Pumps, Compressors, and Auxiliary Equipment
What the exam tests
This domain covers pumps, compressors, fans, and auxiliary equipment used in steam and heating plants. Expect questions on centrifugal and positive-displacement pumps, cavitation, NPSH, affinity laws, compressor cooling, receivers, and the safe operation of each machine.
Pumps
- A centrifugal pump uses a rotating impeller to convert velocity into pressure. Its flow depends on system head; the discharge valve is used to regulate flow, but running for long periods at shutoff can overheat the casing.
- Cavitation occurs when local pressure drops below vapor pressure, forming bubbles that collapse and damage the impeller. It sounds like gravel and reduces flow. NPSH available must exceed NPSH required.
- Positive-displacement pumps move a fixed volume per stroke or revolution. They must have a relief valve on the discharge and must never be run against a closed discharge.
- Affinity laws for a centrifugal pump: flow ∝ speed, head ∝ speed², power ∝ speed³.
Compressors and auxiliaries
- Reciprocating compressors use pistons and valves; clearance volume limits volumetric efficiency. Intercoolers reduce temperature and power, and aftercoolers remove moisture before the receiver.
- Rotary-screw and centrifugal compressors are common in larger plants. Air receivers store pressure, dampen pulsation, and collect condensate; traps remove water.
- Fans and induced-draft equipment handle large gas volumes at low pressure. Vibration, bearing temperature, and lubrication are key checks.
Worked example
A centrifugal pump absorbs 50 kW at its design speed. If speed is increased by 10%, what is the approximate new power?
Power ∝ speed³, so new power = 50 × (1.10)³ = 50 × 1.331 ≈ 66.6 kW. The reasoning is that a small speed increase raises flow and head and raises power much more sharply. This is why variable-speed control saves energy at reduced demand.
Traps that cost marks
- Running a positive-displacement pump with the discharge closed.
- Confusing NPSH available with NPSH required.
- Assuming a centrifugal pump's power rises linearly with speed.
- Neglecting to drain compressor receivers and traps.
- Operating a centrifugal pump at shutoff for long periods.