THE INFLUENCE OF CONSTRUCTIVE AND ENVIRONMENTAL FACTORS ON THE PERFORMANCE OF REFRIGERATION INSTALLATIONS

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1 THE INFLUENCE OF CONSTRUCTIVE AND ENVIRONMENTAL FACTORS ON THE PERFORMANCE OF REFRIGERATION INSTALLATIONS Nicolae BARA S.C FRIGOTEHNICA S.A., Bucuresti, România Rezumat. Prezenta lucrare analizează câteva aspecte care trebuie luate în considerare atunci cand se proiectează o instalaţie de aer condiţionat. Amplasarea instalaţiei în incinta în care aceasta va acţiona joacă un rol important în ceea ce priveşte buna funcţionare şi menţinerea acesteia într-o condiţie tehnică bună. Cuvinte cheie: convecţie,temperatură,transfer de căldură,compresor, vaporizator. Abstract. The present paper analize a few aspects which have to be considered when it has to be design and used a refrigeration instalation. The positioning of this instalation in the space where it will work has a very important role in what concernes a properly function and mentaining it in a suited stage. Keywords: convection, temperature, heat transfer, compressor, evaporator. 1. INTRODUCTION Cooling a body or an inside room or maintaining premises at low temperature can be obtained by heat exchange with the lower temperature environments. Taking heat from low temperature is performed using refrigerants in refrigeration energy consumption for heat transfer environment. In the process for obtaining low temperatures are used several methods which involve physic-chemical refrigerants. The performance of a refrigerating machine mostly depends on the heat transfer coefficients both achieved by the condenser and evaporator. The influence of functional parameters depends on the quality of refrigerating machine service and how the amount of refrigerant is used: small quantity of refrigerant; excessive refrigerant having effects on the running of the refrigerating installation: increasing the repression pressure increasing sucking pressure due to mixture of biofazic refrigerant liquid and steam agent; repression temperature decrease because of unevaporated agent drops. In some cases may occur decreases of performances because of their location, that means insufficient condensation based on placing near a heat source or inappropriate relation between the condenser and the power of the unit. Experimental researches were made on an air cooling evaporator from Termotechnical and Termic Machines of the Mechanical University in Craiova Laboratory. The goal of the evaporator is to cool the air from a room. Heat transfer between the air that must be cooled and the evaporator surface is achieved in the following ways: forced convection between the air and the outside surface of the evaporator. The air is driven by three fans; condensation of moisture in the air or surface freezing pipes; radiation between the walls of refrigerator space and evaporator. To improve heat transfer must be acted on convection coefficient α ext including convection, radiation and mass exchange and the convection coefficient on the refrigerant agent α int. Effect of the ice deposited on the evaporator tubes is taken into account by the thermal resistance factor brought by ice, ε = 0.8, which was determined experimentally. In the case analyzed in the laboratory the evaporator worked with forced convection steam generated by the three fans.cooling process can be represented in the h-x diagram of humid air. The hygrothermal direction establishes the relationship: Q0 tge= (1) G where W is the heat flow for Q 0 ; G 0 the flow of moisture in kg / h. 0 TERMOTEHNICA 2/

THE INFLUENCE OF CONSTRUCTIVE AND ENVIRONMENTAL FACTORS ON THE PERFORMANCE OF REFRIGERATION Point P is obtained at the intersection of the saturation curve φ ε = 100%. Fig. 1. Evaporator for the air cooling.

Condensation heat is transferred by circulating forced air taken through the honeycomb matrix of the capacitor.

2 THE INFLUENCE OF CONSTRUCTIVE AND ENVIRONMENTAL FACTORS ON THE PERFORMANCE OF REFRIGERATION Point P is obtained at the intersection of the saturation curve φ ε = 100%. Fig. 1. Evaporator for the air cooling. The condenser is designed for the refrigerant R-134a. Training of refrigerant is conducted with a compressor type SP-104PK. Condensation heat is transferred by circulating forced air taken through the honeycomb matrix of the capacitor. To assess the influence of how dirty is the surface above the heat exchange it was used the experimental device shown in Fig. 13, because a theoretical assessment through calculation of the intensity of heat exchange on the air is difficult. The main elements of this installation are from the air conditioning in the car Matiz. The considered transfer matrix to study the influence of pollution is made by the condenser system. For measurements of temperature and air velocity performance type kit is used from ICMET Craiova. The parameters of the air heat exchanger exit were determined using a thermo anemometer which allowed simultaneous evaluation of dynamic pressure p d, the air temperature t a and its speed measured in 22 points equally spaced on the surface unit switch to a downstream 100mm of it. Temperature measurement accuracy was 0.1 C. The flow of cooling air over heat is determined by the relationship: a " ' Q V a a c pa ( t a t a ) 4 4 T m T C0S (2) where: V a is the air volume flow in m 3 /s; ρ a - mean air density at constant pressure corresponding to the temperature range t a t a in [J / kg K]; t a, t a air temperature at the entrance to the exit of the exchanger that in C; ε emission factor of the radiator energy; C 0 transmission coefficient of absolute black body; S surface exchange of heat exchanger in m 2 ; T temperature of the environment, K; T m average temperature of the wall, K. Values obtained by direct measurements are presented in Table 3.3. ΔP tmax = 4 % The loss of cooling power measured in conditions (dirty surface, Fig. 3.11) is 4%. Heat loss is a direct consequence of the worsening of heat transfer due to deposits on the outer surface. The general form for calculating the thermal resistance is given by: d R = + å f + (3) S S S n 1 1 j 1 j 1ai j= 1 j+ 1 l j n+ 1ae where: S 1 surface solid fluid separation on the inside of the refrigerant flow, m 2 ; α i coefficient of convection of the refrigerant on the inside surface, W/(m 2 K); S j the interface between the inhomogeneous solid layers, m 2 ; S n +1 air contact surface area, m 2 ; α 2 convection heat transfer coefficient on the air, W/(m 2 K); δ j average thickness of inhomogeneous solid layer, m; λ i thermal conductivity coefficient for each layer, W / (mk); φ j form factors. Fig. 2. h-x diagram of humid air. 62 TERMOTEHNICA 2/2010

Nicolae BARA Fig. 2. Radiation thermometer and pyrometer.

18,3 17,4 18,6 17,7 18 19,1 19,2 20 20,3 20,4 20,7 20,4 T 2a [ o C] 18,4 17,7 17,6 17,4 16,7 17,1 17 17 17,5 17,6 19,1 19,8 T 1V [ o C] -4,7-1,8 1,1 4,2-0,8 9,9 17,9 21,5 22,5 21,8 21,1 19,2 T2 V [ o

11 12 13 14 15 V 1 [ m/s] 3,2 1 3,1 1,8 1,6 1,7 1,8 3 2,7 3,4 3,5 4,1 4,4 3,9 3,5 V 2 [ m/s] 0,8 0,8 1,1 1,3 1 1 1,9 2,2 2,5 0,8 3,5 4 4,4 4,1 3,8 V 3 [ m/s] 1,3 1,9 1,2 1,1 1 1,2 1,4 2,3 2,7 2,7 2,9

3 Nicolae BARA Fig. 2. Radiation thermometer and pyrometer. Speeds evaporator air and wall temperatures V 1 [m/s] 3,6 3,9 3,5 3,3 3,6 3,6 3,4 3,5 4 3,3 3,1 3,3 V 2 [m/s] 2,8 3,5 3,2 3,1 3,1 3,3 3,6 3,5 3,4 3,8 3,2 3,2 T 1a [ o C] 18,3 17,4 18,6 17, ,1 19, ,3 20,4 20,7 20,4 T 2a [ o C] 18,4 17,7 17,6 17,4 16,7 17, ,5 17,6 19,1 19,8 T 1V [ o C] -4,7-1,8 1,1 4,2-0,8 9,9 17,9 21,5 22,5 21,8 21,1 19,2 T2 V [ o C] -4,4-3,1-3,7-5 -2,5-4,3-5 -2,2 8,5 20,5 19,5 19 T amb =22 [ o C] Time index 1; Time index 2 = 35 [%] Tabelul 1 Air speed and evaporator wall temperatures Tabelul V 1 [ m/s] 3,2 1 3,1 1,8 1,6 1,7 1,8 3 2,7 3,4 3,5 4,1 4,4 3,9 3,5 V 2 [ m/s] 0,8 0,8 1,1 1, ,9 2,2 2,5 0,8 3,5 4 4,4 4,1 3,8 V 3 [ m/s] 1,3 1,9 1,2 1,1 1 1,2 1,4 2,3 2,7 2,7 2,9 3,5 4 3,6 3,7 V 4 [ m/s] 2,3 1,6 1,3 1,1 1,4 1,1 1, ,2 3,4 3,3 3,7 3,7 3,7 T o 1a [ C] 14,5 15,5 15,7 15,8 15,6 14,8 14,6 17,3 17,3 17,3 17,6 17, ,4 18,8 T o 2a [ C] 16,6 17,8 15,4 13,9 12,3 11,8 12,1 12, ,3 17,1 17,2 17,2 17,5 17,7 T o 3a [ C] 19 17,3 16,4 16,3 16,3 16,2 16,1 15,4 15,2 15,3 15, ,2 15,4 16,9 T o 4a [ C] 19,2 18,1 17, ,9 16,8 16,9 16,8 16,9 16,9 17,4 17,9 18,1 18,4 18,5 T o 1V [ C] -6-10,9-4,4-5,5-5,2-3,4-4,9-2,1-3,4-2,1 10,6 6,4 18,1 18,8 18,2 T o 2V [ C] -7,3-7,7-6,2-9,9-9,3-8,2-6,8-4,3-5,5-4,3-4,7-0,2 1,4 6,1 13,1 T o 3V [ C] -4,3-6,7-7,9-6 -7,7-7 -6,5-6,5-6,4-3,1-4,1-2,3-1,7-2,8 10,6 T o 4V [ C] -4-7,5-7,9-6,2-7,5-6,9-5,4-5,9-7,4 0,4-5,2-4,3-3,5 3,2 11,5 Fig. 3. Measure points. TERMOTEHNICA 2/

4 THE INFLUENCE OF CONSTRUCTIVE AND ENVIRONMENTAL FACTORS ON THE PERFORMANCE OF REFRIGERATION Fig. 4. Variation of evaporator surface velocities without condensation and frost. Fig. 5. Variation of air temperature. Fig. 6. Variation of evaporator surface temperature. Fig. 7. Air velocity variation to 30 min operation. 64 TERMOTEHNICA 2/2010

Nicolae BARA Fig. 8. The air temperature variation on the first pipe of the evaporator Fig. 9.

on the deposit of ice No. Airspeed Convection coefficient α ext Reynolds, [m / s] [W / m 2 K] Re 1. 0,8 32,34 105,017 2.

2,7 60,9 354,43 9. 3 64,98 393,81 10. 3,2 67,7 420,06 11. 3,4 70,42 446,32 12. 3,7 74,5 485,704 13. 3,9 77,22 511,95 14.

5 Nicolae BARA Fig. 8. The air temperature variation on the first pipe of the evaporator Fig. 9. Variation of air and surface temperatures on the evaporator tubes 3, 4 Table 3 Convection coefficient changes depending on the deposit of ice No. Airspeed Convection coefficient α ext Reynolds, [m / s] [W / m 2 K] Re 1. 0,8 32,34 105, ,06 131, ,1 36,42 144, ,3 39,14 170, ,6 43,22 210, ,8 45,94 236, ,2 54,1 288, ,7 60,9 354, ,98 393, ,2 67,7 420, ,4 70,42 446, ,7 74,5 485, ,9 77,22 511, ,1 79,94 538, ,4 84,02 577,59 Fig. 10. Variation of heat transfer coefficient depending on the speed of air flow. TERMOTEHNICA 2/

THE INFLUENCE OF CONSTRUCTIVE AND ENVIRONMENTAL FACTORS ON THE PERFORMANCE OF REFRIGERATION Fig. 11. The conditioning condenser. Deposits on the exterior and interior surfaces. Fig. 12.

Measurements of relative air* Point of Clean Surface Dirty surface measurement t a [ C] P d [mmh 2 O] W a [m/s] t a [ C] P d [mmh 2 O] W a [m/s] 1. 49,8 11 4,3 48,2 10 4 2. 53,7 9 5,4 51,7 12 5 3.

6 THE INFLUENCE OF CONSTRUCTIVE AND ENVIRONMENTAL FACTORS ON THE PERFORMANCE OF REFRIGERATION Fig. 11. The conditioning condenser. Deposits on the exterior and interior surfaces. Fig. 12. The schematic diagram of air conditioning installation. Measurements of relative air* Point of Clean Surface Dirty surface measurement t a [ C] P d [mmh 2 O] W a [m/s] t a [ C] P d [mmh 2 O] W a [m/s] 1. 49,8 11 4,3 48, ,7 9 5,4 51, ,6 11 5,6 51, ,8 13 4,8 51,2 12 3, ,2 13 5,2 51, ,4 9 5,3 51, ,5 8 5,3 51,5 10 5, ,1 10 3,8 51,4 14 3, ,8 15 5,9 49,1 14 5, ,2 17 5,8 52,1 14 5, ,4 11 6,4 53,4 14 6, ,9 9 5,3 51, ,8 11 6,6 51,4 14 6, ,5 13 5,8 51,2 14 5, ,7 17 5,8 51,3 14 5, ,8 13 5,7 51,4 14 5, ,7 13 4,8 52,2 14 4, ,2 13 5,9 52,4 12 5, ,1 13 5,8 52,6 14 4, ,8 11 5,6 51,4 14 4, ,4 12 5,2 52,3 16 4, ,2 9 5,1 51,7 14 4,6 * t a (1) = 30 C; t a (2) = 30 C; t a (3) = 29 C t a (4) = 29,66 C. The thermophisics properties of the air No. t a, med [ C] ρ a [kg/m 3 ] 10 6 O [m 2 /s] 10 2 λ [W/(mK)] c pa [J/(kg K)] 10 3 η [Ns/m 2 ] Pr Clean surface 41,49 1, ,92 2, ,8 19,126 0,704 Dirty surface 41,070 1, ,91 2, ,8 19,125 0,709 Table 3.3 Table 4 66 TERMOTEHNICA 2/2010

7 Nicolae BARA Temperature-pressure for R-134A Temperature [ C] Pressure [KPa] Temperature [ C] Pressure [KPa] Temperature [ C] Pressure [kpa] -8,89 105,70 18,33 438,18 47, ,12-7,78 114,87 21,11 487,27 48, ,18-6,67 124,32 23,89 539,67 50, ,07-5,56 134,11 26,67 609,38 51, ,72-4,44 144,24 29,44 655,04 52, ,12-3,33 154,65 32,22 718,39 53, ,35-2,22 165,48 35,00 785,61 54, ,35-1,11 176,65 37,78 856,84 57, ,91 0,00 188,16 38,89 886,56 60, ,57 2,22 212,30 40,00 916,35 62, ,62 3,33 224,98 41,11 947,92 65, ,04 4,44 238,08 42,22 979,64 68, ,04 7,22 272,49 43, ,11 71, ,77 10,00 309,58 44, ,21 73, ,29 12,77 349,51 45, ,19 76, ,81 15,56 392,33 46, ,75 Table 5 Thermal power plant air conditioning Clean surface Dirty surface T max = 6395 kw P P tmax = kw Table 6 For the study, φ j = 1 because the radius of curvature of the ribs is greater than the total thickness of the wall. From this relationship results that the overall resistance movement that is greater than each of partial resistance. For decreasing R coefficient, with consequences on the growth of heat transfer will be decreased the largest thermal resistance or increased the lower heat transfer coefficient, α. In the case studied in this paper is the transfer coefficient of air. For building heat exchangers, terms with form or 1 S are greater than the term as αs because is used materials with large thermal conductivity materials and small thicknesses. From measurements conclude that the deposit of stone, thick deposits of soot or other small, however, have an extremely low coefficient of conductivity which leads to decrease high levels of overall heat transfer coefficient. To increase the heat transfer it is good to take action above the factors which influenced the convective transfer coefficient on the air or increases the surface S of heat transfer. Given that on the heat exchange with air is done at lower heat transfer coefficient is preferable to act in this area. Heat transfer coefficient is given by: α = α conv + α rad (4) where α conv is the coefficient of heat transfer by convection is calculated using the criterial relations form No = c (G r, Pr) n or Nu = c (R c, Pr) n chosen according to the flow, laminar or turbulent forced or free flow, the shape of the surface.. Convection can be enhanced by increasing air speed that washes the heat exchange surface. This is done by fans that create a turbulent flow regime. In the case of refrigeration when referring to the α real is ignored. Here we can not talk about painting with paint surfaces or radiant heat and either high temperature. Returning to the intention to increase the heat exchange on the air that is a very effective method to increase the area S, through constructive solutions: o achieve constant or variable cross-section ribs; o acicular protrusions to plates; o acicular protrusions or fins to tubes. Optimal settlement areas (ribs, wings, pins) is to made so that they do not stop the free movement of fluid (in our case air) in order not to create areas of stagnation in which case the conductive layer of air increases lead to reducing heat transfer, Fig Increasing the heat transfer surface can be achieved through the use of corrugated surfaces. TERMOTEHNICA 2/

8 THE INFLUENCE OF CONSTRUCTIVE AND ENVIRONMENTAL FACTORS ON THE PERFORMANCE OF REFRIGERATION Transmitted heat flux does not increase proportionately by increasing the surface because the surface convection coefficient decreases with decreasing the efficiency of the corrugating. These ribs are disposed to one side or surface (e.g. gas-liquid exchangers) or on both surfaces for the gas-gas heat exchangers. Using corrugated surface area change at the same time and temperature of the fluid, which washes the ribs. If we note the λ n - rib thermal conductivity and Λ n - its thickness and through α c - coefficient of convection of the fluid then the upper surface becomes smooth ribbed surface if the next condition is realised: 2 c n n > 5 (5) The distance between the ribs should be greater than twice the thickness of boundary layer, the size of this distance is 10 mm for free convection and a few millimetres for the case of forced convection. It is important to ensure an intimate contact between the core and rib area. For one ridge without resin cement, heat flux transmitted may fall by 35% compared with a welded rib. Ribbed outer surface may be increased upon the surface of the base, up more than 10 times when the pressure losses increase of 2-3 times. Through the use of longitudinal ribs or ribs of the corrugated outer surface of the pipe band may increase 20-fold even when the convection coefficient drops to about one fourth of the initial value. The inner surface of the pipes can be increased through the ribs but increasing the surface is much lower than in the outer ribs. In this case, the convection coefficient increases. Corrugated surfaces can have as a base, round tubes, plane pipes, channels of different sections or plates. These areas can be fitted with different types of ribs, both inside and outside (by type heat exchanger or thermal agents by the way). Increasing the inner surface of pipes shall be made either by the deflection of the inner surface when ribs are formed straight longitudinal or helical or by placing the barrel of the ribs are fixed by soldering. These internal ribs may be straight or wavy bonded, staple, individual, profiled in turn. RÉFÉRENCES/ REFERENCES/ [1] R. Cotta, J. Padet Transient fully developed internal mixed convection with Boussinesq approximation. Congrès ENCIT 2008, Belo Horizonte, Bresil (2008). [2] Anastas Lazaridis, Efstathios Rafailidis, A Plastic Core Compact Heat Exchanger For Energy Conservation [3] R. Grigore, V.Dragusanu, G.Lazaroiu, S.Popa, Study Regarding Simulation and Modelling of Steam Condenser, MOCM -14, Volume 1, 2008, pp [4] *** COSMOS/Flow Technical Reference [5] A. Badea, H. Necula, M. Stan, L. Ionescu, P. Blaga., G.Darie, Echipamente şi instalaţii termice, Editura Tehnică, Bucureşti, 2003 [6] G. Lazaroiu, Sisteme de programare pentru modelare si simulare, Editura Politehnica Press, Bucuresti, (continuare din pag. 23) NATIONAL CONFERENCE OF THERMODYNAMICS WITH INTERNATIONAL PARTICIPATION Craiova, of May, 2011 Scientific Domains: FUEL COMBUSTION THERMODYNAMICS GAS DYNAMICS REFRIGERATION SYSTEMS AND HEAT PUMPS ENERGY HEAT AND MASS TRANSFER ENGINES, TURBINES AND STEAM OVENS Deadlines: Submission of Full Paper for Review 1st of March 2011 Paper Acceptance 21st of March 2011 Final Submission 15th of April 2011 Abstract must be sent by in a maximum of 500 words, including the name of the autors. Contact For any aditional information and abstract paper sending, feel free to contact one of the next persons: Prof. Dr. Eng. - Marin Bică - conat2011@gmail.com Lect. Dr. Eng. - Corina Cernăianu - conat2011@yahoo.com 68 TERMOTEHNICA 2/2010

Seyedeh Sepideh Ghaffari 1 & Seyed Ali Jazayeri 2

Seyedeh Sepideh Ghaffari 1 & Seyed Ali Jazayeri 2 Modern Applied Science; Vol. 9, No. 13; 2015 ISSN 1913-1844 E-ISSN 1913-1852 Published by Canadian Center of Science and Education The Design of a Shell-Tube Heat Exchanger as Evaporator an Absorption