Flair and imagination produce sterile design

Pneumatic actuation is helping improve the efficiency of materials handling at Coventry General Hospital.

Pneumatic actuation is helping improve the efficiency of materials handling at Coventry General Hospital, following the installation of a special airlock-type hatch in its sterile equipment processing department, through which surgical instrument sets, gowns and other medical equipment are passed.

The hatch, at a massive 1.5m high by 1m wide and 0.8m deep, is probably the largest such unit anywhere in the UK medical sector.

With several facilities managers from other hospitals monitoring the performance of the department, similar installations are expected to follow up and down the country in the near future.

Each end of the hatch is fitted with a pair of sliding doors, which are interlocked so that it is impossible for both ends to be open at the same time, which could compromise sterility within the department.

As the doors weigh 50kg each, the hatch developer Goldsworth Engineering called in actuation specialist Hoerbiger-Origa to help identify the best solution for operating the doors.

"We knew that Hoerbiger-Origa has a full range of electric and pneumatic linear drives, so would be objective in identifying the ideal mechanism for us", says Alan Peglar, Managing Director of Goldsworth.

"We have worked with them on similar, although smaller scale projects before, so also knew that they would help with the design of the whole subsystem".

Goldsworth regularly designs and constructs transfer hatches for sterile duty applications.

Over the years the company has perfected a flexible basic design that can incorporate a range of options and operations to suit individual users' specifications.

For instance additional features, such as air flushing, pressure differential monitoring and two-way communications systems are regularly requested.

Manufactured from stainless steel, Goldsworth's standard hatch unit is usually fitted with either two air-tight hinged doors that close against replaceable rubber seals or vertically sliding doors used with bristle seals.

"Increasingly we are asked to make the doors driven", says Peglar, "as was the case with Coventry General".

Hoerbiger-Origa began its design evaluation by noting that the weight of the doors would probably mean that a counterbalance would be required if vertical opening was used, whereas large hinged doors need considerable space to allow for safe opening.

This suggested that horizontal sliding doors were going to offer the best solution, so Hoerbiger-Origa looked into the dynamics of driving them from full open to fully closed in the required 2.5s cycle time.

It was decided that the most elegant design was to provide each of the four doors with its own drive, this being based upon a 50mm bore version of Hoerbiger-Origa's OSP rodless pneumatic cylinder, from which the door is directly hung.

The integral guide option was chosen so that the door is constrained from moving from its vertical position, which would cause sheer force in the drive and could compromise the sterile sealing ability of the hatch.

Hoerbiger-Origa also provide an integral controller with finger trap prevention, status indication, emergency override and many other functions, all preprogrammed so that hospital staff do not have complex operating procedures to learn.

A smaller drive cylinder could have been specified, but it was felt that oversizing lent robustness to the system that would pay dividend as no hospital can ever afford to have its sterile operations capabilities compromised.

"Pneumatic actuation was the preferred solution for Coventry General, where a compressed air supply is available", says Peglar.

"But the design that we developed with Hoerbiger-Origa is such that we can swap freely between their pneumatic and electric drives for future projects without the slightest redesign".

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