Study of a novel silica gel–water adsorption chiller. Part II. Experimental study

The prototype of a novel silica gel–water adsorption chiller is built and its performance is tested in detail. The experimental results show that the refrigerating capacity (RC) and COP of the chiller are 7.15 and 0.38 kW, respectively, when the hot water temperature is 84.8 °C, the cooling water temperature is 30.6 °C, and the chilled water outlet temperature is 11.7 °C. The RC will reach 6 kW under the condition of 65 °C hot water temperature, 30.5 °C cooling water temperature and 17.6 °C chilled water temperature. The results confirm that this kind of adsorption chiller is an effective refrigerating machine though its performance is not as fine as the prediction results. Also it is well effectively driven by a low-grade heat source. Therefore, its applications to the low-grade heat source are much attractive.     

Split heat pipe type compound adsorption ice making test unit for fishing boats

In order to settle the problem of the corrosion between sea water and the steel adsorber for ammonia system, a split heat pipe type adsorption ice making test unit, which use compound adsorbent of CaCl₂ and activated carbon to improve the adsorption performance, is designed and constructed. For this test unit there is mass recovery function between two beds and the CaCl₂ in compound adsorbent per bed is 1.88 kg, and there is only one pump for the whole heating and cooling phase for adsorbers. Performances of this system are tested; the lowest evaporating temperature is as low as −42 °C. At the evaporating temperature of −35 and −25 °C, the cooling powers are 0.89 and 1.18 kW, respectively. At the evaporating temperature of −15 °C, its average cooling power is 1.37 kW, which corresponds coefficient of performance of refrigeration COP=0.41 and specific cooling power per kilogram CaCl₂ of each adsorber SCP=731 W kg⁻¹. The mass recovery process has improved SCP and COP for the system by 15.5 and 24.1%, respectively. Heat transfer performance is also improved by the split heat pipe construction; the average heat transfer coefficient for a whole cycle is 155.8 W m⁻² °C⁻¹.     

Heat transfer of oil-contaminated HFC134a in a horizontal evaporator

The introduction of chlorine-free refrigerants to the market requires experimental investigations of their behaviour in heat pumps and refrigerators. One particular area of interest is the effect of the new oils on the heat transfer in evaporators and condensers. Oil can either increase or decrease the heat transfer coefficient. This paper presents the results from an experimental investigation of the effect of three different ester-based oils on the heat transfer of HFC134a in a horizontal evaporator. The tests were carried out at heat fluxes between 2 and 8 kW m⁻² (corresponding to mass fluxes between approximately 40 and 170 kg s⁻¹ m⁻²). The evaporation temperature was varied from−10 to +10°C. The global oil concentration ranged from 0 to 4.5 mass percentage based on the total liquid flow. The heat transfer coefficient decreased in most of the cases. The results indicate that the decrease seems to depend on the viscosity of the oil. The decrease can fairly well be estimated with the correlation for pure refrigerants by Shah if the viscosity of the mixture is used in the calculations. The data for the oil-contaminated refrigerant also agree well with data for pure refrigerants in a plot of α_tp/α_lo^* versus the inverse Martinelli-Lockhart parameter when α_lo^* is calculated with a modified Dittus-Boelter correlation and the mixture viscosity is used in the calculations. The heat transfer is found to increase when introducing oil in the special cases where the flow rate is low and the viscosity is low (oil A, 2 and 4 kW m⁻² oil B, 6kW m⁻² at +10°C). This is most likely due to surface tension effects. It has been suggested that the increased surface tension leads to a better tube wetting and thus an increased heat transfer.     

Experimental study on forced convective boiling heat transfer of pure refrigerants and refrigerant mixtures in a horizontal tube

Convective boiling heat transfer coefficients of pure refrigerants (R22, R32, R134A, R290, and R600a) and refrigerant mixtures (R32/R134a, R290/R600a, and R32/R125) are measured experimentally and compared with Gungor and Winterton correlation. The test section is made of a seamless stainless steel tube with an inner diameter of 7.7 mm and is uniformly heated by applying electric current directly to the tube. The exit temperature of the test section was kept at 12°C ± 0.5°C for all refrigerants in this study. Heat fluxes are varied from 10 to 30 kW m⁻² and mass fluxes are set to the discrete values in the range of 424–742 kg m⁻² s⁻¹ for R22, R32, R134a, R32/R134a, and R32/R125; 265–583 kg m⁻² s⁻¹ for R290, R600a, and R290/R600a. Heat transfer coefficients depend strongly on heat flux at a low quality region and become independent as quality increases. The Gungor and Winterton correlation for pure substances and the Thome-Shakil modification of this correlation for refrigerant mixtures overpredicts the heat transfer coefficients measured in this study.     

Study of a novel silica gel–water adsorption chiller. Part I. Design and performance prediction

A novel silica gel–water adsorption chiller is designed and its performance is predicted in this work. This adsorption chiller includes three vacuum chambers: two adsorption/desorption (or evaporation/condensation) vacuum chambers and one heat pipe working vacuum chamber as the evaporator. One adsorber, one condenser and one evaporator are housed in the same chamber to constitute an adsorption/desorption unit. The evaporators of two adsorption/desorption units are combined together by a heat-pipe heat exchanger to make continuous refrigerating capacity. In this chiller, a vacuum valve is installed between the two adsorption/desorption vacuum chambers to increase its performance especially when the chiller is driven by a low temperature heat source. The operating reliability of the chiller rises greatly because of using fewer valves. Furthermore, the performance of the chiller is predicted. The simulated results show that the refrigerating capacity is more than 10 kW under a typical working condition with hot water temperature of 85 °C, the cooling water temperature of 31 °C and the chilled water inlet temperature of 15 °C. The COP exceeds 0.5 even under a heat source temperature of 65 °C.     

Seasonal performance of a central air conditioner in Dhahran, Saudi Arabia. A computer simulation study

Seasonal performance of four standard, residential-sized, central air conditioners in a typical hot and humid climate of Gulf environment was measured. A computer model was developed to stimulate the residential building cooling loads and the resulting seasonal energy consumption. For an average 100 m² residence located in Dhahran, equipped with a 17.6 kW air conditioner, simulations show a consumption of ≈22200 kWh of electricity for space cooling over 2600 operation hours, this being within 13% of the measured value. The seasonal performance of three lower capacity (10.5, 12.3 and 14.0 kW) air conditioners for the same duty was also predicted. With the 14.0 kW capacity unit the electricity consumption is reduced by ≈ kWh and can satisfy the load for 96.6% of the time. The smallest (10.5kW) of the four systems can supply the house cooling load for only ≈78% of the time with a seasonal energy consumption of 17 350 kWh.     

Experimental investigation of a micro-combined cooling, heating and power system driven by a gas engine

An experimental investigation of the performance of a micro-combined cooling, heating and power (CCHP) system is described. The natural gas and LPG-fired micro-CCHP system uses a small-scale generator set driven by a gas engine and a new small-scale adsorption chiller, which has a rated electricity power of 12 kW, a rated cooling of 9 kW and a rated heating capacity of 28 kW. Silica gel–water is used as working pair in the adsorption cooling system. The refrigeration COP of the adsorption chiller is over 0.3 for 13 °C evaporation temperature. The test facility designed and built is described, which supplies better test-rig platform for cooling, heating and power cogeneration. Experimental methodology of this system is presented and the results are discussed. An energetic analysis of micro-CCHP system is performed as well. The overall thermal and electrical efficiency is over 70%.     

Surface heat transfer coefficients to stationary spherical particles in an experimental unit for hydrofluidisation freezing of individual foods

Convection heat transfer to spherical particles inside a hydrofluidisation freezing unit was investigated. The unit contained a food tank with a perforated bottom plate to create agitating jets. An aqueous solution of 30% ethanol+20% glucose was used as the refrigeration medium in a temperature range of −20 to 0 °C and flow rates from 5 to 15 l min⁻¹. The lumped capacitance method was applied on cooling profiles of aluminium spheres of 5–50 mm to obtain surface heat transfer coefficients. Coefficients were within a range of 154–1548 W m⁻² °C⁻¹, and depended on diameter, flow rate, refrigeration temperature and fluid agitation level. The agitation due to jets was accounted for by means of an agitation Reynolds number in a Nusselt correlation A large variability of measured surface heat transfer coefficients was observed. This could be attributed to non-constant flow and turbulence fields in the refrigeration medium. The value of the heat transfer coefficient was compared to values determined on strawberries.     

Model development and validation of a solar cooling plant

This paper describes the dynamic model of a solar cooling plant that has been built for demonstration purposes using market-available technology and has been successfully operational since 2001. The plant uses hot water coming from a field of solar flat collectors which feed a single-effect absorption chiller of 35 kW nominal cooling capacity. The work includes model development based on first principles and model validation with a set of experiments carried out on the real plant. The simulation model has been done in a modular way, and can be adapted to other solar cooling-plants since the main modules (solar field, absorption machine, accumulators and auxiliary heater) can be easily replaced. This simulator is a powerful tool for solar cooling systems both during the design phase, when it can be used for component selection, and also for the development and testing of control strategies.     

Evaporation heat transfer for R-22 and R-407C refrigerant–oil mixture in a microfin tube with a U-bendTransfert de chaleur lors de l'évaporation de R-22 et de R-407C mélangés avec de l'huile dans un tube à micro-ailettes avec un coude en U

Evaporation heat transfer experiments for two refrigerants, R-407C and R-22, mixed with polyol ester and mineral oils were performed in straight and U-bend sections of a microfin tube. Experimental parameters include an oil concentration varied from 0 to 5%, an inlet quality varied from 0.1 to 0.5, two mass fluxes of 219 and 400 kg m⁻²s⁻¹ and two heat fluxes of 10 and 20 kW m⁻². Pressure drop in the test section increased by approximately 20% as the oil concentration increased from 0 to 5%. Enhancement factors decreased as oil concentration increased under inlet quality of 0.5, mass flux of 219 kg m⁻² s⁻¹, and heat flux of 10 kW m⁻², whereas they increased under inlet quality of 0.1, mass flux of 400 kg m⁻² s⁻¹, and heat flux of 20 kW m⁻². The local heat transfer coefficient at the outside curvature of an U-bend was larger than that at the inside curvature of a U-bend, and the maximum value occurred at the 90° position of the U-bend. The heat transfer coefficient was larger in a region of 30 tube diameter length at the second straight section than that at the first straight section.     

Experimental performance analysis and modelling of liquid desiccant cooling systems for air conditioning in residential buildings

The paper presents a new desiccant cooling cycle to be integrated in residential mechanical ventilation systems. The process shifts the air treatment completely to the return air side, so that the supply air can be cooled by a heat exchanger. Purely sensible cooling is an essential requirement for residential buildings with no maintenance guarantee for supply air humidifiers. As the cooling power is generated on the exhaust air side, the dehumidification process needs to be highly efficient to provide low supply air temperatures. Solid rotating desiccant wheels have been experimentally compared with liquid sorption systems using contact matrix absorbers and cross flow heat exchangers. The best dehumidification performance at no temperature increase was obtained in an evaporatively cooled heat exchanger with sprayed lithium chloride solution. Up to 7 g kg⁻¹ dehumidification could be reached in an isothermal process, although the surface wetting of the first prototype was low. The process then provides inlet air conditions below 20 °C for the summer design conditions of 32 °C, 40% relative humidity. With air volume flow rates of 200 m³ h⁻¹ the system can provide 886 W of cooling power.A theoretical model for both the contact absorber and the cross flow system has been developed and validated against experimental data for a wide range of operating conditions. A simulation study identified the optimisation potential of the system, if for example the surface wetting of the liquid desiccant can be improved.     

Defrost cycle performance for an air-source heat pump with a scroll and a reciprocating compressor

A 10.6 kW nominal cooling capacity air-source heat pump was tested according to ANSI/ASHRAE Standard 116-1983 for the frost acumulation and defrost cycle. These tests required indoor conditions of 21.1°C (70°F) dry-bulb, 15°C (60°F) maximum wet-bulb, with outdoor conditions of 1.7°C (35°F) dry-bulb, 0.5°C (30°F) wet-bulb. The unit was tested with the original scroll compressor and a reciprocating compressor that yielded similar heating performance. Heating capacity for the scroll system peaked at 8.4 kW (2.38 tons), while the reciprocating system heating capacity peaked at 8.5 kW (2.42 tons) during the frosting period. Heating capacities for the two system configurations differed by less than 1% during the frosting period. Power demand for the scroll system peaked at 2.9 kW, and the reciprocating system power demand peaked at 3.1 kW. During the frosting period, the reciprocating system power demand averaged 7% higher than the scroll system power demand. The reciprocating system completed a defrost 5% faster than the scroll system. Scroll system defrost time was 6.8 min while reciprocating system defrost time was 6.5 min. The volume of condensate produced differed by less than 3% with 1680 ml (102.5 in³) and 1640 ml (100 in³) produced by the scroll and reciprocating systems, respectively. Discharge pressures during defrost were within 3% with peak values of 1315 kPa (191 psia) and 1351 kPa (196 psia) for the scroll and reciprocating systems respectively. The reciprocating compressor produced higher levels of discharge superheat, peaking at 53°C (95°F) compared to the scroll system peak discharge superheat of 47°C (85°F). Overall, discharge superheat for the reciprocating system averaged 18% higher than the scroll system. The reciprocating system produced defrost refrigerant flowrates that averaged 3% higher than the scroll system. Refrigerant flowrates for the scroll and reciprocating systems peaked at 3.7 kg min⁻¹ (8.2 lbm min⁻¹) and 4.0 kg min⁻¹) (8.8 lbm min⁻¹) respectively.     

Two years of operation of a large solar cooling plant

A solar plant was installed with a surface area of 440 m² of evacuated-tube collectors, a reflecting surface of the same area and an absorption refrigeration system for air-conditioning the CERIT building of Pordenone. A 43 m³ heat-storage unit, at temperatures of 80–90°C, and a 150 m³ cold-storage unit were also installed. These components form the largest solar cooling plant in Europe. After two years of operation, it is possible to evaluate the project as a whole, and to appreciate the contribution of the flat solar reflector and the working of the absorption unit.     

Carbon–ammonia pairs for adsorption refrigeration applications: ice making, air conditioning and heat pumping

A thermodynamic cycle model is used to select an optimum adsorbent-refrigerant pair in respect of a chosen figure of merit that could be the cooling production (MJ m⁻³), the heating production (MJ m⁻³) or the coefficient of performance (COP). This model is based mainly on the adsorption equilibrium equations of the adsorbent–refrigerant pair and heat flows. The simulation results of 26 various activated carbon–ammonia pairs for three cycles (single bed, two-bed and infinite number of beds) are presented at typical conditions for ice making, air conditioning and heat pumping applications. The driving temperature varies from 80 °C to 200 °C. The carbon absorbents investigated are mainly coconut shell and coal based types in multiple forms: monolithic, granular, compacted granular, fibre, compacted fibre, cloth, compacted cloth and powder. Considering a two-bed cycle, the best thermal performances based on power density are obtained with the monolithic carbon KOH-AC, with a driving temperature of 100 °C; the cooling production is about 66 MJ m⁻³ (COP = 0.45) and 151 MJ m⁻³ (COP = 0.61) for ice making and air conditioning respectively; the heating production is about 236 MJ m⁻³ (COP = 1.50).     

Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system

Over the past few decades there have been considerable efforts to use adsorption (solid/vapor) for cooling and heat pump applications, but intensified efforts were initiated only since the imposition of international restrictions on the production and utilization of CFCs and HCFCs. In this paper, a dual-mode silica gel–water adsorption chiller design is outlined along with the performance evaluation of the innovative chiller. This adsorption chiller utilizes effectively low-temperature solar or waste heat sources of temperature between 40 and 95 °C. Two operation modes are possible for the advanced chiller. The first operation mode will be to work as a highly efficient conventional chiller where the driving source temperature is between 60 and 95 °C. The second operation mode will be to work as an advanced three-stage adsorption chiller where the available driving source temperature is very low (between 40 and 60 °C). With this very low driving source temperature in combination with a coolant at 30 °C, no other cycle except an advanced adsorption cycle with staged regeneration will be operational. The drawback of this operational mode is its poor efficiency in terms of cooling capacity and COP. Simulation results show that the optimum COP values are obtained at driving source temperatures between 50 and 55 °C in three-stage mode, and between 80 and 85 °C in single-stage, multi-bed mode.     

Nucleate boiling heat transfer coefficients of flammable refrigerants on various enhanced tubes

In this study, nucleate boiling heat transfer coefficients (HTCs) of five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were measured at the liquid temperature of 7 °C on a low fin tube of 1023 fins per meter, Turbo-B, and Thermoexcel-E tubes. All data were taken from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the decreasing order of heat flux. Flammable refrigerants' data showed a typical trend that nucleate boiling HTCs obtained on enhanced tubes also increase with the vapor pressure. Fluids with lower reduced pressure such as DME, isobutene, and butane took more advantage of the heat transfer enhancement mechanism of enhanced tubes than those with higher reduced pressure such as propylene and propane. Finally, Thermoexcel-E showed the highest heat transfer enhancement ratios of 2.3–9.4 among the tubes tested due to its sub-channels and re-entrant cavities.     

Flow boiling heat transfer to carbon dioxide: general prediction method

An updated version of the Kattan–Thome–Favrat flow pattern based, flow boiling heat transfer model for horizontal tubes has been developed specifically for CO₂. Because CO₂ has a low critical temperature and hence high evaporating pressures compared to our previous database, it was found necessary to first correct the nucleate pool boiling correlation to better describe CO₂ at high reduced pressures and secondly to include a boiling suppression factor on the nucleate boiling heat transfer coefficient to capture the trends in the flow boiling data. The new method predicts 73% of the CO₂ database (404 data points) to within ±20% and 86% to within ±30% over the vapor quality range of 2–91%. The database covers five tube diameters from 0.79 to 10.06 mm, mass velocities from 85 to 1440 kg m⁻² s⁻¹, heat fluxes from 5 to 36 kW m⁻², saturation temperatures from −25 °C to +25 °C and saturation pressures from 1.7 to 6.4 MPa (reduced pressures up to 0.87).     

Experimental study on a continuous adsorption water chiller with novel design

A newly developed adsorption water chiller is introduced and tested. In the new adsorption refrigeration system, there are no refrigerant valves, the problem of mass transfer resistance resulting in pressure drop along refrigerant passage in conventional systems when methanol or water is used as refrigerant can be absolutely solved. Silica-gel–water is used as working pair and mass recovery-like process is adopted in order to use low temperature heat source ranging from 70 to 85 °C effectively. The experiment results demonstrate that the chiller (26.4 kg silica-gel in each adsorber) has a cooling capacity of 2–7.3 kW and COP ranging 0.2–0.42 according to different evaporating temperatures. Based on the experimental tests of the first prototype, the second prototype is designed and tested; the experimental data demonstrate that the chiller performance has been greatly improved, with a heat source temperature of 80 °C, a COP over 0.5 and cooling capacity of 9 kW has been achieved at evaporating temperature of 13 °C.     

Nucleate boiling heat transfer coefficients of mixtures containing HFC32, HFC125, and HFC134a

Nucleate boiling heat transfer coefficients (HTCs) of binary and ternary mixtures composed of HFC32, HFC125, and HFC134a on a horizontal smooth tube of 19.0 mm outside diameter were measured. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 kW m⁻² to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool temperature at 7 °C. HTCs of nonazeotropic mixtures of HFC32/HFC134a, HFC125/HFC134a, and HFC32/HFC125/HFC134a showed a reduction of HTCs as much as 40% from the ideal values while the near azeotropic mixture of HFC32/HFC125 did not show the reduction. Four of the well known correlations were compared against the present data for binary mixtures. Stephan and Körner's and Schlünder's correlations yielded a good agreement with a deviation of less than 10% but they can not be easily extended to multi-component mixtures of more than three components. A new correlation was developed utilizing only the phase equilibrium data and physical properties. A regression analysis was carried out to account for the reduction of HTCs and the final correlation, which can be easily extended to multi-component mixtures of more than three components, yielded a deviation of 7% for all binary and ternary mixtures.     

Freezing time and rate of cauliflower florets under different freezing conditions

In this study, the freezing time and rate of 1 cm³ cauliflower floret samples were determined under different freezing conditions in an air blast freezer. Four different air temperatures (−20, −25, −30 and −35°C) and six different air velocities (70, 131, 189, 244, 280 and 293 m min⁻¹) were applied in the freezer, and the freezing rate and time of cauliflower pieces were determined under each condition. The freezing time of cauliflower samples frozen with cold air at −20°C and 280 m min⁻¹ was similar to that of samples frozen with cold air at −35°C and 70 m min⁻¹. When the velocity of air was increased from 70 m min⁻¹ to 293 m min⁻¹, the freezing time was approximately halved.