Thermo-economic optimisation of industrial milk spray dryer exhaust to inlet air heat recovery Timothy G. Walmsley * , Michael R.W. Walmsley, Martin J. Atkins, James R. Neale, Amir H. Tarighaleslami University of Waikato, Energy Research Centre, School of Engineering, Hamilton, New Zealand article info Article history: Received 19 December 2014 Received in revised form 5 March 2015 Accepted 7 March 2015 Available online xxx Keywords: Process integration Heat transfer Particulate fouling Spray dryer abstract This study reports a thermo-economic design optimisation of an industrial milk spray dryer liquid coupled loop exhaust heat recovery system. Incorporated into the analysis is the ability to predict the level of milk powder fouling over time and its impacts on heat transfer and pressure drop. Focus is given to a finned round tube, a bare round tube and a bare elliptical tube. Modelling results show that spray exhaust heat recovery is economically viable for the considered industrial case study. Based on the re- sults, the best liquid coupled loop heat exchange system uses a finned tube heat exchanger to recover heat from the exhaust air with a face velocity of 4 m/s and 14 tube rows, which gives a net present value of NZ$2.9 million and an internal rate of return of 71%. The developed thermo-economic assessment method has the ability to cater to site specific needs that affect the utility savings and the capital cost for implementing exhaust heat recovery. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Spray dryer exhaust heat recovery can typically increase dryer energy efficiency by 10e20 % [1], but it is complicated by the low heat transfer coefficient of air and the presence of powder partic- ulates that may foul the heat exchanger surfaces. Several case studies on spray dryer heat integration for a range of industries from the 1980's, which was subsidised by the UK's energy efficiency demonstration scheme, showed 2e4 years as a typical payback and a steam savings of 10% [1,2]. In general, dryer exhaust heat recovery is applicable to a wide range of other industries. For example, Laurijssen et al. [3] demonstrated that dryer exhaust heat recovery for a conventional multi-cylinder dryer used in the paper industry plays a critical role in lowering thermal energy use by 32%. Han et al. [4] modelled a lignite-fired power station under variable load and proposed to use the hot flue for drying the incoming fuel as a means for heat recovery and boiler efficiency increases in the order of 1e2 %. Tippayawong et al. [5] analysed industrial longan drying practice to show that dryer heat recovery can increase thermal efficiency by 21%. Increasing energy efficiency in milk spray drying is an important topic for New Zealand because the results of the New Zealand dairy industry heavily impacts the national economy. The installed ca- pacity of milk spray drying in New Zealand reached an estimate of 300 t/h in 2013 with a consumption of around 29 PJ/y of thermal energy. Milk powders supply about 20% of New Zealand's exports. As a consequence, energy efficiency in milk powder production is therefore a prime concern for industry and the New Zealand gov- ernment as a means of lifting national economic performance. Spray dryer exhaust heat recovery represents a great remaining opportunity for significantly increasing heat recovery in the milk powder production process. In the United States, many milk pow- der plants have installed exhaust heat recovery systems for pre- heating the inlet air. Besides heat recovery, another benefit for this practice is to minimise inlet dryer air humidity. For these plants, hot air for drying is generated using direct fired natural gas combustion, which combustion reaction increases the air's mois- ture content. Inlet air pre-heating reduces the fuel consumption, which reduces moisture in the air, which maximises the drying capacity of the air. New Zealand plants, however, favour indirect heating methods of the dryer air using steam or indirect gas fired air heaters. The additional benefit of lower inlet air humidity is not present in the New Zealand case. The New Zealand dairy industry has been cautious to uptake spray dryer exhaust heat recovery. In the mid-1980's, the Plains Co- * Corresponding author. E-mail address: timgw@waikato.ac.nz (T.G. Walmsley). Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy http://dx.doi.org/10.1016/j.energy.2015.03.102 0360-5442/© 2015 Elsevier Ltd. All rights reserved. Energy xxx (2015) 1e10 Please cite this article in press as: Walmsley TG, et al., Thermo-economic optimisation of industrial milk spray dryer exhaust to inlet air heat recovery, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.03.102