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1.
This experimental study is carried out to investigate reliability and effectiveness of a new method of using photo-coupler for detecting frost formation in an air source heat pump, and further to determine the most efficient initiation point of the defrost cycle. This new method of using photo-coupler as a frost sensing device is evaluated by comparing its performance with conventional time control defrost system in which defrost cycle is set to start at predetermined interval, e.g. about at every 1–1.5 h. Results indicate that overall heating capacity of photo-coupler detection method (case IV) is 5.5% higher than that of time control method. It is also shown that for maximum efficiency the defrost cycle must be initiated before the frost build-up area exceeds 45% of total front surface of the outdoor coil.  相似文献   

2.
Hot gas defrost model development and validation   总被引:5,自引:0,他引:5  
This paper describes the development, validation, and application of a transient model for predicting the heat and mass transfer effects associated with an industrial air-cooling evaporator during a hot gas defrost cycle. The inputs to the model include the space dry bulb temperature, space humidity, coil geometry, frost thickness, frost density, and hot gas inlet temperature. The model predicts the time required for a complete frost melt as well as the sensible and latent loads transferred back to the conditioned space during the defrost period. The model is validated by comparing predicted results to actual defrost cycle field measurements and to results presented in previously published studies.A unique contribution of the present model is its ability to estimate parasitic space loads generated during a defrost cycle. The parasitic energy associated with the defrost process includes thermal convection, moisture re-evaporation, and extraction of the stored energy in the coil mass following a defrost cycle. Each of these factors contribute to the parasitic load on compressors connected to the defrost return. The results from the model provide quantitative information on evaporator operation during a defrost cycle which forms the basis to improve the energy efficiency of the defrost process.  相似文献   

3.
In almost all domestic refrigerators–freezers all components are assembled in the same relative position since several years ago. It is also known that the condenser releases heat at high temperatures (first law of thermodynamics) as well as the compressor. This heat is rejected to the environment in almost all practical situations partially by natural air convection. However, part of it is due to thermal radiation that causes an overheating of the refrigerator–freezer surfaces adjacent to those equipments. As a consequence there are more heat gains to the refrigerator–freezer through these surfaces and hence higher air temperatures inside. This paper describes how a simple technique can be very useful in order to minimize that part of heat transfer by radiation. The improvement is achieved by placing a radiation shield – a sheet of aluminium foil – over the surfaces close to the condenser and the compressor. For validating this technique a refrigerator–freezer was monitored with thermocouples for the measurements of the inside air temperatures in two situations: with and without the radiation shield. Results show that with this practice the average inside air temperatures in the refrigerator–freezer could decrease to about 2 K. An available commercial code was used in order to simulate the air temperature distribution and air velocities inside the refrigerator cabinet in both situations. Results from the experimental apparatus and from simulations show that there is a good agreement between them which validates the experiments carried out.Also an available commercial code, the Fluent, was used to simulate the internal air temperature in both situations.  相似文献   

4.
This study presents a first-principles mathematical model developed to investigate the thermal behavior of a plate-type, roll-bond evaporator. The refrigerated cabinet was also taken into account in order to supply the proper boundary conditions to the evaporator model. The mathematical model was based on the mass, momentum and energy conservation principles applied to each of the following domains: (i) refrigerant flow through the evaporator channels; (ii) heat diffusion in the evaporator plate; and (iii) heat transmission to the refrigerated cabinet. Empirical correlations were also required to estimate the shear stresses, and the internal and external heat transfer rates. The governing partial differential equations were discretized through the finite-volume approach and the resulting set of algebraic equations was solved by successive iterations. Validation of the model against experimental steady-state data showed a reasonable level of agreement: the cabinet air temperature and the evaporator cooling capacity were predicted within error bands of ±1.5 °C and ±6%, respectively.  相似文献   

5.
Non-azeotropic refrigerant mixtures (NARMs) are investigated for a two-temperature level heat exchange process found in a domestic refrigerator-freezer. Ideal (constant air temperature) heat exchange processes are assumed. The results allow the effects of intercooling between the evaporator refrigerant stream and the condenser outlet stream to be examined in a systematic manner. For the conditions studied, an idealized NARM system will have a limiting coefficient of performance (COP) that is less than that of the best performing pure refrigerant component. However, for non-ideal heat exchange processes (gliding air temperature), the NARM-based system can have a higher limiting COP than a system running on either pure NARM component. Intercooling significantly affects the COP of NARM-based systems; however, depending on the location of ‘pinch points’ in the heat exchangers, only one intercooling heat exchanger may be needed to obtain a NARM's maximum refrigerator COP. The results are presented for mixtures of R22–R142b, R22–R123 and R32–R142b.  相似文献   

6.
The thermal-hydraulic performance under conditions of an initial frost growth on the air-side surface, and for subsequent ‘refrosting’ after a defrost period is experimentally studied for folded-louvered-fin, microchannel heat exchangers. In total, five heat exchangers are considered; the thermal performances during one frost-growth cycle for four different fin geometries are compared in terms of overall heat transfer coefficient, pressure drop, and j and f factors; the defrost and refrost characteristics of two heat exchangers are compared to explore geometry effects. Typically, the performance under refrosting conditions becomes periodic and repeatable after the third or fourth refrosting cycle. The allowable frost growth period (before a defrost is required), the defrost requirement, and the thermal-hydraulic performance depend on heat exchanger geometry for the specimens used in this study.  相似文献   

7.
本文实验研究了陈列柜蒸发器结霜对其风幕送风速度的影响.实验结果表明蒸发器结霜大大降低风幕的送风速度.因此,蒸发器必须定期融霜.本文设定不同的融霜周期考察融霜周期、融霜时间和柜温回升三者之间的关系,为确定最佳的融霜周期提供实验性依据.  相似文献   

8.
为了减缓冷藏陈列柜结霜,在一台中温立式敞开式冷藏陈列柜上加装减湿预冷器,并针对加装前后冷藏陈列柜的性能进行了实验。实验结果表明:在国家标准GB/T21001.1—2007《冷藏陈列柜第2部分:分类、要求和实验条件》中设定的Climate class 3实验条件下(测试环境温度为25℃,相对湿度为60%),加装减湿预冷器使冷藏陈列柜蒸发器的进口空气温度降低2.4℃,含湿量也相应降低;此外蒸发器的结霜速度变缓,在6h的融霜周期内融霜时间减少了28%,系统负荷降低了19.2%。  相似文献   

9.
A field investigation is performed on the frost formation at an evaporator of a commercial full-scale refrigerated container that uses R-12 as the working refrigerant. Results when compared with those from a numerical model presented earlier show that the model is capable of predicting the dynamic behavior of a direct expansion evaporator under both non-frosting and frosting conditions. The air outlet and energy transferred compare well between experiment and model, and within 20% for the air pressure drop. The frost occurrence and propagation agree well generally, with the frost formation first occurring at the first row where the refrigerant enters the evaporator.  相似文献   

10.
A general distributed model with two-phase flow for refrigerant coupled with a frost model is developed for studying the dynamic behavior of an evaporator. The equations are derived in non-steady-state manner for the refrigerant and a quasi-steady state model with permeation for the frost. The complex flow and geometry of the finned tube evaporator lead to uneven wall and air temperature distributions, which in turn affect the rate of frost growth and densification along the coil depth. Results include frost accumulation and its effect on energy transfer, air off-coil temperature, refrigerant liquid dry-out position and propagation of frost formation along the coil.  相似文献   

11.
As per the Montreal Protocol, CFCs and HCFCs are being phased out. HCFC-22 is used in window air conditioners. This paper presents the experimental performance study of a window air conditioner with propane (HC-290), a natural refrigerant, as a drop-in substitute to HCFC-22. Experimental results showed that HC-290 had 6.6% lower cooling capacity for the lower operating conditions and 9.7% lower for the higher operating conditions with respect to HCFC-22. The coefficient of performance for HC-290 was 7.9% higher for the lower operating conditions and 2.8% higher for the higher operating conditions. The energy consumption of the unit with HC-290 was lower in the range 12.4–13.5% than HCFC-22. The discharge pressures for HC-290 were lower in the range 13.7–18.2% than HCFC-22. For HC-290, the pressure drop was lower than HCFC-22 in both heat exchangers.This paper also presents simulation results for the heat exchangers of an HCFC-22 window air conditioner with HC-290 as a drop-in substitute. The simulation has been carried out using EVAP-COND, a heat exchanger model developed by NIST [National Institute of Standards and Technology. EVAP-COND: simulation models for finned-tube heat exchangers, Maryland, USA (2003). http://www2.bfrl.nist.gov/software/evap-cond/ [18]]. The simulated evaporator capacities are within ±4% of the experimentally measured cooling capacities for both refrigerants. Simulation results for HC-290 and HCFC-22 are compared. The exit temperatures of HC-290 are lower by 0.3–1.2 °C in the condenser and are higher by 2.1–2.4 °C in the evaporator than HCFC-22. Evaporating pressures of HC-290 are lower by 2.1–3.3% as compared to HCFC-22. The pressure drops of HC-290 are lower in both the evaporator and the condenser as compared to HCFC-22. The outlet temperatures of air for HCFC-22 and HC-290 in both heat exchangers are nearly the same.  相似文献   

12.
This study proposes dimensionless correlations for predicting the properties of frost formed on a cold plate. Frosting experiments are carried out to obtain the correlations with various environmental parameters including the air temperature, air velocity, absolute humidity, and cooling plate temperature. The thickness, density, surface temperature, effective thermal conductivity, average heat and mass transfer coefficients of the frost layer are correlated as functions of the Reynolds number, Fourier number, absolute humidity, and dimensionless temperature by using a dimensional analysis. The correlations proposed in this study agree well with the experimental data within a maximum error of 10%, and can be used to predict the average frost properties in the following ranges: the air temperature of 5–15 °C, air velocity of 1.0–2.5 m s−1, absolute humidity of 0.00322–0.00847 kg kga−1, and cooling plate temperature of −35–−15 °C.  相似文献   

13.
The present paper advances an in-situ study of frosting and defrosting processes in tube-fin evaporators of household refrigerators and freezers. Frost growth experiments were carried out using a purpose-built test facility comprised of a refrigerated cabinet placed inside a climatized chamber, and a vapor compression refrigeration loop that controls both the evaporating temperature and the evaporator superheating. A first-principles two-dimensional simulation model was put forward to predict the evaporator blocking over time, and also the heat and mass transfer rates. The model validation exercise has shown that the model follows well the experimental trends of pressure drop, frost mass, cooling capacity and air flow rate, with all predictions falling inside ±10% error bounds. The model was then used to assess the impact of several design parameters on both the evaporator blockage and defrost efficiency. It was found that a nearly ideal defrost efficiency is achieved when the defrosting process is performed by two simultaneous heaters.  相似文献   

14.
This study presents a mathematical model to predict the frosting behavior on a cold surface under turbulent flow. The model consists of the standard κε model for turbulent flow and the diffusion equation for the frost layer. The numerical results show that turbulent flow promotes the growth of the frost layer on the cold surface, compared to the laminar flow. Increase in air velocity has little effect on mass transfer under turbulent flow, while frost growth under laminar flow is influenced by the air velocity. With constant air humidity, the frost layer thickness increases with decreasing air temperature, while the relationship for the frost density is reversed. The effect of the air temperature on the mass flux is negligible, compared to the other frosting parameters.  相似文献   

15.
In most domestic and commercial refrigeration systems, frost forms on the air-side surface of the air-to-refrigerant heat exchanger. Frost-tolerant designs typically employ a large fin spacing in order to delay the need for a defrost cycle. Unfortunately, this approach does not allow for a very high air-side heat transfer coefficient, and the performance of these heat exchangers is often air-side limited. Longitudinal vortex generation is a proven and effective technique for thinning the thermal boundary layer and enhancing heat transfer, but its efficacy in a frosting environment is essentially unknown. In this study, an array of delta-wing vortex generators is applied to a plain-fin-and-tube heat exchanger with a fin spacing of 8.5 mm. Heat transfer and pressure drop performance are measured to determine the effectiveness of the vortex generator under frosting conditions. For air-side Reynolds numbers between 500 and 1300, the air-side thermal resistance is reduced by 35–42% when vortex generation is used. Correspondingly, the heat transfer coefficient is observed to range from 33 to 53 W m−2 K−1 for the enhanced heat exchanger and from 18 to 26 W m−2 K−1 for the baseline heat exchanger.  相似文献   

16.
This paper discusses the conservation of energy in a cogeneration system. A steam power cycle (Rankine) produces electrical power 2 MW and steam is bleeded off from the turbine at 7 bar to warm a factory or units of buildings during the winter or to supply a steam ejector refrigeration cycle to air-conditioning the same area during the summer. In the summer this system can be as alternative solution instead of absorption. Certainly the ejector refrigeration unit is more economical than absorption unit. The ratio of electrical power/heat is varied into the region (0.1–0.4) and the evaporator temperature of the ejector cycle is varied into the region (10–16 °C). A computer program has been developed for the study of performance parameters of the cogeneration system.  相似文献   

17.
An innovative photoelectric technology is presented in this paper for detecting frost in the field of defrost-control of refrigerator or freezer applications. Experiments were conducted with a small-scale laboratory test section under natural convection conditions. Two agreeable properties of the new technology, “on–off” and “linear”, have been demonstrated by the experiments. The first property provides an effective judgment of the defrost-control strategy while the second one is suitable for developing an accurate measurement of the frost height. The characteristics (electric current, environment temperature, metal surface temperature, light intensity and sensor location) that affect the properties were investigated for the development of this technology.  相似文献   

18.
This paper describes a field experimental investigation of the effects of frost formation on the performance of a low-temperature large-scale evaporator coil used in industrial refrigeration systems. A series of experiments were conducted to determine the in situ coil cooling capacity of the evaporator over time as frost builds on its surfaces. Field-measured quantities include inlet and outlet air temperatures, inlet and outlet air relative humidity, and air volume flow rate. These measurements provide a baseline set of experimental data that can be used to validate numerical models of industrial evaporators operating under frosting conditions.  相似文献   

19.
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.  相似文献   

20.
A hybrid method for optimizing refrigerant distribution in evaporators is presented that involves the use of small balancing valves in each circuit along with a primary expansion device to control the overall superheat from the evaporator. The flow balancing valves could be located upstream or downstream of the evaporator. This paper presents the results of a study to investigate the benefits of this hybrid scheme for both upstream and downstream flow balancing for the case of air flow mal-distribution. In order to perform this investigation, a simulation model was developed to consider evaporator flow mal-distributions for a 10.55 kW residential R410A heat pump and then validated through comparisons of predicted results with measurements. Simulation results show that there are significant benefits in controlling the superheat of each circuit of evaporators through the hybrid–individual superheat control method. Furthermore, the upstream refrigerant flow control consistently outperforms the downstream refrigerant flow control, and recovers most of the loss in cooling capacity and COP due to non-uniform air flow distribution.  相似文献   

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