Mechanical Systems
What the domain tests
Mechanical Systems covers the heat-rejection chain: chillers, pumps, cooling towers, CRAH/CRAC units, piping, valves, economisers, controls and the redundancy schemes that connect them. The exam probes whether you can size a plant, identify the weakest link, and describe how maintenance is performed without losing cooling.
Plant components
Water-cooled chillers (centrifugal or screw) are more efficient but need towers and water treatment; air-cooled chillers are simpler but capacity falls as ambient rises. CRAH units contain coils, fans and filters and rely on a chilled-water loop; CRAC units contain their own compressors and are typical of small or edge rooms. Pumps are usually configured N+1 on a common header, with variable-speed drives for part-load efficiency. Cooling towers reject condenser heat and consume water; plume, drift and Legionella control are examinable.
Redundancy and distribution
Redundancy must exist in every segment: chiller, pump, tower, pipe header and valve. A common trap is an N+1 chiller plant feeding a single primary loop with no isolation valves, so any pipe repair stops all cooling. Ring headers, dual risers and sectionalising valves deliver true concurrent maintainability. Waterside and airside economisers reduce compressor hours; their availability depends on climate and on the changeover controls working correctly.
Worked example
A facility needs 1,200 kW of cooling. Three chillers are specified at 500 kW each, N+1, rated at 35 C ambient. The site design ambient is 41 C, and the manufacturer derates capacity 3% per degree above rating. Derated capacity = 500 x (1 - 0.03 x 6) = 410 kW. With one chiller down, remaining capacity = 820 kW, well below 1,200 kW. The design fails. Either select larger machines (for example 800 kW nominal giving 656 kW derated, two of three = 1,312 kW) or add a fourth unit. Always check capacity at site design conditions, not catalogue conditions.
Traps that cost marks
Using nominal capacity at 35 C for a hot site; ignoring pump head and pipe friction losses; assuming CRAH units have compressors; forgetting water treatment, blowdown and Legionella risk; and treating a common header as redundancy when it has no isolation valves.