• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.25 no.2
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• pp.101-108
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• 2013

This paper is presented an experimental study of flow boiling heat transfer characteristics of ammonia, and is focused on pressure gradient and heat transfer coefficient of the refrigerant flow inside horizontal small tube with inner diameter of 3.0 mm and length of 2000 mm. The direct heating method is applied for supplying heat to the refrigerant, where the test tube is uniformly heated by electric current. The local heat transfer coefficients were obtained over a heat flux range of 20 to $80kW/m^2$, a mass flux range of 50 to $500kg/m^2s$, a saturation temperature range of 0 to $10^{\circ}C$, and quality up to 1.0. The pressure drops increase with increasing mass flux and heat flux, and with decreasing saturation temperature. The heat transfer coefficients increase with increasing mass flux and saturation temperature in middle and high quality region. And the local heat transfer coefficient increase with increasing heat flux in low quality region. The heat transfer coefficient of the experimental result was compared with six existing heat transfer coefficient correlation. A new boiling heat transfer coefficient correlation based on the superposition model for ammonia in small tubes is developed average deviation of -0.17% and mean deviation of 10.85%.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.2
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• pp.180-186
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• 2001

The condensation heat transfer coefficients of pure refrigerants R-22, R-134a, and a binary refrigerant mixture R-410A flowing in a small diameter tube were investigated. The experiment apparatus consists of a refrigerant loop and a water loop. The main components of the refrigerant loop consist of a variable-speed pump, a mass flowmeter, an evaporator, and a condenser(test section). The water loop consists of a variable-speed pump, an isothermal tank, and a flowmeter. The condenser is a counterflow heat exchanger with refrigerant flowing in the inner tube and water flowing in the annulus. The test section consists of smooth, horizontal copper tube of 3.38mm outer diameter and 1.77mm inner diameter. The length of test section is 1220mm. The refrigerant mass fluxes varied from 450 to 1050kg/(㎡$.$s) and the average inlet and outlet qualities were 0.05 and 0.95, respectively. The main results were summarized as follows ; in the case of single-phase flow, the heat transfer coefficients increase with increasing mass flux. The heat transfer coefficient of R-410A was higher than that of R-22 and R-134a, and the heat transfer for small diameter tubes were about 20% to 27% higher than those predicted by Gnielinski. In the case of two-phase flow, the heat transfer coefficients also increase with increasing mass flux and quality. The condensation heat transfer coefficient of R-410A was slightly higher than that of R-22 and R-134a. Most of correlations proposed in the large diameter tube showed significant deviations with experimental data except for the ranges of low quality and low mass flux.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.9
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• pp.1245-1252
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• 2001

The condensation pressure drop for pure refrigerants R-22, R-134a, and a binary refrigerant mixture R-410A flowing in a small diameter tube was investigated. The test section is a counterflow heat exchanger with refrigerant flowing in the inner tube and coolant flowing in the annulus. The test section consists of 1220 [mm] length with horizontal copper tube of 3.38 [mm] outer diameter and 1.77 [mm] inner diameter. The refrigerant mass fluxes ranged from 450 to 1050 [kg/(㎡$.$s)] and the average inlet and outlet qualities were 0.05 and 0.95, respectively. The main experimental results were summarized as follows : In the case of single-phase flow, the pressure drop of R-134a is much higher than that of R-22 and R-410A for the same Reynolds number. The friction factors for small diameter tubes are higher than those predicted by Blasius equation. In the case of two-phase flow, the pressure drop increases with increasing mass flux and decreasing quality. The pressure drop of R-134a is much higher than that of R-22 and R-410A for the same mass flux. Most of correlations proposed in the large diameter tube showed enormous deviations with experimental data. However, the correlation predicted by Honda et al showed relatively good agreement with experimental data for R-134.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.9 no.3
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• pp.292-299
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• 1997

Ammonia has been used as refrigerant for more than 100 years in absorption as well as in compression systems. Due to its poisonous and inflammable properties, however, its use has been mainly on heavy industrial plants in which regular maintenance are available. For these systems, condensers are generally water-cooled. This is suitable for large systems over 20 RT but is not suitable for small systems. In order to apply ammonia for a small system, it is important to adopt an air-cooled condenser. In this study, simple numerical analysis of an air-cooled condenser for an ammonia refrigeration system has been carried out. The condenser is designed as horizontal tubes with plate fins attached at the outer surface to enhance the air-side heat transfer rate. Effects of fin shape and arrangement are studied in detail. Since the local heat transfer coefficient is highest at the leading edge, heat flux is highest at the edge and decreases along the distance. Conditions of inlet air are also varied in the study and condenser length that is required for full condensation is calculated. The results show that it is important to enhance both the air-side and internal heat transfer coefficients.

• International Journal of Air-Conditioning and Refrigeration
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• v.6
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• pp.104-112
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• 1998

Ammonia has been used as refrigerant for more than 100 years in absorption as well as in compression systems. Due to its poisonous and inflammable properties, however, its use has been mainly on heavy industrial plants in which regular maintenance is available. For these systems, condensers are generally water∼cooled. This is suitable for large systems over 20RT but is not suitable for small systems. In order to apply ammonia for a small system, it is important to adopt an air-cooled condenser. In this study, simple numerical analysis of an air-cooled condenser for an ammonia refrigeration system has been carried out. The condenser is designed as horizontal tubes with plate fins attached at the outer surface to enhance the air-side heat transfer rate. Effect of fin shape and arrangement are studied in detail. Since the local heat transfer coefficient is highest at the leading edge, heat flux is highest at the edge and decreases along the distance. Conditions of inlet air are also varied in the study and condenser length that is required for full condensation is calculated. The results show that it is important to enhance both the air-side and internal heat transfer coefficients.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.7
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• pp.649-658
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• 2005

Flow condensation heat transfer coefficients (HTCs) of R22, R410, Propane (R290) were measured inside a horizontal 9 hole aluminum multi-channel flat tube. The main test section in the refrigerant loop was made of a 0.53m long multi-channel flat tube of hydraulic diameter of 1.4 mm. Refrigerant was cooled by passing cold water through an annulus surrounding the test section. Data were obtained in qualities of $0.1\~0.9$ at mass flux of $200\~400kg/m^2s$ and heat flux of $7.3\~7.7kW/m^2$ at the saturation temperature of $40^{\circ}C$. All popular heat transfer correlations in single-phase subcooled liquid flow and flow condensation originally developed for large single tubes predicted the present data of the multi channel flat tube within $25\%$ deviation when effective heat transfer area was used in determining experimental data. This suggests that there is little change in flow characteristics and patterns when the tube diameter is reduced down to 1.4 mm diameter range. Hence, a modified correlation based on the present data was proposed which could be applied to small diameter tubes with effective heat transfer area. The correlation showed a mean deviation of less than $20\%$ for all data.

• Proceedings of the SAREK Conference
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• 2005.11a
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• pp.249-255
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• 2005

Flow condensation heat transfer coefficients(HTCs) of R22, R4IO, Propane(R290) were measured inside a horizontal 9 hole aluminum multi-channel flat tube. The main test section in the refrigerant loop was made of a 0.53 m long multi-channel flat tube of hydraulic diameter of 1.4 mm. Refrigerant was cooled by passing cold water through an annulus surrounding the test section. Data were obtained in qualities of 0.1 ${\sim}$ 0.9 at mass flux of $200{\sim}400$ $kg/m^2s$ and heat flux of $7.3{\sim}7.7$ $kW/m^2$ at the saturation temperature of $4^{\circ}C$. All popular heat transfer correlations in single-phase subcooled liquid flow and flow condensation originally developed for large single tubes predicted the present data of the multi channel flat tube within 25% deviation when effective heat transfer area was used in determining experimental data. This suggests that there is little change in flow characteristics and patterns when the tube diameter is reduced down to 1.4 mm diameter range. Hence, a modified correlation based on the present data was proposed which could be applied to small diameter tubes with effective heat transfer area. The correlation showed a mean deviation of less than 20% for all data.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1270-1275
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• 2004

Flow condensation heat transfer coefficients(HTCs) of R22 and R134a were measured on horizontal aluminum multi-channel tube. The experimental apparatus was composed of three main parts ; a refrigerant loop, a water loop and a water-ethylene glycol loop. The test section in the refrigerant loop was made of aluminum multi-channel tube of 1.4 mm hydraulic diameter and 0.53 m length. The refrigerant was cooled by passing cold water through an annulus surrounding the test section. The data scan vapor qualities $(0.1{\sim}0.9)$, mass flux ($200{\sim}400$ $kg/m^{2}s$) and heat flux ($7.3{\sim}7.7$ $kW/m^{2}$) at $40{\times}0.2^{\circ}C$ saturation temperature in small hydraulic diameter tube. It was found that some well-known previous correlations were not suitable for multichannel tube. So, It must develop new correlations for multi-channel tubes.

• Proceedings of the SAREK Conference
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• 2006.06a
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• pp.657-662
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• 2006

The present paper dealt with an experimental study of condensation heat transfer coefficients with refrigerant R-22, R-407C and R-410A, and was focused on pressure gradient and heat transfer coefficient in horizontal tube-in-tube heat exchangers using inner diameter of 4 mm, 3 mm and 2 mm in a 16.91 mm tube and length of 3,000 mm. Experiments were performed at inlet saturation temperature of 35 to $45^{\circ}C$ and mass flux ranges from 200 to $600 kg/m^2s$. The pressure gradient with inner tube diameter of 4.0 mm is higher 2.5 times than that of 8.0 mm. In tube-in-tube HEX, the pressure gradient of R-410A were lower than those of R-22 and R-407C. The condensation heat transfer coefficients increased with mass flux increase, but they decreased with saturation temperature increased. Condensation heat transfer coefficients of R-410A were a little higher than those of R-22 and R-407C. The condensation heat transfer coefficients of tube-in-tube HEX were about 40% higher than those of double tube HEX.

• Clean Technology
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• v.13 no.2
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• pp.151-158
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• 2007

An experimental study to investigate the condensing heat transfer characteristics of small diameter horizontal double pipe heat exchangers with R-22 and R-410A was performed. Experimental facility was constructed to calculate and observe HTC(heat transfer coefficients), flow patterns and pressure drop. The main components include a liquid pump, an evaporator, a condenser(test section), a sight-glass, pressure taps and measurement apparatus. Two pipes of different diameters are tested; One 5.35 mm ID 0.5 mm thick, the other 3.36 mm ID 0.7 mm thick. The mass flow rate ranged from 200 to $500\;ks/m^2{\cdot}s$ and heating capacity were form 1.0 to 2.4 kW. The flow patterns of R-22 and R-410A were observed with a high speed camera through the sight-glass. The tests revealed that HTC of R-410A was higher than that of R-22 by maximum 5%. Annular pattern was observed for the most cases but stratified flow was also detected when x<0.2. The pressure drop in 3.36 mm ID pipe was higher than that of 5.35 mm by $30{\sim}50%$. Comparing with previous correlations such as Shah, Fujii and Soliman's, Fujii' showed the best good agreement with my data with a maximum deviation of 40%.