Despite the 10 air changes/hour requirement and the need to adequately purge any accidental releases, the design of the intake duct and the diameter of the stack should be sized to limit air velocity to a maximum of 15 m/s since higher velocities not only create a feeling of being in a wind inside the shelter, but also can cause quite a bit of noise. Any ducting to the analyzer house which passes through hazardous areas should be leak-tight. Ducting through Zone 1 hazardous areas should be avoided where possible.
All the above needs to occur while the HVAC unit maintains the temperature inside the house within limits specified by the user and required by the analyzer itself. Rate of change of temperature in the building is also a consideration, since some analyzers are affected by and unable to compensate for changes in ambient temperature.
The housing needs to be designed to permit maintenance, adjustments and repairs to be carried out quickly and preferably without affecting analyzer operations. All components likely to require attention should be accessible without the aid of portable ladders or other temporary means, and shall have mountings and fittings located so that they are front-accessible. In the case of cabinet-mounted systems, this includes sufficient room within the cabinet for the technicians' hands, tools and light to see what they are doing or working on. All shelters and houses should have unobstructed internal headroom of at least 2 m, as well as a workspace (bench as a minimum) for support staff to perform maintenance tasks.
Dead zones such as corners and trenches that may collect gas should be avoided. One way this is accomplished is to mount the majority of the sample system "outside" the shelter containing the analyzer(s) proper and then putting in restriction orifices to limit the maximum flow rate through the bulkhead in the event of a catastrophic failure. For this reason equipment such as sample conditioning units, gas cylinders, calibration sample containers and laboratory sampling points are normally located outside the housing with appropriate weather protection.
Besides being protected from exposure to gases that may be in the shelter, personnel working with analyzer systems must be protected from injury from other hazards, such as burns (sample or process lines greater than 40 ºC should be insulated), electric shock (proper grounding of all circuits and the building itself) and cuts from exposed sharp edges.
Manual shutdown devices for the incoming power, sample, carrier gas and other potentially hazardous utilities should be clearly identified outside the housing. These isolation devices should be fitted close to the analyzers. Also, a separate shutdown device should be fitted for any associated house ventilation fans.
Fire detection may be provided in the form of smoke or heat detectors and should be included for all liquid service analyzers. Because not all fires burn the same, be sure to select the right type of fire detector, as you may need to use UV and IR detectors, depending on the fluid. Smoke detectors often help detect fires early in the fire cycle, especially for non-fluid materials.
Of course, if something happens while someone is in the shelter, they'll need a safe exit. Outward-opening doors should be provided at both ends of the housing to permit easy escape in an emergency and include safe egress to the unit's muster point. The position, size of doors and access to them should permit removal of equipment housed. Lockable doors should be avoided, or if they are used, the doors must be capable of being opened from the inside in the locked condition, and they should have crash bars. Manual call points should be provided on the outside of the house next to the doors, so operators can initiate an alarm should the automated systems fail.
Protecting your analyzer investment involves more than "just putting the equipment in a box," regardless of the size and type of box. It also includes providing a safe work environment for the people required to keep that equipment in good working order so that you continue to get highly reliable process analyzer systems as well.