Topic of the month
The power of diagrams and words
Some decades ago a consulting engineering company had a large industrial project which included some sophisticated air-conditioning systems. The relative humidity in the air-conditioned spaces had to be controlled and the air-handling units were designed to be site built-up units. These units each consisted of a large plenum housing all components including primary and secondary air filters, a supply fan, chilled water cooling coil, an evaporative type air washer, and a steam heating coil. The plenum consisted of galvanized steel sections which were joined together on site and then sealed off to prevent air and water leakage. The supply air fan was pushing air through the air-handling unit thereby pressurizing the plenum. Each air-handling unit was built on top of a concrete plinth, with the plinth forming the floor of the unit. The air-handling plant rooms were located at ceiling level with ducting running horizontally out into the ceiling void from the air-handling plant rooms. The air-handling plant rooms were located directly above the factory floor.
In those days CAD drawings were still a futuristic dream and the engineering drawings were done by hand. A single line drawing of a typical air-handling unit was drawn up indicating the plenum and all internal components. This drawing was titled “Detail of typical air-handling unit” and was included in the consultant engineering drawings and specifications. The drawing indicated the air-washer consisting of sump, re-circulating pump and a water spray tree fitted with spray nozzles. The drawing did not indicate a separate enclosure for the air-washer.
The contractor duly constructed the air-handling units exactly as per the single line drawing. The air-washers therefore were not provided with own water tight enclosure and consequently relied on the built-up plenum walls to prevent water from leaking to the outside.
During commissioning of the plant two problems became apparent:
• The plenum walls were not water tight against the supply fan delivery pressure and water leaked through the plenum panel joints to the outside of the plenum.
• The concrete plinths (and floors around plinths) were also not water proofed and water leaked through the plinths onto the factory floor below.
The client was naturally not amused since the water leakage damaged factory process equipment and building structures. After repeated fruitless efforts to seal off the plenum walls and plinth a separate fiber glass plenum was installed with the air-washers inside this plenum, thereby providing the required water seal.
Unfortunately the above rectification took some time and nearly resulted in the consultant being taken to court by the client. Fortunately a settlement was reached.
During the legal proceedings blame was thrown around by the various parties with the consultant maintaining that the above single line drawing was not a manufacturing drawing and that some further engineering had to be done by the contractor, and with the contractor maintaining that the drawing should have shown all components including a separate air washer enclosure.
The lawyers eventually ruled that both consultant and contractor were negligent. It transpired that the problem on the consultant’s side was that the drawing title contained the term “detail”, which in legal terms means as good as a manufacturing drawing! If the drawing had been titled “schematic” or “scheme” or similar the consultant would have been off the hook! The contractor, on the other hand, did not perform any detailed engineering design work, which was a contractual requirement, but merely copied the single line drawing.
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