An investigation of Cooling Unit Performance on the London Underground

Railway Network

FRANCESCO DRAGONI2, SACHIO BAIG2, DAVE PERRIS2, NICK BOOT-HANDFORD2, ALEX PAURINE1, MARK GILBEY2, MATT WEGNER1,2 AND GRAEME MAIDMENT1

1London South Bank University, 103 Borough Road, London, SE1 OAA. maidmegg@LSBU.ac.uk

2London Underground Ltd, 200 Buckingham Palace Road, London, SW1W 9TJ Nicholas.Boot-Handford@tube.tfl.gov.uk

Keywords, Underground, Air Handling, Air-cooling, Performance

AbstractIn 2012 London Underground successfully installed two new innovative cooling technologies at Green Park and Oxford Circus. These cooling systems utilise bespoke platform air handling units (AHU’s) designed by London Underground for specific application in a tunnel environment. The units intake warm air dissipated by the trains as they stop at the stations. The air is cooled as it passes through specially designed water based cooling coils and then exhausted from the units directly onto the platforms where passengers are waiting.

Building on this initial success London Underground has implemented a new R&D project to assess design options for the next generation of AHUs with the focus being on reducing whole life costs of future installations. One particular aspect of this R&D project was to investigate cleaning of the internal components of the units, which are subjected to air borne dust particles and therefore prone to performance losses. The aim of this investigation was to identify design options which reduce the maintenance requirements of the units and the need for regular track access, whilst maintaining the high cooling performance, thus providing significant cost savings to London Underground. The study of differences between dust accumulation patterns on various heat exchanger coil configurations and the resultant impact of the air side fouling on the coil heat exchange performance is the subject of this paper.

London Underground has trialled six different design concepts of AHUs, which were installed in a disused station. The disused station is on an operational line and therefore ensured testing of the AHUs performance in an operational LU environment, akin to the environment in which they will be required to operate in future. Measurements were taken to monitor the performance of the units as well as the rate of build-up of the dust. The findings presented in this paper provide valuable information for the

development of the next generation of AHUs which will be deployed on the LU network.

1.0 Introduction to the StudyDelivering cooling through conventional air handling units is a challenge on the London Underground (LU) network. Existing applications have encountered reliability issues due to build-up of dust on the cooling coils which has limited the heat transfer and therefore cooling performance. The purpose of this study was to understand the operation of the Air Handling Units (AHU) and any factors limiting their performance to inform their future design. AHUs needed to be capable of delivering the cooling required whilst not involving costly maintenance or excessive energy use over their life. The original brief investigated a range of different options for filtration including electrostatic precipitators, inertial separators, wet scrubber, fabric/panel filtration, etc. Two options which met the brief in mitigating the problem of dust build up in a complex environment were 1) adding fabric/ panel filter or 2) redesign of the coil to allow the particles to pass through by increasing the fin pitch and size of the coil. This research investigated the experimental performance of a number of AHUs configurations and used the results to investigate their whole life cost. This paper describes this study including the aims and objectives, development of the test facility, detailed results and analysis from the experiment, a life cycle cost study and next steps in the project.

2.0 The Trial Method2.1 Aims and Objectives The overall purpose of the trial was to develop an AHU solution that ensured the current design cooling performance was not compromised, whilst reducing the whole life cost of the units, through removing any need for an intensive cleaning regime and excessive power consumption. The trial aimed to achieve this by either protecting the coil with filters or employing an alternative coil technology which would allow much of the dust to pass through it instead of becoming lodged within the coil fins.

2.2 Success CriteriaThe following success criteria for the AHUs were identified:- A successful unit will maintain the cooling capacity by protecting the

coil with a filter or an effective coil. The cooling coil should be able to return to the design performance

after 6 months and no further degradation in performance after another 6 months.

Maintenance and energy use shall be as low as practicably possible, such that the whole life cost is minimized.

The weight, unit size and noise produced should be no greater than today.

2.3 Details of the Trial2.3.1 Site LocationThe location of the trial was critical to ensure that the environment was representative of the AHUs location in a station. After analysis based on ease of access for installation, tunnel temperature, ability to collect data during the day, and the airborne environment, a disused station (on an operational line) was chosen. The trial was set up in the station at platform level, in an area where the platform used to be. This is shown in Figure 1. The units were installed with sufficient clearance to prevent short circuiting of air between adjacent units.

Figure 1 Photograph showing layout of the trial AHUs

2.3.2 Dust AnalysisSamples of the dust at the station were taken from the air and from surfaces during both traffic and engineering hours to ensure that the trial was representative. This analysis was then compared to previous samples taken at a central London operational station and a line wide study

Blockwork wall separating trial site from trains

conducted in 2008 by LU. This was investigated using gravimetric, Grimm laser scatter and SEM x-ray elemental analysis.

The findings were compared to previous published work on the dust in the LU environment (Seaton, et al., 2005 and Adams, et al., 2001) and small differences were found but it was agreed that this site was representative of the network. This is validated in Section 3.

2.3.3 AHUs and Filters Tested In total six AHUs were developed and tested. These were then compared to a control unit based on the design of the existing units installed and operational. Table 1 shows details of each unit in terms of the coil and fan configuration.

AHU 1 - Control

AHU 2 AHU 3 AHU 4 AHU 5 AHU 6

Coil Type

Rippled Fins - 8 fins/inch, 12 rows

Flat Fins - 4 fins/inch, 12 rows

Flat Fins – 4 fins/inch, 12 rows

Flat Fins -4 fins/inch, 8 rows

Flat Fins4 fins/inch, 12 rows

Flat Fins4 fins/inch, 12 rows

Fan Axial fan, fixed speed

Axial fan, external VSD

EC plug fan, integral speed control

Axial fan, fixed speed

EC plug fan, integral speed control

EC plug fan, integral speed control

Table 1 Details of the AHUs tested

A review of filtration in similar contexts was undertaken, as stated in Section 1 and it was established that a number of fabric/ panel filters were suitable. Calculations were then undertaken to identify the pressure drop that would be created by fitting the different filters and therefore the impact that would have on a number of parameters such as noise, power, air flow, cooling output and weight. After this exercise had been completed the following were selected as shown in Table 2.

Type of Filter Filter Class based on EN779:2012

Filter type

Metal Mesh Panel G2 CoarseMetal Mesh Panel G3 Coarse

Delbag Panel G4 CoarseDelbag Bag Filter G4 CoarseAAF Panel Filter M6 MediumAAF Bag Filter M6 Medium

Table 2 Details of the filters tested

2.3.4 Test configuration A schematic of one of the AHUs under test is shown Figure 2 below. These units are normally supplied with chilled water that is generated by either conventional chillers housed in the open atmosphere, borehole water or seepage water as proposed by Ampofo and Maidment (2011). For the purposes of this trial an external air cooled chiller was used to chill water to a flow temperature of 15oC. Based on previous applications and calculations using temperature and humidity data for the platforms this was determined to be the optimum temperature for cooling in the LU tunnel environment through sensible cooling. This maximises sensible cooling and ensures that the cooling system is operating at a high efficiency or COP. The conditions in the test environment were not controlled and were affected by variations including ambient temperature and humidity, and were therefore an accurate representation of a typical LU platform environment.

Figure 2 Schematic Showing AHUs and measurement points.

The variables measured and how they are used to calculate the performance of each AHU as defined in the success criteria are detailed in Table 3. Specifically, cooling capacity (thermal performance) was measured by calculating the heat transfer on the water side. Filter and coil degradation (hydraulic performance) was measured using the pressure drop across the coil and filter respectively.

Parameters measured/ Investigated

Measuring Equipment/ tool used

Data collected used to calculate

Water volume flow rates (m3/s)

Digital Manometer Establishing AHUs’ cooling coils capacity

Air inlet and outlet temperatures (°C)

Thermocouple measurement recorded

Establishing AHUs’ cooling outputs,Establishing LMTDs,Establishing the UA Values

Water inlet and outlet temperatures (°C)

Thermocouple measurement recorded

Establishing AHUs’ cooling outputs,Establishing LMTDs, Establishing the UA Values1

Pressure differential on air side (Pa)

Digital Manometer Establishing the level of clogging on filters and coils

Tunnel’s/ Platform’s Ambient Dry Bulb Temperature (°C)

Thermocouple measurement recorded

Establishing the effect of AHUs on the platform

Tunnel’s/ Platform’s Relative Humidity (%)

Thermocouple measurement recorded

Establishing the environmental conditionsEstablishing the effect of AHUs on the platform

Fan speed (rpm) Tachometer For ensuring the set parameters are not varying with time

Fouling of Cooling Coils

Visual Establishing the rate of fouling

Table 3 Parameters measuredData was recorded on the units every 5 minutes. This was then collected weekly. It was found that the affect of the trains running caused the pressure drop across the coil to vary significantly. Therefore an average of the values from 0100-0300 each night was taken as the daily reading.

3.0 Initial ResultsThis section describes the validation of the experiment as a realistic trial and the hydraulic and thermal performance of AHUs with different types of filter and coil configurations to minimize maintenance and maximise cleanability compared to the control AHU. This section also describes a study of the mechanism of fouling of an unfiltered coil and suggests opportunities for new filter constructions. These are discussed in Section 4.

3.1 Experimental ValidationIt was important to validate that the disused station site was similar to other sites used to trial AHUs. The build-up of dust on this unit was compared to previous trial results using the increase in pressure drop across the coil over time. This is shown in Figure 3 below and indicates

1 UA is U times A; where U is the overall heat transfer coefficient and A is the heat transfer area.

that the trial control performance is similar to AHU 1 to 4 from previous trials.

Figure 3 Coil pressure drop build up in AHUs1 compared to other sites

3.2 Hydraulic and Thermal Performance of AHUsTable 4 shows a summary of the initial results for the six AHUs configurations over approximately a three month period.

As can be seen AHUs 2, 5 and 6 have been tested with one specific filter configuration, AHU 3 has been tested with three different filter configurations and AHU 1 and 4 did not have any filter fitted. Initial results showed that fouling had only a small impact on heat transfer with the exception of AHU 2. On inspection of the AHU 2, it was observed that this was caused by fouling acting as an insulation layer on the coil as shown in Figure 3a. This can be seen clearly compared to fouling of the control AHU 1 coil shown in Figure 3b. These figures show the difference in fouling between the two AHUs, with AHU 2 showing insulating fibrous type fouling compared to more conductive metallic dust fouling with AHU 1. This explains the reduction in thermal performance with AHU 2 compared with the other AHUs.

Table 4 Summary of initial results from first three months of trial

Figure 3a AHUs 2 coil 7 July 2015

Figure 3b AHUs 1 coil 7 July 2015

Table 4 also shows the performance of the filters over three months. This shows that with the exception AHU 2 filter, no filter could achieve a ten week duration between filter changes, with a maximum of 5.4 weeks being achieved. This is too short and would require many filter changes over the season which would be operationally and financially prohibitive for LU. Therefore the use of these filters is precluded. The filling of the AHU filters can be seen in Figure 4, which shows the increase in filter pressure drop as the filter loses capacity.

Figure 4 Pressure drop for across filters. Note erroneous data removed.

Figure 5 shows the pressure drop across the coil and clearly indicates that the coils in AHUs 3, 5 and 6 are being very well protected by their filters. AHU 2 is not experiencing any significant increase in coil pressure drop despite the observation of fibrous material on coil. The pressure drop across AHU 1 is beginning to increase, whereas the wider fin spacing of AHU 4 means that the coil pressure drop has not increased.

Figure 5 Coil pressure drops Note: erroneous data has been removed.

3.3 The Mechanism of FoulingThe fouling mechanism of AHUs 1 and 4 was studied qualitatively by visually recording the build of material over time. This is shown in Figures 6 and 7. Figure 6 shows the build up with the control unit with narrow fins (AHU 1). As can be seen fouling begins to have an impact when bridging across the fins occurs. This can be caused by either fibres or bridging taking place at the shoulders of coil tubes, as indicated. However, with AHU 4 (as shown in figure 7) with wider fin spacing, bridging at the collar does not occur and build-up of particles is prevented as the coil appears to “air wash” or self-clean once a specific thickness of particles is achieved. Bridging of fins can be caused however by fibres, such as the Dandelion seed shown in Figure 7.

8 June 2015 –Surface starting to foul 21 July 2015 – Fibres caught

Fibres

5 October 2015 – bridging 2 November 2015– fibres and bridging between fins occurring

Figure 6 Fouling process of AHUs 1

8 June 2015 –Starting to stick on tubes. 22 June 2015 – Increased fouling on tubes and starting to stick to fins

13 July 2015 – self cleaning 28 September 2015 – Bridging to fibres

Figure 7 Fouling process of AHUs 4

3.4 Limitations of the studyThis investigation chose to focus more on the location of the trial being representative of the AHUs final location above the platform rather than laboratory conditions to control the variables. Although this achieved empirical findings that ensured significant stakeholder support and results that can be relied upon for the future applications of AHU, it did mean that

Bridging taking place on coil fin

shoulders

When fouling reaches a specific thickness debris is being air washed

Bridging due to fibres

aspects of the trial were subjected to operational and environmental factors that had an influence on the trial. These included:

- Response time to faults: with the units being located at the bottom of a disused station, faults required logistical arrangements from suppliers and the project team to attend and fix. Although this didn’t result in significant down time there were a few days where units were turned off to reduce the impact of any fault.

- Choice of sensor: the sensors chosen to monitor the pressure drop across the filters become clogged at times which resulted in the control mechanism increasing the fan speed and therefore filling up the filter quicker.

- The chiller: as the investigation was a temporary installation, the chiller was hired rather than purchased and this resulted in the available unit having a larger chilling capacity than was required. This created a situation whereby if a AHU unit stopped for any length of time the chiller would stop working as the demand dropped below its minimum operating capacity.

These influences were mitigated by a greater site presence and support to ensure the trial had limited down time.

4.0 Further Results and Next StepsThe investigation has led to some interim findings which are presented in this paper. As with much research it has also led to further questions which are now being investigated. They build on the original objective for the trial and the successes found so far. In the case of the alternative coil configuration, the reducing of the rows has been found to lengthen the time it takes for hydraulic performance to be affected by dust, however this design reduces the cooling performance. To develop this further, AHU 2 which has the wider fin spacing but 12 rows rather than the 8 which were tested unfiltered will be left unfiltered to monitor the dust build up to identify how well it will perform compared to AHU 4 but without the reduced cooling performance experienced in the trial.

In addition, a different type of filter is now being trialled. Rather than a single type installed to prevent the dust build up, dual filters have now been fitted to two of the units in an attempt to prolong the life of both by harnessing the strengths of them both. Therefore a unit which removes the large particles is partnered with one than removes smaller ones so that they both operate more closely to their design parameters. Currently both of the additional investigations are producing promising results.

In acknowledgement of the limitations of the trial location another next step is to remove the control unit and the unit with wider fins with reduced rows (AHU 4) and return them to the factory. This will enable them to undergo testing laboratory conditions within which the variables can be easily controlled. Before the units were released from the factory during manufacture they underwent testing to confirm their performance. This data will then be used to benchmark the units from the trial against to see how degraded their performance has become due to the dust build up. Following this, a cleaning trial will be undertaken which will simulate some of the constraints that maintenance will face such as time, access and equipment. A number of tests will be undertaken on a separate unit to establish the exact cleaning method as both high pressure water and air have been used in the past to different levels of success. Once a cleaning test has been conducted, the units will be tested again in the factory to evaluate their hydraulic and thermal performance.

6.0 Conclusions This paper describes the investigation of six different concepts of air handling units (AHUs) installed in a disused station to test their performance in a fully working LU environment. Measurements were taken to monitor the performance of the units as well as the rate of build-up of the dust and other airborne materials that adversely affect the performance of the units. The output from these trials will inform the final design of cooling solution to cool sites in the future and be used at multiple locations across the network.

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AcknowledgementsThis work would not be possible without the input of others. We specifically acknowledge GEA-Searle, Mendage Projects Ltd, Z-Tech Control Systems, the LU Operations team and the AHUs Steering Group.