Frost retardation of an air-source heat pump by the hot gas bypass method

This study is concerned with a hot gas (refrigerant) bypass method to retard the formation and propagation of frost in an air-source heat pump. The feasibility of the hot gas bypass method was investigated experimentally and the method's performance is compared with that of a normal, 1.12 kW capacity air-source heat pump system with no defrost equipment such as an electric resistance heater. Results indicate that the hot gas bypass method is useful for retarding the formation and growth of frost at the outdoor coil. The best performance is shown under a bypass refrigerant flow rate of 0.2 kg/min (20% of the whole system refrigerant flow rate). During 210 min of heat pump operation, the hot gas bypass method improved COP and heating capacity at an average of 8.5% and 5.7%, respectively, relative to the normal system.     

Hot gas defrost model development and validation

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.     

Frost, defrost, and refrost and its impact on the air-side thermal-hydraulic performance of louvered-fin, flat-tube heat exchangers

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.     

Refrigerant flow instability as a means to predict the need for defrosting the evaporator in a retail display freezer cabinet

For refrigerated display cabinets to perform their function of keeping food cold, there must be free movement of air through the evaporator. The moisture in the ambient air entrained in the cabinet forms frost on the evaporator. It is traditional for heat to be applied to the evaporator at regular intervals to melt this frost. The frequency, typically 3–4 times per day, is enough to avoid the frost becoming excessive even in extreme conditions. For much of the time defrosting is not always necessary. A large portion of the energy used during a defrost is an overhead – heating and then cooling the metal and the food rather than melting the frost. The effect of this is examined in the paper along with the results from testing an algorithm that detects the need for a defrost from the pattern of refrigerant flow (or evaporator exit superheat). The algorithm allows the number of defrosts to be reduced without excessively raising the temperature of food stored in the cabinet. The reduction in energy and carbon dioxide emission were examined and were shown to be substantial.     

Air-side heat transfer enhancement of a refrigerator evaporator using vortex generation

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.     

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.     

风冷热泵除霜过程动态特性模拟和实验研究

风冷热泵热气除霜过程动态特性的研究是风冷热泵(机组)研究的一个重要课题。在热气除霜实验研究的基础上，从基本守衡定律出发，提出了风冷热泵热气除霜过程的动态特性模型，重点模拟了这一过程中霜层侧的传热传质和制冷剂侧压力变化情况。回顾了热气除霜过程的研究现状，详细介绍了采用分布参数求解模型的过程，以及实验结果和模拟结果的比较。尽管在一定程度上这是一个理想化的动态特性模型，但实验结果和模拟结果的良好吻合证明了这一模型的有效性，可以应用于风冷热泵的全过程仿真研究。     

An analysis of the feasibility and characteristics of photoelectric technique applied in defrost-control

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.     

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 theoretical study of a novel regenerative ejector refrigeration cycle

There has been a demand for developments of the ejector refrigeration systems using low grade thermal energy, such as solar energy and waste heat. In this paper, a novel regenerative ejector refrigeration cycle was described, which uses an auxiliary jet pump and a conventional regenerator to enhance the performance of the novel cycle. The theoretical analysis on the performance characteristics was carried out for the novel cycle with the refrigerant R141b. Compared with the conventional cycle, the simulation results show that the coefficient of performance (COP) of the novel cycle increases, respectively, by from 9.3 to 12.1% when generating temperature is in a range of 80–160 °C, the condensing temperature is in a range of 35–45 °C and the evaporating temperature is fixed at 10 °C. Especially due to the enhanced regeneration with increasing the pump outlet pressure, the improvement of COP of the novel cycle is approached to 17.8% compared with that in the conventional cycle under the operating condition that generating temperature is 100 °C, condensing temperature is 40 °C and evaporating temperature is 10 °C. Therefore, the characteristics of the novel cycle performance show its promise in using low grade thermal energy for the ejector refrigeration system.     

Analysis of early-stage frost formation in natural convection over a horizontal cylinder

Frost growth process on a cold surface consists of two stages: The early-stage or one-dimensional growth of ice columns and multidimensional growth in the form of a porous structure. The transition time which marking these two stages is important for any numerical modeling of frost formation. This paper proposes a mathematical model to predict the transition time and frost properties in natural convection of frost formation over a cooled horizontal cylinder in the first stage of growth period. Comparison is performed among the results of this model and experimental observations reported in the literatures. It is observed that the presented model can be used more efficiently to determine transition time and frost properties in the early-stage of frost formation. Based on the obtained results a new correlation is developed for the duration time of early-stage of frost formation process (transition time) in natural convection.     

Simulation based identification of the ideal defrost start time for a heat pump system for electric vehicles

Resistance heating with PTC elements to cover the heat demand of electric vehicles reduces significantly the cruising range at low outside temperatures. Reversible heat pump systems are one of the most promising solutions for this problem. However, in heat pump mode the frost formation on the exterior heat exchanger reduces the performance and efficiency of the system. Therefore, an efficient defrost method is crucial to benefit from the heat pump also under frosting conditions. In the present paper, a transient Modelica simulation model of a reversible CO₂-heat pump system with hot gas defrost was set up in order to assess the impact of different defrost start times. The model is able to handle frost growth on the exterior heat exchanger as well as defrosting. The simulation results showed an optimal point of time to conduct defrost at chosen operating conditions in order to maximize the average COP including the frosting and defrost period.     

Modeling of frosting behavior on a cold plate

This paper proposes a mathematical model to predict the frost properties and heat and mass transfer within the frost layer formed on a cold plate. Laminar flow equations for moist air and empirical correlations for local frost properties are employed to predict the frost layer growth. Correlations for local frost density and effective thermal conductivity of the frost layer, derived from various experimental data, are expressed as a function of the various frosting parameters: the Reynolds number, frost surface temperature, absolute humidity and temperature of the moist air, cooling plate temperature, and frost density. The numerical results are compared with experimental data to validate the proposed model, and those agree well with the experimental data within a maximum error of 10%. Heat and mass transfer coefficients obtained from the numerical analyses are also presented. The results show that the model for the frost growth using the correlation of the heat transfer coefficient without considering the air flow has a limitation in its application.     

Effects of fan-starting methods on the reverse-cycle defrost performance of an air-to-water heat pump

The effects of fan starting methods on the defrost performance in an air-source heat pump were investigated experimentally. Experiments were conducted on a 50 kW unitary air-to-water heat pump. The dynamic characteristics of both the coil fan pre-start and normal-start tests during the defrost cycle were discussed. The peak of discharge pressure for the fan normal-start test was up to 2595.6 kPa during defrost termination and recovery time, which was close to the discharge protection value (2650 kPa). The discharge pressure for the fan pre-start test was 742.3 kPa lower at the end of the drain time, so the peak of the discharge pressure was 687 kPa lower during the defrost termination and recovery time than that for the fan normal-start test. We found the phenomenon seemed to be related to both the small inner volume of the plate heat exchanger and larger refrigerant flow rate during the defrost termination and recovery time. Pertinent performance data (pressure, temperature, superheat, sub-cooling, etc) was plotted and discussed to determine the effects of the coil fan pre-start.     

Frost formation on a cold surface under turbulent flow

This study presents a mathematical model to predict the frosting behavior on a cold surface under turbulent flow. The model consists of the standard κ–ε 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.     

Modeling of frost growth and frost properties with airflow over a flat plate

A physical model of frost layer growth and frost properties with airflow over a flat plate at subfreezing temperature was developed. Frost roughness was measured, and an empirical correlation for the average frost roughness was suggested. Heat and mass transfer coefficients were calculated using the modified Prandtl mixing-length scheme containing the effects of both frost roughness and turbulent boundary layer thickness. Frost thermal conductivity was theoretically analyzed by solving the combined equations of air equivalent conductivity and thermal conductivity of the frost inner layer. Based on the present model, heat and mass transfer coefficient, frost thermal conductivity, frost thickness, frost mass concentration and frost density with time and space were estimated. The model showed good agreement with the basic trends of the test data taken from other literature. Spatial and temporal changes of heat flux and frost surface temperature were also investigated.     

Methodology for thermal design of novel combined refrigeration/power binary fluid systems

Refrigeration cogeneration systems which generate power alongside with cooling improve energy utilization significantly, because such systems offer a more reasonable arrangement of energy and exergy “flows” within the system, which results in lower fuel consumption as compared to the separate generation of power and cooling or heating. This paper proposes several novel systems of that type, based on ammonia–water working fluid. Importantly, general principles for integration of refrigeration and power systems to produce better energy and exergy efficiencies are summarized, based primarily on the reduction of exergy destruction. The proposed plants analyzed here operate in a fully-integrated combined cycle mode with ammonia–water Rankine cycle(s) and an ammonia refrigeration cycle, interconnected by absorption, separation and heat transfer processes. It was found that the cogeneration systems have good performance, with energy and exergy efficiencies of 28% and 55–60%, respectively, for the base-case studied (at maximum heat input temperature of 450 °C). That efficiency is, by itself, excellent for cogeneration cycles using heat sources at these temperatures, with the exergy efficiency comparable to that of nuclear power plants. When using exhaust heat from topping gas turbine power plants, the total plant energy efficiency can rise to the remarkable value of about 57%. The hardware proposed for use is conventional and commercially available; no hardware additional to that needed in conventional power and absorption cycles is needed.     

An experimental study on minimizing frost deposition on a cold surface under natural convection conditions by use of a novel anti-frosting paint. Part I. Anti-frosting performance and comparison with the uncoated metallic surface

In this study, an experimental investigation is made of the anti-frosting performance of our newly developed anti-frosting paint. By coating the paint on a cold metal surface the onset of the frost formation may be delayed at least 15 min and the thickness of the deposited frost layer may be reduced by at least 40% and thus the weight of the deposited frost may be reduced by more than 40% compared with that on the uncoated copper surface. Under some preferable conditions (air relative humidity <60%, cold plate surface temperature >−10 °C), the coating surface can be kept free of frost at least for 3 h.     

X-ray investigation of a domestic refrigerator. Observations at 25°C ambient temperature

In this study, the transient behavior of a domestic refrigerator is investigated by the use of an X-ray system. The studies are made on a two-door upright freezer with a volume of 435 liters, and which has an automatic defrost feature. The refrigerant is R134a. During the experimental study, ambient temperature is held at 25±2 °C. Real time X-ray video images of the refrigeration circuit are taken during the pull-down (cooling down of the refrigerator from ambient temperature) and cyclic periods as well. X-ray images are recorded by focusing to on the dryer, capillary exit, evaporator inlet, and accumulator regions specifically. In order to watch evaporator and dryer sections continuously, two identical experiments are made while the probe is focused on either the evaporator or dryer sections each time. By matching the video images and temperature data, the flow regimes, charge inventory, accumulator functioning, and changes of subcooling degree at dryer inlet are explained. Possible flow induced noise mechanisms are identified.     

Dimensionless correlations of frost properties on a cold plate

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 kg_a⁻¹, and cooling plate temperature of −35–−15 °C.