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...
There are several common types of electrical earthing (grounding) used in buildings, industries, hospitals, and power systems.
There are several common types of electrical earthing (grounding) used in buildings, industries, hospitals, and power systems.
1. Plate Earthing
A copper or galvanized iron plate is buried in the ground and connected to the equipment using an earthing conductor.
- Copper plate: commonly used where higher conductivity is required.
- GI plate: economical and widely used.
- Suitable for buildings and industrial installations.
2. Pipe Earthing
A GI or copper pipe is installed vertically into the ground and connected to the equipment.
- One of the most common methods.
- Economical and relatively easy to install.
- Suitable for many building and industrial applications.
3. Rod Earthing
A copper, GI, or galvanized steel rod is driven into the ground.
- Requires relatively little excavation.
- Suitable where soil conditions permit deep electrode installation.
- Multiple rods can be interconnected to reduce earth resistance.
4. Strip/Wire Earthing
A metal strip or wire is buried horizontally in a trench and used as the earth electrode.
- Common for large installations.
- Useful where a long horizontal earth electrode is practical.
- Often used around substations and industrial facilities.
5. Chemical Earthing
An earthing electrode is installed with conductive backfill material around it to improve soil conductivity and maintain lower earth resistance.
- Useful in high-resistivity soil.
- Requires appropriate electrode and backfill selection.
- Maintenance and environmental considerations should be addressed.
6. Grid/Mat Earthing
A network of interconnected conductors is buried below the ground.
- Common in substations, power plants, and large industrial facilities.
- Controls touch and step potentials.
- Provides multiple paths for fault current.
7. Foundation Earthing
Earthing conductors or dedicated foundation electrodes are incorporated into the building foundation.
- Provides an extensive earth-electrode system.
- Particularly useful in new construction.
- Must be coordinated with structural and electrical design.
Quick Comparison
| Type | Typical Application |
|---|---|
| Plate Earthing | Buildings/industrial installations |
| Pipe Earthing | General building installations |
| Rod Earthing | Buildings, equipment, outdoor systems |
| Strip/Wire Earthing | Industrial installations/substations |
| Chemical Earthing | High-resistivity soil |
| Grid/Mat Earthing | Substations/power plants |
| Foundation Earthing | New buildings |
Important: Earthing requirements, electrode dimensions, conductor sizes, and acceptable earth resistance should be designed and tested according to the applicable local electrical code and standards such as IEC 60364, IEEE 80, and NFPA 70 (NEC), as applicable.
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