lie xu, hai dong Feng,Chun guang yang. 8)Aprea, C., Reno, C., 2002. A numerical approach to very fast thermal transient in an air cooling evaporator. Applied Thermal Engineering 22, 219 228 9) ASHRAE, 1976. Thermo-physical Properties of Refrigerants. American Society of Heating, Refrigerating and Air Conditioning Engineers, New York, USA. 10) Study material from Whirlpool university and Product development center, Whirlpool of India Ltd Ranjangoan. 2015, IERJ All Rights Reserved Page 6