English version machine-translated from the Vietnamese original — under review.
Based on the source documents, the design of the inlet duct and discharge duct of a VAV box plays a very important role in system performance (including pressure drop, life-cycle cost and noise).
Below is a detailed analysis of the influence of these factors:
I. VAV Box Inlet Duct Design (VAV Box Inlet Ducts)
Inlet duct design directly affects the overall pressure drop of the VAV box, the first cost and the accuracy of the airflow sensor.
1. Avoid Flexible Duct
- Negative impact: Flexible duct should not be used at the VAV box inlet.
- Reason: Flexible duct causes a high pressure drop, especially if it is kinked. Furthermore, the roughness of flexible duct can cause errors in velocity pressure sensor readings.
2. Choice of Hard Duct Type and Connection Shape
The choice between different hard duct configurations (conical, 45°) has a significant impact on First Cost and Total Pressure Drop:
| Option | Relative First Cost | Total Pressure Drop (in.w.g.) (For 10 ft of duct) | Application Recommendation |
|---|---|---|---|
| B. Conical, Hard Duct (Conical, Hard) | Base | 0.31 | Recommended for most VAV boxes. |
| C. 45°, Hard Duct (45°, Hard) | Higher ($180) | 0.24 | Recommended when the VAV box is at a 45° angle to the main duct. |
| D. Oversized Conical, Hard Duct (Oversized Conical, Hard) | Highest ($235) | 0.19 | Recommended for VAV boxes in "obviously critical zones". |
3. Oversizing the Inlet
- Role: Significantly reduces the total pressure drop (e.g. from 0.38 in.w.g. (Option A/B) to 0.19 in.w.g. (Option D) for 10 ft of duct).
- LCC recommendation: Although the oversized duct option (Option D) offers pressure drop benefits, applying it to every zone in a large system would result in a very long payback period (e.g. 75 years).
- Optimisation: The inlet duct should be upsized only for zones considered to be obviously critical zones. To achieve a payback period of about 15 years, oversized ducts should not be applied to more than 20% of the candidate zones.
- Low pressure drop in the main duct: The inlet duct should be upsized where the main duct velocity is low (inflection starts when duct main velocity ~60% of tap duct velocity).
II. VAV Box Discharge Duct Design (VAV Box Discharge Ducts)
Discharge design focuses on minimising the pressure drop through the reheat coil and the plenum, while also considering noise and cost.
1. Make Use of an Oversized Reheat Coil
This is the most important factor in reducing discharge pressure drop and Life-Cycle Cost (LCC).
- Definition: An oversized reheat coil is a standard factory option that uses a larger box and coil (e.g. the next larger standard VAV box size) while the damper and velocity pressure sensor remain at the smaller size.
- Performance benefits:
- Lower air pressure drop: The pressure drop across the reheat coil is significantly reduced (e.g. from 0.30 in.w.g. to 0.15 in.w.g.).
- Lower total pressure drop: The total discharge pressure drop is reduced from 0.35–0.43 in.w.g. to 0.18–0.22 in.w.g..
- Hot water system benefits: Allows a larger hot water temperature difference (HW ΔT) (~10°F), helping to reduce pump and piping sizes and improve the efficiency of condensing boilers.
- Recommendation: This option offers major benefits and should be clearly specified in the schedules and specifications.
2. Discharge Configuration (Discharge Plenum)
- Recommendation: A rectangular discharge plenum should be used, followed by all-round ducts.
- LCC-optimal options:
- Option D (Unlined Plenum, Oversized HW Coil): Recommended if the acoustic criteria are met without lining. This option has the lowest Total Pressure Drop (0.18 in.w.g.) and a relatively low cost.
- Option E (Lined Plenum, Oversized HW Coil): Recommended if lining is needed for acoustic purposes and is permitted by local codes/practice.
3. Box / Branch Layouts
Discharge layout options include Plenum with Round Ducts, All Round Ducts, or Plenum with Rectangular Ducts.
