Design Ventilation Systems

Overview

Engineering reference data for Design Ventilation Systems in HVAC systems.

Key Formulas

Sensible Heat

$Q = \dot{m} c_p \Delta T$

Heat causing temperature change.

Latent Heat

$Q = \dot{m} h_{fg} \Delta\omega$

Heat causing moisture change.

COP (Cooling)

$COP = Q_c / W$

Coefficient of performance.

Variables

Symbol	Description	Unit
$Q$	Heat transfer	W
$\dot{m}$	Mass flow rate	kg/s
$c_p$	Specific heat of air	J/(kg·K)
$\Delta T$	Temperature difference	K

Design Procedure

The systematic approach to designing a ventilation system includes the following key steps:

1. Calculate Heat or Cooling Load: Determine both sensible and latent heat loads.
2. Calculate Required Air Shifts: Based on the number of occupants, their activity level, and any special processes within the space.
3. Calculate Air Supply Temperature.
4. Calculate the Required Air Quantity.
5. Calculate Temperature Loss in Ducts.
6. Select Component Outputs: Size heaters, coolers, washers, and humidifiers based on air quantity and capacity from manufacturer catalogs.
7. Size the Boiler or Heater.
8. Design and Calculate the Duct System.

Air Supply Temperature Guidelines

The following are common guidelines for selecting air supply temperatures, which are critical for system efficiency and occupant comfort.

Additional Formulas and Relations

Combined Duct Temperature Loss: The heat loss from a duct and the temperature change in the air flow can be combined into a single equation. $H = A k \left(\frac{t_1 + t_2}{2} - t_r\right) = 1000 q c_p (t_1 - t_2) \quad \text{(5c)}$ Note: For larger temperature drops, the logarithmic mean temperature difference should be used for greater accuracy.

Duct Friction Resistance: The major pressure loss in ducts due to friction can be calculated as: $dp_f = R \cdot l$ where $R$ is the duct friction resistance per unit length (Pa/m) and $l$ is the length of the duct (m). The value of $R$ is typically obtained from duct friction charts or formulas based on duct size, air velocity, and surface roughness.

References

• Original Source