Playing it cool

For one of the most impressive laser experiments in the world - the key to success is all about maintaining the environmental status quo…

A new laser laboratory has been installed at Imperial College London to enable scientists in the College's Department of Life Sciences to study light activated processes such as photosynthesis. The laboratory is run by scientist Dr Jasper van Thor from the Division of Molecular Biosciences and has made Imperial a world leader in this type of technology.

The Ultrafast Spectroscopy Laboratory houses three state-of-the-art laser systems that can analyse the movements of proteins, and discover how they can become activated, at a molecular level. For example, one experiment has involved looking at a protein called phytochrome, which senses light and bacteria in plants, to discover how it changes shape after it absorbs light.

The lasers work by generating intense light pulses that last only femtoseconds, or 0.000000000000001 of a second. Controlling the temperature environment of the laser equipment was imperative when constructing the laboratory because each laser sits on an optical table, and a temperature change can cause thermal movement in these tables and affect the lasers. This would severely disrupt any experiment that is being carried out1.

Crofton, the consulting engineer, was bought in to provide the appropriate temperature environment for the laser equipment. Barry Henson of Crofton said: “We needed to ensure that the temperature and humidity levels of the equipment were maintained in very close tolerances within +/- 1°C all year round, under dynamic load (temperature) conditions between each laser. Finding the right solutions to achieve this was a very challenging process.”

The lasers are contained within separate cells or booths, and are used to carry out different experiments. The individual experiments produce different amounts of heat and therefore require different cooling.

The cells are sub-divided using laser safety curtains that are suspended from the ceiling. The curtains bind the three areas to allow individual experiments to take place, and importantly, prevent any stray laser light from escaping. In the strategy that has been adopted they also assist in segregating the thermal loads between the cells.

Another major challenge was keeping the cells free of the plant equipment that was needed to heat, cool and control the humidity of the air within each cell. There was very little space to house plant equipment within the cells and the bulk of equipment needed to be located outside the active laboratory so any maintenance that was required did not interrupt experiments. 

Crofton solved this problem by installing an air handling plant in a service cupboard adjacent to the laboratory. The equipment chills the air using cool water from the Imperial chilled water infrastructure. The water flows through heater exchanges within the air handling plant, which comprises sensors and controllers that cool the air to the necessary temperature. Air is distributed to each cell in the laboratory through steel duct work. Centralising the plant in this way helped overcome the problem of having to house a major cooling plant within the individual cells, which would not allow temperature control at an individual cell level.  

A heater battery system is used to trim the temperature and the supply of air in the individual cells. Henson explains: “There is a high degree of variability between the heat loads (temperatures) in the different cells which is caused by the equipment being switched on and off for different experiments. Because of this we needed to ensure that the air supply temperature could be modulated within each separate zone to respond to the varying heat loads.”

“We therefore gave each cell its own heater battery that controls the air supply temperature within its zone. This is a cooling and ventilation system, and ensures that the temperature and humidity environment for each separate experiment is kept constant and is not affected by the other lasers.”

Maintaining low humidity was another major issue for Crofton, to avoid moisture within the air at high humidity having an impact on the laser beams. The experiments are sensitive to humidity and favour as low a humidity as possible. Ensuring low humidity was challenging because of the available chilled water temperatures, which limit the extent to which air can be dehumidified.

“We controlled humidity by cooling the supply air to around 9°C which condenses any moisture in the air. The condensate is drained away, and the dried air is then re-heated through a hot water heater battery to a temperature that is suitable to serve the room,” Henson said.

High air change rates were required to address high heat loads but also in order to deal with the use of nitrogen in the laboratory. Nitrogen is used to purge the experiments and create the dry conditions that they require. In order to manage the high air volumes Crofton installed laminar flow panels to introduce air into the laboratory. The air is duct from the air handling plant and supplied into the laboratory through the laminar flow panels, which have the ability to handle high air volumes without creating draughts so they were ideal for this application.

Henson explains: “The supply air from the laminar flow panels distributes around the laser equipment, picking up heat, and is then drawn back to the air handling plant, through steel duct work which draws air from a low level in the laboratory.”

The air circulated within the laboratory comprises 90% re-circulated air and 10% fresh air. The fresh air constitutes approximately 10 air changes per hour which is required to dissipate the nitrogen safely. The total air change rate is 50 per hour comprising 40 changes with re-circulated air and 10 with fresh air.

Another difficulty was ensuring that air was cleaned to HEPA standards and that it was pressurised to a level that would prevent the ingress of dirty air from the surrounding areas which would affect the experiments. Therefore filter packs were incorporated into the air handling plant to clean the air.

“The laboratory itself had a number of space restrictions, with downstand beams, waste pipe work, and low ceilings adding to the difficulties of fitting all the equipment in the space. A carefully coordinated and detailed design was undertaken, accounting for the multitude of power supplies, trunking, pipes, ducts and laboratory equipment, computers within the space. It took a great deal of time and consideration to finally resolve how everything would fit,” Henson said.

Developing high performance laboratories are never without problems and this installation was no different. Crofton has maintained a relationship with the scientists post occupation to help improve the environmental conditions within the laboratory; the following works having been undertaken:

The space temperature was originally being measured in the air as it returned from the individual laser cells to the central plant allowing for individual cell control. The point at which the air is removed from the cell is in the ceiling above the laser table. Much better sensing of the conditions at the experiment was needed and space temperature sensors are being installed to allow the temperature readings to be taken on the laser table at the location that the experiments are being undertaken.

At certain times of the year humidity levels were higher than originally planned; investigations discovered that the dampers that control the mix of fresh air and re-circulated air were not able to close to a sufficient point to reduce the amount of fresh air to the 10% required. In hot humid conditions this was adding additional demand onto the plant that was not initially included within the plant sizing calculations. Additional fixed dampers have been installed to overcome this problem.

References:
1.    Imperial website: http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/newssummary/news_18-5-2010-17-30-6