Evaporator Air Research Articles

Experimental study on evaporator performance in mobile air conditioning system using HFO-1234yf as working fluid

The widely used evaporators with two different type were experimentally studied in psychrometric calorimeter room using HFO-1234yf as working fluids. The evaporators types are laminated plate and microchannel parallel flow (PF) type. The experimental results compared with R134a system show that cooling capacity has different features in laminated plate evaporator and microchannel PF evaporator under different refrigerant pressure at expansion valve inlet. In general, cooling capacity of laminated plate evaporator of HFO-1234yf is reduced up to 8.0%. But in microchannel PF evaporator, cooling capacity is comparable and/or larger than that of R134a up to 6.5%. The HFO-1234yf evaporator air off temperature deviation is larger than that of R134a evaporator. The air side pressure drop is very close for both evaporators and refrigerants. HFO-1234yf refrigerant side pressure drop is larger than that of R134a for both evaporators.

Experimental study of a novel capacity control algorithm for a multi-evaporator air conditioning system

The use of a multi-evaporator air conditioning (MEAC) system is advantageous in terms of installation convenience, high design flexibility, being easy to maintain and commission, better indoor thermal comfort control and higher energy efficiency. While MEAC units worth billions of dollars are sold worldwide, the detailed accounts on compressor capacity control and refrigeration flow distribution amongst evaporators remain unavailable in public domain, mainly due to commercial confidentiality. Limited control algorithms for MEAC systems have been developed based on system simulation, and no experimental-based capacity controller developments and their controllability tests may be identified in open literature. In the study reported in this paper, a novel capacity control algorithm, which imitated On–Off control of a single evaporator air conditioning (A/C) system in each indoor unit of a MEAC system by using variable speed compressor and electronic expansion valves (EEVs), was developed. Controllability tests under various settings for experimentally validating the novel capacity control algorithm were carried out and the control algorithm was further improved based on the experimental results.

Thermoeconomic design and optimization of frost-free refrigerators

An optimization methodology aiming at energy and cost savings in frost-free refrigerators is advanced. A steady-state simulation model was devised and its predictions for energy consumption were compared with experimental data obtained elsewhere, with differences not exceeding a +/− 9% error band. The model was used to find out the evaporator air flow rate, the number of evaporator and condenser fins, the compressor size and efficiency, and the insulation thicknesses of both fresh- and frozen-food compartments which yield the minimum energy consumption when the total cost is constrained. The optimization led to a refrigeration system with energy consumption 18% lower than that observed for the baseline system without any cost penalty. The system performance for four different compressors, including a high efficiency, a large capacity and a variable-speed one, was also analyzed. For the refrigerator under analysis, it was found that, to ensure an economically feasible design, the variable-speed compressor should not cost 26% more than a single-speed one with the same piston displacement.

Modelling and simulation of a heat pump for simultaneous heating and cooling

The heat pump for simultaneous heating and cooling (HPS) carries out space heating, space cooling and hot water production for small office and residential buildings. It works under heating, cooling and simultaneous modes to produce hot and chilled water according to the thermal demand of the building. A subcooler connected to a water tank is placed after the condenser to recover some energy by subcooling of the refrigerant during a heating mode. The water loop at a higher temperature than ambient air is used subsequently as a source for a water evaporator. Average winter performance is improved compared to a standard reversible heat pump (HP). The air evaporator is defrosted by a two-phase thermosiphon without stopping the heat production. The operation of the HPS is modelled using TRNSYS software. The model is validated using results of an experimental study carried out on a HPS prototype working with R407C. Annual simulations of the HPS coupled to a hotel are run in order to evaluate annual performance and energy consumption of the system. The results are compared to the ones of a standard reversible HP. Depending on the scenario, savings in electric energy consumption and annual performance improvement can reach respectively 55% and 19%.

Experimental Study of an Internal Heat Exchanger Influence on R134a Automobile Air Conditioning System

Liquid-suction or internal heat exchanger (IHX) is installed in vapor compression refrigeration systems to exchange energy between cool low-pressure gas and warm high-pressure liquid refrigerant. The aim of this research is to experimentally evaluate the effect of IHX adaptation in an automotive air conditioning system focusing on evaporator working conditions. In this new design of IHX, the high-pressure liquid passes through the central channel and the low-pressure vapor flows in several parallel channels in the opposite direction. The experimental set-up has been made up of original components of the air conditioning system of a medium sedan car, specially designed and built to analyze vehicle A/C equipments under real work conditions. The results show that low pressure drop will be imposed on the cycle using this type of IHX. Also, they confirm considerable decrease of compressor power consumption; it is intensified at higher evaporator air flow. A significant improvement of the coefficient of performance will be achieved with IHX too. The influence of operating conditions has been also discussed in this paper.

Experimental study of an air-source heat pump for simultaneous heating and cooling – Part 2: Dynamic behaviour and two-phase thermosiphon defrosting technique

This article presents the concepts of an air-source heat pump for simultaneous heating and cooling (HPS) designed for hotels and smaller residential, commercial and office buildings in which simultaneous needs in heating and cooling are frequent. The main advantage of the HPS is to carry out simultaneously space heating and space cooling with the same energy input. Ambient air is used as a balancing source to run a heating or a cooling mode. The second advantage is that, during winter, energy recovered by the subcooling of the refrigerant is stored at first in a water tank and used subsequently as a cold source at the water evaporator to improve the average performance and to carry out defrosting of the air evaporator using a two-phase thermosiphon. Unlike conventional air-source heat pumps, defrosting is carried out without stopping the heat production. A R407C HPS prototype was built and tested. The basic concepts of the HPS are detailed in part1 of this article [1]. Its performance on defined operating conditions corresponds to the data given by the selection software of the compressor manufacturer. In the present part of this article, the operation of the high pressure control system, the transitions between heating, cooling and simultaneous modes and the defrosting sequence are analysed and validated experimentally.

Experimental study of an air-source heat pump for simultaneous heating and cooling – Part 1: Basic concepts and performance verification

This article presents the concepts of an air-source heat pump for simultaneous heating and cooling (HPS) designed for hotels and smaller residential, commercial and office buildings in which simultaneous needs in heating and cooling are frequent. The main advantage of the HPS is to carry out simultaneously space heating and space cooling with the same energy input. Ambient air is used as a balancing source to run a heating or a cooling mode. The second advantage is that, during winter, energy recovered by the subcooling of the refrigerant is stored at first in a water tank and used subsequently as a cold source at the water evaporator to improve the average performance and to carry out defrosting of the air evaporator using a two-phase thermosiphon. Unlike conventional air-source heat pumps, defrosting is carried out without stopping the heat production. A R407C HPS prototype was built and tested. Its performance on defined operating conditions corresponds to the data given by the selection software of the compressor manufacturer. The operation of the high pressure control system, the transitions between heating, cooling and simultaneous modes and the defrosting sequence were validated experimentally and are presented in the second part of this article [1].

Performance assessment of a direct expansion air conditioner working with R407C as an R22 alternative

Abstract This paper presents an experimental investigation of a direct expansion air conditioner working with R407C as an R22 alternative. Experiments are conducted on a vapor compression refrigeration system using air as a secondary fluid through both the evaporator and the condenser. The influences of the evaporator air inlet temperature (20–32 °C), the evaporator air flow rate (250–700 m 3 /h) and the evaporator air humidity ratio (9 and 14.5 g wv /kg a ) at the condenser air temperature and volume flow rate of 35 °C and 850 m 3 /h, respectively on the system performance are investigated. Experimental results revealed that the evaporator air inlet temperature has pronounced effects on the air exit temperatures, pressures of the evaporator and the condenser, cooling capacity, condenser heat load, compressor pressure ratio and the COP of both refrigerants at humidity ratios of 9 and 14.5 g wv /kg a . Significant effects of the evaporator air flow rate are also gathered on the preceding parameters at the same values of mentioned-humidity ratios. The best performance, in terms of operating parameters as well as COP, can be accomplished using R22 compared to R407C. The inlet humidity ratio affects dramatically the performance of vapor compression system using R22 and R407C. The raising up humidity ratio from 9 to 14.5 g wv /kg a leads to an augmentation in the average cooling capacity by 29.4% and 38.5% and an enhancement in the average COP by 30% and 24.1% for R22 and R407C, respectively.

Experimental investigation on a solid desiccant system integrated with a R407C compression air conditioner

In the present work, experimental performance data of a solid desiccant based hybrid air conditioning system are presented. The system consists of a packed bed solid desiccant integrated with a R407C conventional vapor compression refrigeration system. Experiments are carried out during dehumidification operation mode for various operating parameters such as; desiccant mass on shelves (5, 10 and 15 kg), air mass flow rate (7.4 and 10.2 kg/min), shelves number (1, 2 and 3) and three values of shelves span (7, 14 and 28 cm) at evaporator air inlet conditions of 28 °C DBT and 66% RH, condenser air inlet volume flow rate of 850 m 3/h and temperature of 35 °C. The reactivation of the desiccant at different regeneration temperatures and air flow rates as well as desiccant masses is also investigated. During the dehumidification mode, the average system coefficient of performance increases by 6.2% and 1.61% when the mass of desiccant increases from 5 to 10 kg and from 10 to 15 kg, respectively. The enhancement in the coefficient of performance is 6.2% due to increasing the air mass flow rate from 7.4 to 10.2 kg/min. Increasing both shelves number and span yields to a reduction in the adsorption rate that can be extracted by the desiccant material in the ranges of considered operating conditions. The regeneration temperature and the air flow rate of regeneration have significant effects on the reactivation process. It was found that, with increasing the mass flow rate of regenerated air from 7.4 to 10.2 kg/min produces a reduction in regeneration time by 87.5% and an augmentation in the desorption rate by 16% after 10 min of regeneration. In addition, with escalating the regeneration temperature from 45 to 55 °C, the reactivation time reduces by 25%. Reported results revealed that solid desiccant based hybrid air conditioning system reduces the compressor electric power and the number of electric unit (kW h) by 10.2%.

Design and simulation of a heat pump for simultaneous heating and cooling using HFC or CO 2 as a working fluid

This article presents a Heat Pump for Simultaneous heating and cooling (HPS) designed for hotels, luxury dwellings and smaller office buildings. The main advantage of the HPS is to carry out simultaneously space heating and space cooling in a dual mode. The ambient air is used as a balancing source to run a heating or a cooling mode. The HPS also participates to domestic hot water preparation all year round. The second advantage is that, during winter, some energy recovered by subcooling of the refrigerant is stored at first in a cold water tank that is not used for cooling. This energy is used subsequently as a cold source at the water evaporator in order to improve the average coefficient of performance and to run a defrosting sequence at the air evaporator. Two refrigerants are studied: HFC R407C and carbon dioxide. HFCs provide good performance, but new restrictive regulations on F-gases lead us to study low-GWP refrigerants as well. Highly efficient models of compressors and heat exchangers have been defined. Annual simulations show that CO 2 is a refrigerant which adapts rather well to the operation of the HPS thanks to the higher amount of energy available by subcooling and the large temperature glide at heat rejection used for DHW production.

Determination of optimum working conditions R22 and R404A refrigerant mixtures in heat-pumps using Taguchi method

In this study, refrigerants R22 and R404A five of their binary mixtures which contain about 0%, 25%, 50%, 75% and 100% mass fractions of R404A were tested. It is investigated experimentally the effects of gas mixture rate, evaporator air inlet temperature (from 24 to 32 °C), evaporator air mass flow rate (from 0.58 to 0.74 kg/s), condenser air inlet temperature (from 22 to 34 °C) and condenser air mass flow rate (from 0.57 to 0.73 kg/s) on the coefficient of performance (COP) and exergetic efficiency values of vapor compression heat-pump systems. To determine the effect of the chosen parameters on the system and optimum working conditions, an experimental design method suggested by Genichi Taguchi was used. In this study, it was observed that the most effective parameters are found to be the condenser air inlet temperature for COP and exergetic efficiency.

Transcritical CO2refrigerator and sub-critical R134a refrigerator: A comparison of the experimental results

SUMMARY This paper describes experiments comparing a commercial available R134a refrigeration plant subjected to a cold store and a prototype R744 (carbon dioxide) system working as a classical ‘split-systems’ to cool air in residential applications in a transcritical cycle. Both plants are able to develope a refrigeration power equal to 3000 W. The R744 system utilizes aluminium heat exchangers, a semi-hermetic compressor, a back-pressure valve and a thermostatic expansion valve. The R134a refrigeration plant operates using a semi-hermetic reciprocating compressor, an air condenser followed by a liquid receiver, a manifold with two expansion valves, a thermostatic one and a manual one mounted in parallel, and an air cooling evaporator inside the cold store. System performances are compared for two evaporation temperatures varying the temperature of the external air running over the gas-cooler and over the condenser. The refrigeration load in the cold store is simulated by means of some electrical resistances, whereas the air evaporator of the R744 plant is placed in a very large ambient. The results of the comparison are discussed in terms of temperature of the refrigerants at the compressor discharge line, of refrigerants mass flow rate and of coefficient of performance (COP). The performances measured in terms of COPs show a decrease with respect to the R134a plant working at the same external and internal conditions. Further improvements regarding the components of the cycle are necessary to use in a large-scale ‘split-systems’ working with the carbon dioxide. Copyright r 2009 John Wiley & Sons, Ltd.

Theoretical and experimental simulation of roof-top bus multiple-circuit air-conditioning system performance

Many air-conditioning (AC) systems are designed to operate at maximum cooling capacity regardless of the variation in the daily cooling load. At low loads, the conditions can be uncomfortably cold and the overcooling is an unnecessary waste of energy. To address these two issues, a multiple refrigeration circuit concept is proposed and applied to a roof-top bus AC system. A two-circuit model is proposed for a standard bus size in which each circuit has two evaporators of equal sizes arranged in parallel and installed on each passenger row, respectively. This means that each passenger row is served by two different evaporators sharing a common heat exchanger box. Depending on the cooling load, this concept allows one or both circuits (compressor motors) to be switched on and during either modes, it also allows one or more sets of evaporator blowers to be switched on. A steady-state computer model has been developed to simulate the performance of the proposed two-circuit AC system. A two-circuit air conditioner is also designed to form a roof-top bus AC system, fabricated, and installed on to an experimental rig. The experimental data are used to validate the computer model. The validation is on the system thermal performance and on the evaporator air outlet conditions (dry bulb temperature and relative humidity) at different modes of system operation, either at full or partial cooling loads. The simulated results gave satisfactory agreement with those obtained from the experimental work. Maximum absolute deviations are within the range of 19.3 per cent, although most of the simulated results are less than a 10 per cent range from the experimental ones, which validates the computer program. The paper describes the modelling work carried out and the results obtained are presented in comparison with the experimental data.

Defrost cycle performance for a circular shape evaporator air source heat pump

Air source heat pumps have numerous advantages in many applications over other heating equipment with regard to energy efficiency. However, there are two main problems with air source heat pumps: (1) heating capacity decreases when the outdoor air temperature becomes lower and (2) when there is frost formation on the outdoor heat exchanger surfaces in humid climates. This paper will examine the defrost cycle for a residential heat pump with circular shaped evaporator coil in more detail paying special attention to the high humidity conditions encountered in maritime climates. The investigation was to optimise the efficiency of an air source heat pump operating under a range of conditions that would include defrost. Performance optimisation was achieved through a series of experiments carried out to the EN14511 test standard from which it was possible to note the best defrost initiation condition, defrost operating time and intervals between defrosts that most benefited the performance of the heat pump.

A simulation model for the performance of a hybrid liquid desiccant system during cooling and dehumidification

A finite difference model describing simultaneous heat and mass transfer in a hybrid liquid desiccant cooling system is presented in this paper. This type of system provides cooled and dehumidified air by a combination of a packed absorber tower and a conventional vapour compression system. The model will allow the prediction of the rate of condensation in the dehumidification tower and the evaporator of the vapour compression system, the conditions of the air and liquid desiccant leaving the tower, the conditions of the air leaving the evaporator, and other important parameters. The mathematical model results were validated with existing experimental data for a similar system with an uncertainty of 10.5% for the total rate of condensation and 0.8°C for the evaporator air exit temperature. Copyright © 2005 John Wiley & Sons, Ltd.

R134a and various mixtures of R22/R134a as an alternative to R22 in vapour compression heat pumps

The performance of an air to water vapor compression heat pump has been investigated experimentally. The main purpose of this study was to investigate the possibilities of using R134a as a working fluid to replace R22 for vapor compression heat pumps. Pure R22, pure R134a and some binary mixtures of R22/R134a were considered as working fluids. The performance of the system was characterized by mixture ratio, COP and evaporator air inlet temperature. Comparisons are made between the pure refrigerants and refrigerant mixtures on the basis of the COP. Experimental results show that the mixture ratio affects the COP significantly, and the COP could be improved by using pure R134a or an appropriate mixture of R134a/R22 instead of pure R22. The maximum COP occurred at a mixture ratio of around 50/50% R134a/R22. For a mass percentage of 50% of R134a, the COP was enhanced by about average 25%.

Testing of a heat-pump-assisted mechanical opener dryer

In this study a heat-pump-assisted dryer with an opener mechanism was constructed and tested at the Energy Laboratory, Mechanical Engineering Department, Balikesir University, Turkey. The test material used was wet wool, which is a fibrous material. The variables in the experiments were the air rate, the recirculation air ratio (RAR) and the bypass air ratio. In the experiments, the air mass velocities ranged from 0.78 to 1.50 kg/m 2 s. The bypass air and recirculation air ratios varied from 20% to 80%. For this system both coefficient of performance of the whole system (COP ws) and the specific moisture extraction rate of the whole system (SMER ws) increased as evaporator air ratios increased. COP ws was found to range from 2 to 3.5 while SMER ws varied between 1.5 and 2.8 kg/kW h depending on RAR.

On the steady-state modelling of a two-stage evaporator system

We develop and validate against experimental measurements a steady-state two-stage flooded refrigerant evaporator model for a heat pump drying system. A prototype two-stage heat pump dryer test facility was designed, built and instrumented to provide the required measurements for the validation of the model. Repeatability and data quality tests were conducted to evaluate the accuracy of measurements. Experimental data could be reproduced to within ±6.5 per cent of replicated air and refrigerant side measurements for the same evaporator's air inlet conditions while the discrepancy of energy balance at the air-side and refrigerant-side was observed to be within ±8.9 per cent. The two-stage evaporator model predicted the air-side total heat and latent heat transfer of the two-stage evaporator to within (−6.3 per cent, 7.6 per cent) and (−11.5 per cent, 9.5 per cent), respectively. On the refrigerant-side, the model enabled the calculation of the degree of superheat to within (−10.6 per cent, 1.7 per cent). The model has shown that there is significant improvement in the heat recovered from a two-stage evaporator system compared to a single evaporator system. In addition, the model demonstrated that the improvement in total heat recovery could be as high as 40 per cent over its base-value when the latent to total load at the two-stage evaporator is increased. Copyright © 2001 John Wiley & Sons, Ltd.

Modeling of a Heat Pump With Evaporator Air Dehumidification for Reduced Frost Formation

This paper presents an analysis of a heat pump that uses a solid desiccant dehumidifier to reduce the humidity of the ambient air that flows into the evaporator, with the purpose of decreasing frost formation on the evaporator. The heat pump is analyzed by adding a dehumidifier model to a previously developed heat pump model that includes frost formation. The dehumidifier reduces the amount of energy required for evaporator defrosting, but introduces the need for energy for regenerating the desiccant. The purpose of the analysis is to search for operating conditions and optimum dehumidifier designs for which the use of the dehumidifier results in energy savings. The results show that the use of a dehumidifier may reduce energy consumption if the energetic cost of defrosting the evaporator is high. Other benefits of dehumidification include an increase of the time intervals between defrost cycles, a better stability in the conditions in the controlled space, and the potential for increased reliability and reduced maintenance of the heat pump.

Calorimeter measurements of a heat pump dehumidifier: Influence of evaporator air flow

The performance characteristics of a Rankine cycle heat pump dehumidifier have been measured in a psychrometric calorimeter which provides close control of the air state. The quality of the measurements was monitored by applying energy and mass balance test criteria. The results show that the influence of the evaporator air flowrate on the moisture extraction rate, and on the energy efficiency of the drier, depends on the relative humidity of the evaporator inlet air stream. When the relative humidity is high the dehumidification efficiency is maximized when the evaporator air flow is maximized, within the range of air flowrates investigated. Below 50% relative humidity the specific moisture extraction rate exhibits an increasingly peaked maximum as a function of the evaporator air flowrate. This maximum moves to lower air flows when the relative humidity is lower.