THE MULTI-FUNCTIONAL CLEANROOM OF TNO BUILDING AND CONSTRUCTION RESEARCH AT DELFT

INTRODUCTION

For studies in rooms in which dust-free air is important, the department of Indoor Environment, Building Physics and Systems of TNO Building and Construction Research has access to a multi-functional cleanroom annex operating theatre [OT], which has been built in co-operation with sponsors .

To allow testing of medical equipment, the room is designed as a medical room Class S3 to NL-NEN 3134 [1] to realise electrostatic discharge control [ESD]; for this purpose the room is fitted with two semi-conducting floors [ground floor and raised floor].

The room is constructed in a big hall of the TNO Building and Construction Research premises at Leeghwaterstraat 5, Delft [NL], figure 2 shows the floor plan. The room is fitted with an airlock entrance with automatic sliding doors and an airlock for goods with double doors. Some glass panels extend from floor to ceiling and act as observation areas. The room has a raised floor with tight or perforated elements for floor exhaust as required. HEPA filters can be placed in any location in the suspended ceiling.

The air handling unit is based on an adjustable air supply up to 14,500 m3/h with a fresh-air part of maximal 4,500 m3/h. In the room the air supply takes place over up to 12 HEPA-filters (11.2 x 0.6)m, at any location in the suspended ceiling. The exhaust takes place over three adjustable grilles in each corner of the room; just below the ceiling and above and below the raised floor in order to realise floor-exhaust if necessary. The fresh-air part cares for the pressure hierarchy by using adjustable wall mounted grilles for overflow through the airlocks to the hall. The system is controlled by a computer system that also can be controlled from outside the building by telephone.

DETERMINATION OF THE AIR-TIGHTNESS [Leak proof]

For measurements to determine the air-tightness of the room, a measuring method to NL-NEN 2682 is used [2]. In this method, a room to be monitored for air leaks is exposed to high or low pressure via a controlled ventilator. The air flow supplied by the ventilator is established at the various pressure levels via a measuring flange.

MONITORING OF THE S3 CLASSIFICATION

One condition for the construction of the cleanroom was that this must also be suitable for tests on medical instruments and techniques. The room must be classed as a room for a medical purposes with S3 classification to NEN 3134 111. This describes a patient area in which all metal parts must be earthed separately according to a specific protocol .

Although the patient area in an operating theatre is limited to a distance of max. 1.50m from the operating table, in general all metal parts of the partition walls and air outlet grilles must comply with the same requirements. These metal parts may have no galvanic contact with earth other than via the separate earth line.

To comply with this, during the construction great attention was paid to the isolation of the wall construction. Almost daily checks were carried out to ensure that there was no galvanic contact with the building earth. On completion of the room, the initial conditions appeared to have been well fulfilled where the earth resistance of the room was established at >>50 MW . This shows that with the necessary attention, it is perfectly possible to meet the requirements of NEN 3134 with the system wall construction used.

To prevent static charges, according to NEN 3134, at least one of the following measures must be taken for operating the theatre:

• installation of an air conditioning system which achieves a relative humidity of at least 50%
• use of a semi-conducting floor.

As is usual practice, it was decided to apply both measures. The air conditioning unit is fitted with a humidifier and dehumidifier to control the relative humidity in the room. The requirements in NEN 3134 for the semi-conducting floor are as follows:

• the mean continuity resistance must be at least 50kW
• at each measurement point, the continuity resistance must not be less than 20kW and not more than 1MW .

Provisional measurements have shown that the continuity resistance at various measurement points on the floor is 200kW . The ground floor construction thus complies perfectly with the requirements imposed.

The raised floor showed a continuity resistance of 266MW which is too high according to NEN 3134 specifications but is sufficient to fulfil the ESD demand of <1GW .

MONITORING OF DUST CLASSIFICATION

The aim was to realise a cleanroom with class-100 qualification at maximum air supply conditions. This was checked in the as-built situation according to the methods, described in the Federal Standard 209E. The standard gives the maximum acceptable particle concentration, the minimum number of measuring points and the minimum for the measuring volume for each class. According to these rules the minimum number of measuring points was determined as 73 with a minimum measuring volume of 0.23ft³. The air exhaust took place through the raised floor in which 1/3 part of the elements were perforated.

The measurements were carried out at 100 measuring points at a height of 1.20m with a measuring volume of 0.50ft³ in 30 seconds using an Airborne Particle Counter. The counter determines the number and dimensions of the particles >=0.3mm.

The measurements were carried out twice and the average of the two results were evaluated. The average particle concentration >=0.3mm was determined as 7.5 particles/ft³. The cleanroom clearly meets the class 100 criteria.

With these results one could possibly claim that even class-10 is realised, but according to the rules of the Federal Standard 209E the number of measuring points and measuring volume should then be increased.

THE USE OF TRACER GAS TECHNIQUE

In an operating room there are many particles in the air arising from the skin of the operating team many of which will be carrying bacteria. The wound area is therefore at risk of contamination. The air supply system is supposed to create a clean area on the operating table to reduce this risk. The presence of the operating lamp and the operating team however disrupts the airflow around the table and generates a temperature gradient which increases the probability of contamination. This is one of the options to be investigated in this cleanroom.

In order to make a quantitative evaluation of this effect there are four mannequins placed around the operating table in the room which represent the operating team. The mannequins are proportionally heated inside to give them the same skin temperature as a human being. Also on the table a heated mannequin represents the patient. The four standing mannequins are equipped with a tubing system under their operating clothing by which a tracer gas [SF6] can be distributed separately to the under and upper part of their body. By measuring the tracer gas concentration in the wound area one can determine if there is a possibility for an air transport from the skins of the operating team to the wound area of the patient. By measuring the mean concentration in the exhausted air from the wound area, the relationship between both measured concentrations will give a valuation of contamination risk for the chosen configuration of air supply system in combination with the lamp choice and position.

A comparison between the calculated contamination risk with CFD-technique [Computational Fluid Dynamics] and the measured values in the cleanroom has been made. It appears that concentrations in the operating field are approximately 5 times higher than in the exhaust air that can be found when a big round operating lamp is placed horizontally under a downflow ceiling.

LITERATURE