Preheating

The preheating section consisted of components that can be assembled in a variety of configurations depending on the nature of the experiment. The standard assembly is shown in Figure 3.1. The process fluid (milk supplied from a local dairy factory (properties reported in Appendix F.3), reconstituted whey (Appendix F.2) or water) was stored in an 850 litre refrigerated tank (SV) with agitator (SVA). The fluid could be pumped at flow rates in the range 30-2 000 l/h from the vat using a centrifugal pump (PM1) controlled by a variable speed drive (VSD1). A commercially manufactured plate heat exchanger (PHE) brought the temperature of the process fluid from the refrigerated temperature (e.g. 4°C) to the target experimental temperature usually in the range 30-70°C. Direct steam injection (DSI) units (DSI1) could be used in conjunction with the PHE to provide instant temperature elevations of 5-20°C depending on the experimental flow rate. DSI was used predominantly in microbial type experiments particularly when the growth of thermophiles was targeted into specific items of equipment and growth in connecting pipes was discouraged. DSI was generally not used in fouling experiments because the injected steam introduced flow rate spikes detrimental to the successful monitoring of fouling rates. Two research heat exchangers (THE and MPHE) followed and the process fluid could be routed through either one first depending on the nature of the experiment. Before entering the evaporator feed tank (FT) the process fluid was heated (via a second DSI unit: DSI2) and could be held at constant temperature (in a holding tube bank: HT) if required.

3.2.1.1 Miniature plate heat exchanger (MPHE) rig

Custom built fouling modules were designed for the pilot plant to provide a suitable surface for the study of fouling and bacterial contamination during processing. Six modules were built into an array so that a number of heat exchange surfaces could be studied in any one run. The array was known as the miniature plate heat exchanger (MPHE) rig. The design of the rig was based on the following objectives:

• To provide an easily accessible fouling surface which could be inspected at any

stage during the operation of the plant. The surface also had to be removable so that the fouling could be photographed and examined under such equipment as a microscope before being disturbed from the surface.

• To allow the independent isolation of each module from the plant flow at any time during processing so that the heat exchange surface could be inspected without stopping the product flow.

• To ensure each module did not significantly increase the temperature of the fluid

moving through the process side. The aim was to have the temperature profile across the rig as uniform as possible so that the modules would be comparable in terms of process fluid temperature.

• To allow the heat exchange surface to be subject to relatively high and stable

heat fluxes. This meant that a liquid to liquid heat transfer scenario with the two fluids separated by a wall similar to that found in a plate heat exchanger was preferred to liquid to air heat transfer.

• To allow the installation of sensors to monitor heat flux and temperature.

Figure 3.3 shows a three-dimensional model of the module developed. Each module consisted of two stainless steel chambers separated by a thin (0.6 mm) removable stainless steel plate. The process fluid flowed through one side and hot water (heating medium) passed through the other. To create a tight seal between the three components two silicone rubber gaskets were inserted, one on either side of the removable plate. When the module was packed the gaskets lined up with the chambers’ flanges. The packed module was held together by 4 wing-nuts and bolts, one located at each corner of the flanges. One module provides approximately

10 cm2 _{of heat exchange surface.}

Figure 3.3 A three-dimensional representation of a module developed for the study of fouling and bacterial contamination.

The MPHE rig was constructed from six modules connected together in series. A photograph of the array is shown in Figure 3.4. A detailed piping and instrumentation diagram of the MPHE rig is provided in Figure A.2 of Appendix A. The rig can be described as two headers (one each for the processing and heating sides) with twenty-four branches (four for each module). During start up, the process and heating fluids initially flowed through the headers while the branches are isolated by a series of valves (i.e. no fluid entering the module). To activate a module the corresponding valves on the header lines are shut while the valves on the branches are opened. In this state the entire volumes of the processing and heating fluids passed through the module.

Figure 3.4 Photograph of the miniature plate heat exchanger rig.

Through the same system of valves a single module could be isolated (essentially reversing the activation process) at any point during operation allowing the run to continue uninterrupted. After isolation, the MPHE could be disassembled allowing the test plate to be inspected and physical measurements such as deposit weight to be made. To allow isolation, the utility side of the module was deliberately connected to the rig using a flexible plastic hose fitting. This gave the heating medium chambers some movement when the wing-nuts were removed.

The heating medium used in the current research was hot water generated by an electric element (HE2) installed in a 50 litre stainless steel tank (HM). The water was recirculated through a circuit that provided heating to both the MPHE rig and the tubular heat exchanger by a centrifugal pump (PM2) as shown in Figure A.1 of Appendix A.

A heat flux sensor (e.g. H1) could be attached to the test plate within the heat exchange surface area. The installation and description of the heat flux sensor is discussed in detail in section 3.3.2.2.

A temperature sensor was installed within the process fluid chamber. Initially, a resistance temperature device (RTD) was installed in the MPHE using a tube socket weld union but this was later replaced by a type-T thermocouple (e.g. T9) due to lag and error in the RTD-well measurement as discussed further in section 3.2.2.2.

3.2.1.2 Tubular heat exchanger (THE) rig

The tubular heat exchanger (THE) rig was designed to study fouling on heat exchange surfaces in a similar manner to the MPHE. The THE rig had a much larger heat exchange surface area than the MPHE so sufficient amounts of fouling could be produced to be chemically analysed. As well as meeting the objectives of the MPHE the THE had to provide a surface area at a controlled temperature for the colonisation of thermophiles. This allowed the release of thermophiles into the bulk milk flow to be studied at the optimum range of thermophile growth.

Figure 3.5 shows the tubular heat exchanger rig installed in the pilot plant. A detailed piping and instrumentation diagram of the THE rig is provided in Figure A.3 of Appendix A. The THE was arranged into two parallel banks of three tubes in series. The tubes were designed concentrically with three tubes inside one another. The process fluid flowed through the central chamber while the heating medium could flow on either or both sides of the product, providing heating on both the inner and outer surfaces if necessary. A schematic drawing of a single tubular heat exchanger is given in Figure 3.6, which shows this configuration. For this project heating was provided only by the inner tube to avoid complexities in the monitoring of fouling. In this case, fouling would only develop on the outer surface of the inner tube which will be referred to as the test tube for the remainder of this thesis.

Figure 3.5 Photograph of the tubular heat exchanger rig.

Figure 3.6 The assembly of an individual tubular heat exchanger.

Temperature sensors (RTDs or thermocouples) were installed at the inlets and outlets of each tube on both the heating and process fluid sides. Combined with a flow meter (F1) installed at the heat exchanger inlet, these sensors were used to monitor the onset and build up of fouling in each individual tube. Section 3.3.2 provides further detail on these sensors and the monitoring of fouling.

The THE rig was especially designed for easy disassembly for examination of the test tube surfaces after a run. This enabled visual and analytical study of the fouling

produced. This design also allowed individual test tubes to be isolated through a system of piping bypasses, without stopping the process flow. Any test tube could therefore be removed and studied at any time during a run. This is shown in the piping and instrumental diagram of the THE in Figure A.3 of Appendix A. This design feature was used during Hinton’s study but not in the current project.