Gökçe PEKER
Firat University
Faculty of Engineering, Mechanical Engineering Department, Elazig, Turkey
Cengiz YILDIZ
Firat University
Faculty of Engineering, Mechanical Engineering Department, Elazig, Turkey
Yusuf BİLGİÇ
Firat University
Baskil Vocational School, Elazig, Turkey
Ahmet YILDIZ
Firat University
Faculty of Engineering, Mechatronics Engineering Department, Elazig, Turkey
Gülşah ÇAKMAK
Firat University
Faculty of Engineering, Mechanical Engineering Department, Elazig, Turkey
THE EFFECTS OF SPRINGS INSERTED VERTICALLY INSIDE THE PLATE HEAT EXCHANGER TO THE
HEAT TRANSFER AND PRESSURE DROP
Keywords: new type plate heat exchanger, heat transfer, pressure drop
Abstract
Plate heat exchangers are equipments which can exchange heat by circulating the homogenous or different fluid types with different temperatures without intermixing.
These systems are required to be as small as possible and to achieve a high rate of heat transfer. While it is possible to increase the heat transfer to higher values in the improvement studies, high pressure losses arise, which have a negative effect on the heat transfer improvement techniques. These negative effects somewhat limit the use of improvement techniques.
In this study, an experimental study was done by designing a new type plate heat exchanger. In the plate heat exchanger developed for this purpose, the springs were placed vertically between the plates to increase the turbulence effect of the fluid. Water at
1 gokce5581@gmail.com
different flow rates was used as fluid on both sides of the heat exchanger. Fluid temperatures and pressure losses were measured in the study. In this context, the effect of springs used in plate heat exchangers on heat transfer and pressure drop were investigated.
It was observed that the heat transfer in the spring plate type heat exchanger was higher than that of the non-spring plate type heat exchanger. As a result, heat transfer was improved with the newly designed heat exchanger and a heat exchanger design that is easy to manufacture was realized. Thus, it was found that it is possible to obtain a smaller size heat exchanger in the same capacity by using turbulence generating springs in plate heat exchangers.
1. Introduction
Heat exchangers are devices that provide heat exchange between two fluids with different temperatures and prevent fluids from mixing. Heat exchangers are widely used in a wide variety of application areas, from home heating and ventilation systems to chemical processing and power generation in large factories. The difference of heat exchangers from mixing chambers is that they do not allow mixing of two fluids [1].
Heat transfer in a heat exchanger generally includes convection on each fluid side and conduction in the wall separating the two fluids. When analysing a heat exchanger, it is appropriate to work with the total heat transfer coefficient, which takes into account the contribution of all these effects on heat transfer. The rate of heat transfer between two fluids at one location of a heat exchanger depends on the magnitude of the temperature difference at that location; this difference changes throughout the heat exchanger [2].
Plate heat exchangers are equipments which can exchange heat by circulating the homogenous or different fluid types with different temperatures without intermixing. Thanks to the special design on the plates, the speed of the fluid constantly changes during the flow. Therefore, it transfers the required heat with a smaller volume thanks to the agitation and high turbulence obtained. The surfaces where the main heat transfer occurs in plate heat exchangers are generally made of thin metal plates.
These metal surfaces can be straight or wavy. As with tubular type heat exchangers, they are less resistant to high pressure and temperatures. Plates can be made of aluminium, zirconium, titanium, nickel, or stainless steel.
With the replacement of tube type heat exchangers with plate heat exchangers day by day, plate heat exchangers have gained a rapidly increasing market share in the entire sector. The wide selection of plates in various sizes and materials gives the plate heat exchangers superior
flexibility. This flexibility provides a great advantage to plate heat exchangers in many thermal processes. Plate heat exchangers generally have higher overall heat transfer coefficient than shell and tube heat exchangers [3, 4, 5].
In this regard, heat exchangers systems, which are widely used in industry, are required to have as small dimensions as possible and to achieve a high rate of heat transfer [6]. A more efficient heat transfer is realized with plate type heat exchangers. One of the methods used to achieve high efficiency in plate type heat exchangers is to increase the turbulence values of the flow between the plates by designing a plate heat exchanger with high turbulence density. With increasing turbulence density, the overall heat transfer coefficient and thus the efficiency of the heat exchanger increases and its dimensions become smaller.
For this purpose, some of the studies on improving the heat transfer and increasing the efficiency in plate heat exchangers with different types of geometry are presented below.
Aliabadi and Hormozi [7] experimentally investigated the effects of the simultaneous use of pin channel and copper water nano fluid on the performance of plate fin heat exchangers and compared the results with the results obtained for the base fluid flowing in a straight channel. The experimental results clearly showed that the pin channel increased the thermal-hydraulic performance of the plate fin heat exchanger by about 38% when compared to the flat channel. It was observed that the heat transfer coefficient and pressure drop increased when nano fluids were used instead of base fluids, and an average performance factor of 1.65 was obtained with the simultaneous use of the pin channel and nano fluid in the plate finned heat exchanger.
Pantzali et al. [8] investigated both experimentally and numerically the effect of using nano fluids in a miniature plate heat exchanger with a modulated surface. First, the thermo-physical properties (thermal conductivity, thermal capacity, viscosity, density and surface tension) of a typical nanofluid (CuO / Water at 4% volumetric ratio) were systematically measured, then the effect of surface modulation on enhancement of heat transfer and friction losses was investigated by conducting experimental studies on the existing miniature heat exchanger and modelling a conceptually similar plate heat exchanger with the help of CFD. As a result, the modulation of the surface and the use of nanofluids significantly increased the heat transfer.
Aliabadi et al. [9] investigated experimentally the copper / water nanofluid flow in different plate fin heat exchangers. Seven plate-fin
channels, including plain, perforated, offset strip, louvered, wavy, vortex generator, and pin were fabricated and tested. Copper / water nano fluids were produced with five nanoparticle weight ratios (0%, 0.1%, 0.2%, 0.3%
and 0.4%) by electroplated wire technique, a one-step method, and the necessary properties of nano fluids systematically were measured and empirical correlations were proposed. The results showed that both the convective heat transfer coefficient and the pressure drop values of all channels increased with increasing the nanoparticle weight ratio. Suitable thermal-hydraulic performance and maximum surface area reduction were found for the vortex generator channel. The performance factor for the vortex generator channel increased up to 1.65 at medium and high flow rates. Finally, correlations were developed to estimate the Nusselt number in the plate finned channels and the fan friction factor of the base fluid and nanofluids.
Stogiannis et al. [10] designed a miniature plate heat exchanger with a modulated surface that uses nano fluids as the working fluid, aiming for a more compact and efficient cooling for low temperature applications. The recommended working fluid was SiO2 / water nanofluid with 1%
volumetric ratio that could increase the heat transfer rate up to 35%
compared to water. Relevant CFD simulations conducted in this study showed that less coolant was needed for a given operating temperature, and as a result, less pumping power was needed when this nano fluid was used instead of water. Since SiO2 nanoparticles are relatively inexpensive and nanofluids are easy to prepare, their use was a good solution for mini-scale devices or low-power applications.
In this study, it was aimed to increase the heat transfer and to reduce the pressure losses to a minimum value by providing turbulence and vortex formation with plate type heat exchangers designed as springs. Springs were placed vertically inside the plates manufactured for this purpose.
Temperature and pressure drop measurements were made by flowing water at various flow rates in plate heat exchangers. The effects of the geometries of the springs vertically mounted on the plates on the temperature and pressure drop were investigated. With the geometry of the newly designed plate heat exchanger, more turbulence and vortex were created and more heat energy was tried to be used, unlike the existing heat exchanger systems. In this study, a new heat exchanger with an easy-to-manufacture plate was designed and a more efficient heat transfer was achieved with less pumping power. When the studies in the literature were examined, it was seen that classical type chevron type plate heat exchangers were
generally used. The newly designed plate heat exchanger added a unique value to the study.
2. Material and Method
In this study, a new type plate heat exchanger was designed. The newly designed heat exchanger is 120x500mm in size. Channel Spacing is 7mm, Seal Thickness is 2mm and Port Diameter is 15mm. Plate thickness used in the heat exchanger is 2mm (Fig.1). The plate material is St.37 structural steel and the plate is galvanized against rusting. In order to increase the turbulence intensity of the fluid inside the heat exchanger, springs were placed between the plates to increase the heat transfer. The wire thickness of the springs used is 1 mm, the spring diameter is 6mm and the spring pitch is 5mm. Springs were placed perpendicular to the plate. The 3- dimensional drawing of the plate is shown in Fig. 2, and the photograph of the manufactured plates is shown in Fig. 3.
Fig. 1. Schematic representation of the plate heat exchanger.
Fig. 2. 3D view of the designed new type plate heat exchanger
Fig. 3. Production photo of the designed new type plate heat exchanger
As seen in Fig. 4, there are two cycles in the experimental setup, one of which is a cold cycle and the other is a hot cycle. Distilled water circulates in hot cycle and mains water circulates in cold cycle. Distilled water is heated in the hot water tank and then pumped to the new type plate heat exchanger with the help of a pump. Here, the hot distilled water gives its heat to the cold mains water sent from the cold cycle to the new type heat exchanger with the help of a pump. In the cold cycle, the water taken from the network is heated in the cold water tank to the appropriate temperature and then pumped to the heat exchanger. In order for the starting temperature value for cold water to be the same in all experiments, the water taken from the network is not used directly, it is pumped to the heat exchanger after it is brought to the desired starting temperature in the cold
water heating tank. A 1 kW resistance was used to heat the water in the hot water tank, and a 6 kW resistance was used to heat the water in the cold water tank. These resistances were controlled by contactors. PID controlled temperature control thermostat was used to control the temperature, thermocouple was used for temperature measurement of the thermostat. In addition, in the experimental setup (Fig. 4), the temperature of the hot distilled water and cold water was measured by the thermocouples placed at the inlet and outlet of the heat exchanger. The pressure drops in the heat exchanger was measured with the differential pressure gauge placed at the inlet and outlet of the heat exchanger and the flow rate was measured with the flowmeters on the pipe line before the cold water and hot distilled water enter the heat exchanger. Throttling valves were used to adjust the flow rate of the pumps. System pipelines were selected as PVC and PVC pipes in hot line were insulated to prevent heat loss. In addition, the new type plate heat exchanger was insulated to reduce heat losses.
Fig. 4. Schematic view of the experimental setup
In order to check the stable working condition of the experimental setup, the test of the empty plate heat exchanger, which wasn’t changed, was performed. Distilled water was used as hot fluid and tap water was used as cold fluid. Experiments were carried out for the hot fluid at 6 different flow rates (𝑚ℎ𝑜𝑡̇ =1lpm, 2lpm, 3.3lpm, 4.3lpm, 5.5 lpm, 6.3 lpm) and the cold fluid flow rate was kept constant (𝑚𝑐𝑜𝑙𝑑̇ ==3.3lpm). The hot fluid inlet
temperature was kept constant around 60oC and the cold fluid inlet temperature around 25oC. The measured temperature, flow rate and differential pressure values were recorded. The data obtained at this stage were used as a reference value to determine the effect of the parameters to be modified on heat transfer.
3. Results
With the data obtained as a result of this study, temperature difference graphs were drawn for the hot fluid depending on the flow rate. As seen in Fig. 5, as the flow rate increased, the fluid temperature difference decreased considerably. In the new type spring plate heat exchangers, the temperature difference is higher than the empty plate heat exchanger. In the plate heat exchanger with springs placed vertically (90o), the temperature difference was 2.9 oC compared to the empty plate while the hot fluid flows with a flow of 1 lpm, this difference decreased to 0.4 oC for 6.3 lpm. Although the temperature difference decreases at high flow rates, more heat transfer was achieved due to the intensity of the turbulence intensity.
Fig. 5. Temperature difference-flow rate graph for hot fluid (𝑚𝑐𝑜𝑙𝑑̇ =3.3lpm, Thot inlet= 60 oC, Tcold inlet=25oC)
With the data obtained, a graph of pressure drop depending on the flow rate for plate heat exchangers was drawn. As can be seen in Fig. 6, the pressure drop for the same flow rates was found higher for the spring plate heat exchanger, but this pressure drop wasn’t not too much. It is thought that this is due to the fact that the springs placed vertically didn’t show much resistance to the flow. In the plate heat exchanger with vertical springs, the pressure drop at a flow of 6.3 lpm was 400 Pa more than the empty plate, and 25 Pa more than the empty plate at 1 lpm. The new type plate heat exchanger can be easily operated at low flow rates, since the pressure drops are not more than the empty plate at low flow rates. Since the required pump power will not increase at these flow rates, a good thermal-hydraulic efficiency can be obtained.
Fig. 6. Pressure drop curves for hot fluid
(𝑚𝑐𝑜𝑙𝑑̇ =3.3lpm, Thot inlet= 60 oC, Tcold inlet=25oC)
4. Conclusions
In this study, the temperature change and pressure drop in the plate type heat exchanger in which springs were placed vertically were experimentally investigated.
As a result of the studies, it was determined that the temperature difference in the spring heat exchanger increased compared to the empty plate heat exchanger and that the heat transfer can be improved by giving
turbulence effect to the flow with the help of the springs placed in the heat exchanger. Springs and flow rate was effective in the improvement in heat transfer. With the increase in heat transfer, there was a certain increase in pressure loss.
A temperature increase was achieved in all flow rates studied in the plate heat exchanger with a spring inside, compared to the empty plate heat exchanger. As a result of the study, the heat exchanger, whose springs are placed vertically (90o), seems advantageous due to both the temperature difference it provides and the pressure drop not much compared to the empty plate heat exchanger.
Acknowledgments
This study was funded by the Scientific and Technological Research Council of Turkey (TUBITAK ARDEB 1002 Project Number: 218M931).
The authors would like to express their thanks to TUBITAK for their financial supports the set-up, fabrication and research implementation.
References
[1] Zohuri B., 2017, Compact Heat Exchangers, Springer.
[2] Çengel, Y. A. ,1998. Heat Transfer, New York: Mc Graw Hill.
[3] Jun, D.C.L., 1999. Optimum design and running of PHEs in geothermal district heating. Heat transfer engineering, 20(4), pp.52-61.
[4] Gut, J.A. and Pinto, J.M., 2004. Optimal configuration design for plate heat exchangers. International Journal of Heat and Mass Transfer, 47(22), pp.4833-4848.
[5] Genceli, O., 1999. Heat Exchangers. İstanbul: Birsen Press.
[6] Çakmak, G. and Yıldız, C., 2007. The influence of the injectors with swirling flow generating on the heat transfer in the concentric heat exchanger. International communications in heat and mass transfer, 34(6), pp.728-739.
[7] Khoshvaght-Aliabadi, M. and Hormozi, F., 2015. Heat transfer enhancement by using copper–water nanofluid flow inside a pin channel. Experimental Heat Transfer, 28(5), pp.446-463.
[8] Pantzali, M.N., Kanaris, A.G., Antoniadis, K.D., Mouza, A.A. and Paras, S.V., 2009.
Effect of nanofluids on the performance of a miniature plate heat exchanger with modulated surface. International Journal of Heat and Fluid Flow, 30(4), pp.691-699.
[9] Khoshvaght-Aliabadi, M., Hormozi, F. and Zamzamian, A., 2014. Experimental analysis of thermal–hydraulic performance of copper–water nanofluid flow in different plate-fin channels. Experimental thermal and fluid science, 52, pp.248-258.
[10] Stogiannis, I.A., Mouza, A.A. and Paras, S.V., 2015. Efficacy of SiO2 nanofluids in a miniature plate heat exchanger with undulated surface. International Journal of Thermal Sciences, 92, pp.230-238.
