Dennis L. O'Neal

Effect of frost growth on the performance of louvered finned tube heat exchangers

Abstract An experimental investigation of the effects of frost growth on the performance of heat exchangers with louvered fins has been conducted. Frost accumulation, pressure drop across the heat exchanger and an energy transfer coefficient based on a logarithmic mean enthalpy difference (LMED) were quantified under frosting conditions as functions of the air humidity, air face velocity and fin spacing. Higher air humidity, air face velocities and smaller fin spacing all led to increased frost growth, higher pressure drops and higher energy transfer coefficients. As frost accumulated on the heat exchanger, the overall energy transfer coefficient eventually dropped. These trends are consistent with those reported in the literature.

Frosting performance of tube fin heat exchangers with wavy and corrugated fins

Abstract An experimental investigation was made into the effects of frost growth on the thermal performance of fin tube heat exchangers with wavy and corrugated fins. Although previous studies on frosting in heat exchangers have dealt with flat fins, there are few data on wavy or corrugated fins. Frost accumulation, pressure drop across the heat exchanger, and an energy transfer coefficient based on a logarithmic mean enthalpy difference (LMED) were quantified in terms of air humidity, fin type, and fin spacing. It was found that higher air humidity and fin density lead to more frost growth and higher pressure drops. These general trends are consistent with what has been reported in the literature. Contrary to earlier literature reports, the energy transfer coefficient was approximately constant for the duration of the test. In addition, it was found that the latent portion of the overall energy transfer process was approximately 40% of the total.

Effect of short-tube orifice size on the performance of an air source heat pump during the reverse-cycle defrost

Abstract Many air source heat pumps use the reverse-cycle defrost to eliminate frost that forms on the outdoor heat exchanger during normal winter operation. During the defrost, the heat pump is switched from the heating to the cooling mode to provide heat to the outdoor heat exchanger to melt the frost. Once the frost is melted and drained from the heat exchanger, the unit is switched back to the heating mode. The objective of this research was to characterize the effect of short-tube orifice diameter on the response of a heat pump during the reverse-cycle defrost. An experimental apparatus was constructed containing a nominal 3 ton (cooling capacity) residential air source heat pump. A manifold was constructed which allowed for the switching of different sized orifices by turning a shut-off valve. Refrigerant temperature and pressure measurements were made throughout the system as well as refrigerant flow-rates, air-side capacity, compressor/outdoor fan power and refrigerant level in the accumulator. A detailed comparison of the effect on defrost performance is provided.

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.

Performance of finned-tube heat exchangers under frosting conditions: I. Simulation model

Abstract This paper describes an analytical model that was developed to predict the performance of finned-tube heat exchangers under frosting conditions. The method models the frost growth mechanism and heat exchanger performance in a comprehensive manner. The results include frost accumulation and its effect on energy transfer in relation to varying humidities, fin densities and ambient conditions.

System performance characteristics of an air conditioner over a range of charging conditions

Abstract A popular type of residential air conditioner in the USA is the split system. These systems have an outdoor unit which houses the condenser, condenser fan, compressor and controls. A separate indoor unit houses the evaporator, expansion device and evaporator blower. As the refrigerant charge of these systems is adjusted during field installation, the potential exists for not setting the charge exactly to the manufacturers specifications. The objective of the work reported here was to quantify the influence of the refrigerant charge on the steady-state and cyclic operation of a residential split-system air conditioner. A 3 ton unit with capillary tube expansion was tested. The charge was systematically varied to determine its effect on system variables such as capacity, flow-rate, evaporator superheat, power consumption and seasonal energy efficiency ration (SEER). The results indicated a large degradation in capacity for undercharging than for overchar at a given temperature. However, the trends in capacity as a function of temperature for undercharging were different from those for overcharging. The difference in trends was found by close examination of the refrigerant flow control provided by the capillary tube. Other results indicated that small amounts (5%) of undercharging caused as much as a 6% drop in the SEER.

The effect of void fraction model on estimation of air conditioner system performance variables under a range of refrigerant charging conditions

Abstract One variable required for modelling the heat transfer and pressure drop of refrigerant inside air conditioner evaporators and condensers is the void fraction. Eight void fraction models were used in a refrigeration system model to evaluate their impact on the estimation of important system variables in an air conditioner as a function of charging. The void fraction models included: (i) homogeneous, (ii) Lockhart and Martinelli, (iii) Thom, (iv) Zivi, (v) Baroczy, (vi) Hughmark, (vii) Premoli and (viii) Tandon. The system variables considered included: power, capacity, refrigerant flow, subcooling and superheat. Comparisons were made with a 10.6 kW capacity air conditioner with capillary tube expansion. Results indicate that the Hughmark void fraction model appeared to provide the best comparison to measured data over the range of charging conditions considered.

A quasi-steady-state model of attic heat transfer with radiant barriers

Abstract During the cooling season, heat transfer from the attic into the conditioned space of a residence can represent a significant portion of the total envelope heat transfer. Radiant barriers are one method used to reduce this heat transfer. A quasi-steady-state model was developed for predicting attic heat transfer in residences with radiant barrier systems. The model was used to estimate the reduction in cooling load that would occur with a radiant barrier and to identify important construction and environmental parameters that influence this cooling load reduction. The models output consisted of hourly ceiling heat fluxes inside the house based on hourly weather data inputs. Model results were compared with detailed experimental results from two small test houses. The model predicted typical summer heat flux reductions of between 35 and 43% with different radiant barrier configurations and levels of insulation. These compared to measured heat flux reductions of between 29 and 37% in attics under the same conditions. Sensitivity studies were also conducted to show the effect of uncertainty in several of the important physical attic parameters on the final heat flow predictions of the model.

A comparison of critical flow models for estimating two-phase flow of HCFC22 and HFC134a through short tube orifices

Abstract An experimental study to investigate the critical flow of refrigerants through short tube orifices has been performed by measuring the mass flowrates and pressure profiles along the short tube orifice. Eight critical flow models have been examined and their results compared with the experimental data for HCFC22 and HFC134a. These models include four homogeneous equilibrium models, two homogeneous frozen models, and two non-homogeneous equilibrium models. The data indicate that the flow was choked when downstream pressures were lower than the saturation pressure corresponding to the upstream temperature. The observed flows through short tube orifices included a lack of equilibrium due to short time of expansion and homogeneous mist flow at the exit plane. These flow trends would be more consistent with the basic assumptions of the homogeneous frozen models. Based on the comparison of the existing critical flow models and experimental data, the homogeneous frozen models showed the best agreement with the measured data except for exit qualities below 0.06.

A semi-empirical model of two-phase flow of refrigerant-134a through short tube orifices

Abstract Measurements were conducted on Refrigerant-134a flowing through short tube orifices with length-to-diameter ( L / D ) ratios ranging from 5 to 20. Both two-phase and subcooled liquid flow conditions entering the short tube were examined for upstream pressures ranging from 896 to 1448 kPa and for qualities as high as 10% and subcoolings as high as 13.9°C. Data were analyzed as a function of the main operating variables and tube geometry. Semi-empirical models for both single- and two-phase flow at the inlet of the short tubes were developed to predict the mass flow of Refrigerant-134a through short tube orifices. Choked flow conditions for Refrigerant-134a were typically established when downstream pressures were reduced below the saturation pressure corresponding to the inlet temperature. The flow rate strongly depended on the upstream pressure and upstream subcooling/quality. The mass flow also depended on cross-sectional area and short tube length. The mass flow model utilized a modified orifice equation that formulated the mass flow as a function of normalized operating variables and short tube geometry. For a two-phase flow entering the short tube, the modified orifice equation was corrected using a theoretically derived expression that related the liquid portion of the mass flow under two-phase conditions to a flow that would occur if the flow were a single-phase liquid. It was found that for sharp-edged short tubes with single- and two-phase flow, approximately 95% of the measured data and models prediction were within ±15% of each other.