Subcooling

About: Subcooling is a research topic. Over the lifetime, 6150 publications have been published within this topic receiving 99125 citations.

Design and simulation of a heat pump for simultaneous heating and cooling using HFC or CO2 as a working fluid

Abstract: 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 CO2 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.

Subcooled Pool Boiling Experiments on Horizontal Heaters Coated With Carbon Nanotubes

Abstract: Pool boiling experiments were conducted with three horizontal, flat, silicon surfaces, two of which were coated with vertically aligned multiwalled carbon nanotubes (MWCNTs). The two wafers were coated with MWCNT of two different thicknesses: 9 μm (Type-A) and 25 μm (Type-B). Experiments were conducted for the nucleate boiling and film boiling regimes for saturated and subcooled conditions with liquid subcooling of 0―30°C using a dielectric fluorocarbon liquid (PF-5060) as test fluid. The pool boiling heat flux data obtained from the bare silicon test surface were used as a base line for all heat transfer comparisons. Type-B MWCNT coatings enhanced the critical heat flux (CHF) in saturated nucleate boiling by 58%. The heat flux at the Leidenfrost point was enhanced by a maximum of ∼150% (i.e., 2.5 times) at 10°C subcooling. Type-A MWCNT enhanced the CHF in nucleate boiling by as much as 62%. Both Type-A MWCNT and bare silicon test surfaces showed similar heat transfer rates (within the bounds of experimental uncertainty) in film boiling. The Leidenfrost points on the boiling curve for Type-A MWCNT occurred at higher wall superheats. The percentage enhancements in the value of heat flux at the CHF condition decreased with an increase in liquid subcooling. However the enhancement in heat flux at the Leidenfrost points for the nanotube coated surfaces increased with liquid subcooling. Significantly higher bubble nucleation rates were observed for both nanotube coated surfaces.

Capillary tube selection for HCFC22 alternatives

Abstract: In this paper, pressure drop through a capillary tube is modeled in an attempt to predict the size of capillary tubes used in residential air conditioners and also to provide simple correlating equations for practicing engineers. Stoecker's basic model was modified with the consideration of various effects due to subcooling, area contraction, different equations for viscosity and friction factor, and finally mixture effect. McAdams' equation for the two-phase viscosity and Stoecker's equation for the friction factor yielded the best results among various equations. With these equations, the modified model yielded the performance data that are comparable to those in the ASHRAE handbook. After the model was validated with experimental data for CFC12, HFC134a, HCFC22, and R407C, performance data were generated for HCFC22 and its alternatives, HFC134a, R407C, and R410A under the following conditions: condensing temperature; 40, 45, 50, 55°C, subcooling; 0, 2.5, 5°C, capillary tube diameter; 1.2–2.4 mm, mass flow rate; 5–50 g/s. These data showed that the capillary tube length varies uniformly with the changes in condensing temperature and subcooling. Finally, a regression analysis was performed to determine the dependence of mass flow rate on the length and diameter of a capillary tube, condensing temperature, and subcooling. Thus determined simple practical equations yielded a mean deviation of 2.4% for 1488 data obtained for two pure and two mixed refrigerants examined in this study.