Technology

Determination of flow regime in AHUs

AHU

This article delves into determining flow regimes in AHUs, particularly in pharmaceutical settings. It explores the formulas and calculations to classify laminar, transient, and turbulent flows in compressible fluid flows. Additionally, it contains the relevance of these classifications in ensuring compliance with regulatory standards and maintaining a controlled environment in pharmaceutical facilities.

Air Handling Units (AHUs) are a crucial component of Heating, Ventilation, and Air Conditioning (HVAC) systems in various industries, including pharmaceuticals. The flow regime in AHUs, whether laminar, transient, or turbulent, significantly impacts the unit’s performance, efficiency, and ability to maintain a controlled environment. In pharmaceutical applications, where strict regulations and guidelines govern the production and storage of medicinal products, the classification of AHU flow regimes is paramount.

Why does Flow Regime classification matter in pharmaceutical AHUs?

AHUs must maintain a controlled environment in pharmaceutical applications that meets strict regulatory requirements. The flow regime in AHUs directly impacts the unit’s ability to control temperature, humidity, and air quality. For instance:

  • Laminar flow is essential in pharmaceutical cleanrooms, where the risk of contamination must be minimised.
  • Turbulent flow can lead to increased energy consumption, reduced system efficiency, and compromised air quality.
  • Transient flow can result in unstable system performance, affecting the overall controlled environment.

The classification of flow regimes in AHUs is crucial in ensuring compliance with regulatory standards, such as:

– FDA’s Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing (2004)

– EU’s Good Manufacturing Practice (GMP) guidelines for medicinal products

– ASHRAE Standard 170-2017: Ventilation of Health Care Facilities.

Formulas and Calculations for Flow Regime Classification

Engineers use various formulas and calculations to classify the flow regime in AHUs. The following sections outline the key equations and parameters to determine laminar, transient, and turbulent flows in compressible fluid flows.

Laminar Flow

Laminar flow occurs when the Reynolds number (Re) is lesser than 2000. The Reynolds number is calculated using the following formula:

Re = ρUL/μ

Where:

– ρ = fluid density (kg/m³)

– U = fluid velocity (m/s)

– L = characteristic length (m)

– μ = fluid viscosity (Pa·s)

For laminar flow, the friction factor (f) can be calculated using the following equation:

f = 64/Re

Transient Flow

Transient flow occurs when the Reynolds number (Re) is between 2000 and 4000. During this regime, the flow is unstable and can exhibit laminar and turbulent flow characteristics.

The friction factor (f) for transient flow can be calculated using the following equation:

f = 0.316/Re^0.25

Turbulent Flow

Turbulent flow occurs when the Reynolds number (Re) exceeds 4000. The friction factor (f) for turbulent flow can be calculated using the following equation:

f = 0.184/Re^0.2

Variation of Friction Factor (f) with Reynolds Number (Re) for Different Flow Regimes

  • X-Axis: Reynolds Number (Re) (logarithmic scale for clarity across regimes)
  • Y-Axis: Friction Factor (f)

The above graph visually demonstrates the transitions between laminar, transient, and turbulent flow regimes.

Importance of Flow Regime Classification in Pharmaceutical AHUs

Classifying flow regimes in AHUs is critical in pharmaceutical applications, where the controlled environment must meet strict regulatory requirements. By determining the flow regime, engineers can:

– Ensure compliance with regulatory standards and guidelines

– Optimise AHU performance and efficiency

– Maintain a controlled environment that meets pharmaceutical production and storage requirements.

In conclusion, determining flow regimes in AHUs is a critical aspect of pharmaceutical applications. By understanding the formulas and calculations used to classify laminar, transient, and turbulent flows, engineers can ensure compliance with regulatory standards, optimise AHU performance, and maintain a controlled environment that meets pharmaceutical production and storage requirements.

References:

Expertise shared by-

Harsh Bardhan

HVAC Technical Consultant.

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