ENERGY SAVINGS FROM AN AUTOMATIC TUBE CLEANING SYSTEM ATCS
The successful operation of a water cooled HVAC chiller requires controlling the growth of biofilm and deposition of scale on the internal tubes of the condenser heat exchange surface. The formation of fouling reduces heat transfer efficiency and subsequently increases chiller work load and energy consumption. Commercially available ATCS specify replacement times for the sponge projectiles at every 1000 hours of chiller run time of operation based on application of 30 minute injections intervals. This paper presents research undertaken in conjunction with The University of Adelaide and focused on endeavours to understand what the impact of physical aging (number of cycles) has on the contact area, stiffness (shear force) and the diameter of the projectiles. The experimental results show that the projectile diameter and contact area increased with the number of cycles, while the shear force can drop in magnitude by approximately half. Separately, analysis of the mean fouling factor from on/off trials of a commercial chiller fitted with ATCS is presented. The mean fouling factor was in the order 15% lower when the ATCS is in operation. Furthermore, results from two case studies for the side by side comparison of ATCS installed on one of two identical 200 ton and 300 ton chillers operating in parallel yielded energy savings in the range of 13% to 19% INTRODUCTION The fouling of heat exchangers is estimated to cost industrialized countries 0.25% of Gross Domestic Product (GDP) and it is responsible for 2.5% of the total equivalent anthropogenic emissions of carbon dioxide (Steinhagen et al. 2011) In Australia, this equates to approximately $3.5 billion dollars annually, and it is evident that effective mechanisms of defouling heat exchangers would result in considerable expenditure savings as well as reductions in greenhouse gas emissions (Thulukkanam, 2013) The physical properties of projectiles such as their size, shape and surface roughness as well as their cleaning performance properties such as shear force and contact area are important attributes to their effectiveness in cleaning. While the ATCS is in operation, the contact area provides a point of constant physical contact between the cleaning projectiles and the tube inner wall. These physical areas of contact determine the extent of the cleaning performance, as projectiles brush and sweep the walls of the tube while moving, and therefore a larger area of contact would indicate a greater removal of foulant material. The stiffness of cleaning projectiles produces radial stress towards the tube inner wall, and these stresses act as a normal force, thus forming a shear force between projectiles and the tube inner wall while the projectiles are moving in the tubes. In theory, a larger stiffness would produce a greater shear force and thus provide a better cleaning performance. Projectiles are usually larger in diameter than the tubes, and this is important as it allows projectiles to have larger physical contact with tube inner walls. The performance of various projectiles has been extensively reviewed by Jalalirad et al. 2013a; Jalalirad et al. 2013b; Malayeri et al. 2014. It was found that flexible and rough projectiles are undoubtedly better in cleaning than stiff and smooth projectiles. These authors developed a contact stability criterion, i.e. the Z factor, which relates the cleaning ability of the projectile to its stiffness in a qualitative manner. The Z factor is low for hard projectiles and vice versa. However, while a hard projectile can exert a shear force many times larger than a soft projectile, its cleaning ability is not appreciably better. The reason was attributed to poor contact stability with the tube surface. Instead, they showed that the size of projectiles plays an important role to provide a continuous cleaning. More recently, Abd Elhadya et al, (2015) investigated the impact of hardness and surface texture on cleaning action of four different projectiles (1) rubber, (2) sponge, (3) notched and (4) ribbed. Their performance to clean a tubular heat exchanger was investigated as a function of the injection rate. A CaSO4 solution was used as the foulant. The cleaning ability of the different types of projectiles was also observed to vary with the injection rate. One factor that was not evident in the open literature reviewed to date is the anticipated changing physical performance due to ageing/degradation of the projectiles over time. In commercial chiller applications of the ATCS, sponge ball projectiles operate at a minimum of 1000 hours of chiller runtime before the manufacturer's requested point of replacement. Based on 30 minute injection cycles, the projectiles will undertake at the minimum for a single pass shell and tube heat exchange some 2000 injection cycles. It is standard operating procedure for the projectiles used in such applications to be 1 mm larger than the tube internal diameter. Heat Exchanger Fouling and Cleaning 2017 ISBN: 978 0 9984188 0 3; Published online www.heatexchanger fouling.com 221 The study also reports the results from the analysis undertaken for several water cooled condenser shell and tube heat exchangers operating with the ATCS. Two distinct types of tests were performed on different systems. Firstly, a sequence of ATCS on/off trials and its impact on the fouling factor analysed during the operating period. Secondly, a side by side comparison of two identical chillers, one fitted with ATCS and one without are presented for a medical centre and casino hotel. EXPERIMENTAL The properties of cleaning projectiles are crucial to the ability to perform their cleaning action. This study investigated commercially available sponge ball cleaning projectiles obtained from Ningbo Everfly Industry Co. Ltd. (China) The manufacturer specifications in terms of the exact chemical composition and the overall physical properties of the sponge balls were not subscribed. As no detailed manufacturer specifications were enclosed, various physical properties of the sponge balls were investigated to enable distinction between each respective ball size. Physical properties refer to the: diameter, width of the ball at its reference line; shape, the observed measure of the ball's overall spherical symmetry; surface roughness, the observed measure of the ball's roughness. The performance properties include the contact area, which refers to the surface area of the sponge ball with which it is in contact with the inner surface of the condenser tube and the shear force. The full account of the experimental procedures for the contact area and shear force have been documented elsewhere (Malayeri et al. 2014) In brief, the reported contact areas were measured in a transparent tube with similar inner diameter to that of the tube (15mm) used in ageing experiments. Thus, the measured contact area would be similarly defined as the dynamic contact area (Malayeri et al. 2014) The hydrodynamic force and shear are determined from a test rig equipped with a pressure transducer, a flow meter and the facility for injecting projectiles. The projectile pass through the tube by the flow while the pressure in the back of the projectile and respective flow velocity were recorded. Considering the cross section of the pipe, the measured pressure represents the required force for pushing the projectile by the force of the flow. When the hydrodynamic force is divided by the contact area between the projectile and tube, then the respective shear can be determined. However, the contact area between the projectile and the tube while under the force of the flow cannot easily be measured. To obtain reliable and consistent results Malayeri et al. (2014) required the soft projectiles to operate at the minimum flow velocity of 0.4 m/s. As this is considerably lower than the typical 1 m/s specified for the operation in commercial applications of the ATCS, undertaking analysis at this minimum would not yield the required representation of normal operation. While this represents a limitation of the present study, in the context of the findings it is believed this shortcoming is minimal in comparison to the implied assumption of consistent projectile properties when discussing fouling. Especially the potential extrapolation of short run experimental results conducted over several to tens of hours compared with 1000 plus hours in commercial operations. The projectiles pass repeatedly through a tube testing system designed to quantify the degradation of the physical and performance properties. Wieland Werke AG provided the testing tubing and the test length of the tube was 1.6 m with a 15 mm inner diameter. Three projectile sizes (15 mm, 16 mm and 17 mm) are subject to 2000 cycles. The physical and performance properties of the sponge balls are obtained in intervals of 50 cycles. This means that a total of 40 measurements are taken on each sponge ball for each property investigated. The second component of this study involved the investigation of an ATCS on/off trial as part of the Westmead Hospital's Chiller system located in Parramatta, Sydney. The unit fitted with the ATCS is a 19XR High Efficiency Hermetic Centrifugal Liquid Chiller manufactured by Carrier (model 19XR 70 71475 DJ S 854. The characteristics of the condenser tubes are summarized in Table 1. Table 1. Tube parameter for Carrier chiller at Westmead. It was agreed with the Westmead Hospital's technical operators that the chiller would remain as the lead chiller and operational for 24 hours per day for the full 6 week trial, commencing on the 14th of August 2015. Prior to this period, the hospital's chiller had been idle for 4 months (low season load as southern hemisphere winter) and the chiller had not been cleaned since the beginning of its last operation. This reflects that the ATCS did not start from a clean state. However, based on fouling theory, as the surface temperature of th