Davide Del Col, Marco Azzolin, Stefano Bortolin, Alberto Cavallini
Dipartimento di Ingegneria Industriale – University of Padova
Via Venezia, 1 – I 35131 Padova, Italy
Abstract: The worldwide alert about global warming has led to an increasing interest in new HVAC (heating, ventilation and air conditioning) technologies with low environmental impact. When considering this impact, both an indirect effect due to the energy consumption, and the consequent carbon dioxide emissions caused by the electricity production process, and a direct effect due to leakages of refrigerant must be taken into account. A way to achieve high heat transfer rates in compact heat exchangers is using minichannel technology. In addition to high heat flux applications, minichannels are viewed as appropriate options to reduce the charge of refrigerant, minimize the problems of release of potentially hazardous fluids in the atmosphere and allow to use natural fluids such as hydrocarbons and carbon dioxide.
Keywords: minichannels, charge reduction, void fraction, heat exchangers
A significant and still growing part of the engineering research community has been devoted in the last few decades to scaling down devices, while keeping or even increasing their functionality. The introduction of minichannels in the field of enhanced heat transfer and in the refrigeration and air conditioning sectors is surely one of those attempts (Fig. 1). Minichannels allow to develop compact elements which work with reduced refrigerant charge and can withstand extremely high system pressures. Both for HFC and for natural refrigerants, such as hydrocarbons and ammonia, the minimization of the charge is a key point to reduce global warming impact or even solve safety problems.
2. Heat transfer in minichannels
Proper understanding of microscale transport phenomena is fundamental for the designer of microscale heat exchangers. The transition between micro- and macro-scale has been a recurring theme in the literature; however so far there is no established criterion to properly define the transition between conventional ducts to minichannels, particularly in the case of boiling and condensation heat transfer.
With regard to condensation, some researchers observed flow regimes with R134a in minichannels, but no general flow regime map is available. Matkovic et al. (2009) presented heat transfer data taken inside a 0.96 mm diameter single circular channel. They measured the heat transfer coefficient during condensation of R134a at 40°C saturation temperature, and found an experimental trend of the heat transfer coefficient as one would expect from macroscale condensation, at least at mass velocity equal or higher than 200 kg m-2 s-1.
Some numerical studies have been presented in the literature reporting that the channel shape may have great influence on the heat transfer coefficient during condensation inside minichannels. In fact, the surface tension is supposed to enhance the heat transfer in the presence of corners as compared to the case of circular channels. The effect of the minichannel shape is particularly interesting since most of the minichannels used in practical applications have non circular cross sections. Del Col et al. (2011) compared the heat transfer coefficients measured inside a 1.23 mm square section channel against data by Matkovic et al. (2009) taken inside a circular channel. The heat transfer is found to be enhanced in the square channel at 200 kg m-2 s-1 and this must be due to the effect of surface tension, whereas this enhancement is null at higher mass flux.
A validated experimental technique has been employed by Del Col et al. (2012) to assess the effect of channel orientation on heat transfer coefficients during the condensation inside a square cross section minichannel, with a hydraulic diameter of 1.23 mm. No differences have been noticed with mass velocities down to 200 kg m-2 s-1, whereas the condensation heat transfer is controlled by the shear stress. Some effect of inclination was found at lower mass fluxes.
A criterion to identify the transition between micro- and macro-scale has been proposed for flow boiling: the transition occurs when, reducing the channel dimension, the tube diameter reaches a value which is of the same order of magnitude of the bubble size.
Several recent studies have been reported on vaporization in minichannels. Some experiments show that the heat transfer coefficients obtained during vaporization in minichannels are not a function of vapor quality nor mass velocity (in contrast with the macro-channel trend), but are a function of heat flux and saturation pressure. Other experimental studies demonstrate that the heat transfer coefficient also depends on vapor quality and mass velocity. Some experimentalists conclude that flow in small channels is dominated by nucleate boiling while forced convection evaporation is less important.
A phenomenon that must also be accounted for in evaporators design is the onset of dryout in flow boiling that causes a sharp decrease of the heat transfer coefficient due to a change in the heat transfer mechanism. Liquid film dryout is the result of the gradual disappearance of the liquid film adjacent to the wall. Much recent activity has been carried out in order to investigate the behaviour of flow boiling heat transfer and critical heat flux in minichannels, but there is still a lack of information and reliable data, if compared to the wide range of engineering design and possible applications.
Fig. 1. Right side: enlarged image of a 1.4 mm hydraulic diameter multiport minichannels test tube. Left side: square channel cross section with 1.23 mm hydraulic diameter.
3. Systems using minichannelS with low specific charge
If the aim is to maintain high performance and to reduce the refrigerant charge, it can be useful to have a parameter to compare the performance of different systems. The comparison is done by using the specific charge which is the ratio of the refrigerant charge to the heating/cooling capacity.
A 100 kW water-to-water heat pump working with 30 g/kW of propane is installed at the University of Padua and presented in Cavallini et al. (2010). The unit is devoted to testing applications and two prototypes heat exchangers using minichannels can be used as a condenser and as an internal heat exchanger. Minichannels are used in order reduce the internal volume of the heat exchangers and then to minimize the refrigerant charge of the system. The prototypes are segmentally baffled shell-and-tube heat exchangers using copper minitubes with an internal diameter of 2 mm (Fig. 2). The authors reported that the experimental specific charge measured in the heat pump without a liquid receiver was about 30 g/kW using a plate condenser and a plate evaporator and a 0.8 kg reduction could be achieved using the minichannel condenser with a negligible COP performance variation, around 2%.
Fernando et al. (2004) reported the experimental measurements of the charge in a minichannel condenser and in a minichannel evaporator simulating a water to water heat pump working with propane. The two heat exchangers, designed with the priority of the minimization of the refrigerant charge, are composed of 30 and 36 multiport aluminum tubes in two parallel rows, respectively. Every tube had six channels of about 1.42 mm internal hydraulic diameter and a length of 651 mm. Tests were conducted at different conditions and run to find the maximum COP. In the evaporator and in the condenser the optimum measured refrigerant charge resulting in the highest COP vary respectively between 23 and 27 g and between 69 and 93 g. The minimum specific charge that could be achieved is about 37 g/kW considering 7.23 kW of heating capacity.
An ammonia chiller has been experimentally investigated in Hrnjak and Litch (2008). They tested one plate evaporator and two air cooled condensers, one with parallel tube arrangement between headers and multiport microchannel tubes with an hydraulic diameter of 0.7 mm, and the other with a single serpentine microchannel tube with a hydraulic diameter of 4.06 mm. The condenser had louvered fins, each tube had triangular minichannels with a length of 698.5 mm. This system could work with a very low specific charge of 20 g/kW.
Fig. 2. Left side: tube bundle of the minichannel condenser. Right side: front view of a minichannel evaporator without header.
In order to make an accurate estimation of the refrigerant charge in a minichannel heat exchanger a model that uses void fraction correlations must be developed. Void fraction is a dimensionless quantity defined as the ratio of the cross sectional area occupied by the vapor compared to the total cross sectional area. To calculate the void fraction it is necessary to use proper correlations (Da Riva et al., 2010).
The refrigerant charge reduction in refrigerating systems, along with the substitution of HCFCs and high-GWP HFCs, is a goal for the urgent need to reduce their contribution to the greenhouse effect and to reduce atmospheric emission. In this paper minichannels technology and various applications are presented. Minichannels are used in order to reduce the internal volume of the heat exchangers without decreasing performance. The understanding of dominant mechanisms during boiling and condensation in minichannels is the fundamental basis for the development of accurate design methods. Systems using minichannels heat exchangers that could work with a specific charge of 30 g/kW with propane, and 20 g/kW with ammonia have already been developed. For the evaluation of the refrigerant charge in the heat exchangers there is need to calculate the volume that is occupied by the liquid and by the vapour and this can be achieved by using void fraction correlations available in the literature.
Cavallini, A., Da Riva, E., Del Col, D., 2010. Performance of a large capacity propane heat pump with low charge heat exchangers. Int. J. of Refrig. 33, 242-250.
Da Riva, E., Del Col, D., Cavallini A., 2010. Modeling of performance and charge in minichannel heat exchangers. IIR 2th Workshop on Refrigerant Charge Reduction, KTH, Stockholm, Sweden.
Del Col, D., Bortolin, S., Cavallini, A., Matkovic, M., 2011. Effect of cross sectional shape during condensation in a single square minichannel. Int. J. Heat Mass Transfer 54, 3909-3920.
Del Col, D., Bortolato, M., Bortolin, S, 2012. Experimental study of condensation inside a square minichannel: effect of channel orientation. ECI 8th International Conference on Boiling and Condensation Heat Transfer, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
Fernando, P., Palm, B., Lundqvist, P., Granryd, E., 2004. Propane heat pump with low refrigerant charge: design and laboratory tests. International Journal of refrigeration 27, 761-773.
Hrnjak, P., Litch, A.D., 2008. Microchannel heat exchangers for charge minimization in air-cooled ammonia condenser and chillers. Int. J. of Refrigeration 31, 658-668.
Matkovic, M., Cavallini, A., Del Col, D., Rossetto, L., 2009. Experimental study on condensation heat transfer inside a single circular minichannel. Int. J. of Heat and Mass Transfer 52, 2311-2323.