Engineering: One System, Multiple Design Variables An AHU is not just a box with a fan and cooling coil. A small design mistake can increase: ✗ Fan power ✗ Pressure drop ✗ Noise ✗ Operating cost ✗ Maintenance frequency 1. Typical AHU Air Path Fresh Air → Louver → pre-Filter → Heat Recovery → Cooling/Heating Coil → Fan → Silencer → Supply Air Return/exhaust air paths must also be considered, especially where heat recovery, pressure control and IAQ are important. 2. Airflow Sizing The basic relationship is: Q = A × V Where: • Q = Airflow (m³/s) • A = Face area (m²) • V = Air velocity (m/s) Example: 20,000 m³/h = 5.56 m³/s At 2.5 m/s face velocity: Required area ≈ 2.22 m² Lower face velocity generally means a larger AHU footprint but can reduce component pressure drop and fan energy. ASHRAE notes that designers should not automatically default to 2.5 m/s; lower coil/filter velocities can provide energy benefits. (ASHRAE Handbook) 3. Pressure Drop = Hidden Energy Cost Typical AHU res...
HYDROTEST – AHU & CHILLER PIPING SYSTEM
A hydrostatic pressure test (hydrotest) is performed on chilled-water piping to verify the strength, integrity, and leak-tightness of the AHU and chiller piping system before insulation and commissioning.
🔧 Typical Hydrotest Procedure
1. Preparation
- Review approved drawings, specifications, and test pressure requirements.
- Isolate chillers, AHU coils, control valves, pumps, expansion tanks, and other equipment not rated for the test pressure.
- Ensure all pipe joints, valves, flanges, and welds are accessible for inspection.
- Install calibrated pressure gauges at suitable locations.
2. Filling
- Fill the piping system slowly with clean water.
- Open high-point vents to remove trapped air.
- Ensure the system is completely filled before pressurization.
3. Pressurization
- Use a calibrated hydrostatic test pump.
- Raise pressure gradually to the specified test pressure.
- Avoid sudden pressure increases.
4. Test & Inspection
- Stabilize the system and maintain the specified pressure for the required duration according to the project specification/approved method statement.
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Inspect all:
- Welded joints
- Flanges
- Grooved couplings
- Threaded connections
- Valves
- Drain points
- Pipe supports and connections
- Check for visible leakage, pressure loss, or deformation.
5. Acceptance
The system should be accepted when:
- No visible leakage is observed.
- No unacceptable pressure loss occurs.
- No pipe, joint, valve, or fitting shows deformation or damage.
- Test results comply with the approved project specification and applicable standard.
6. Completion
- Depressurize the system safely.
- Drain or retain water as specified.
- Remove temporary test equipment.
- Restore valves and equipment to their normal operating configuration.
- Complete the hydrotest report and inspection sign-off before insulation.
Typical Flow
Isolation → Filling → Air Venting → Pressurization → Pressure Stabilization → Inspection → Acceptance → Drain/Restore → Handover
Important: Test pressure and test duration must not be assumed. They should be taken from the approved project specifications, piping design, equipment ratings, and applicable code/standard.
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