duct model

7/30/2019 Duct Model

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D1: MODEL-BASED COMMISSIONING METHODOLOGY FOR

SIMPLE DUCT SYSTEM (In Progress)

Takao Odajima

Design Department Takenaka Corporation

Introduction

For a central air distribution system to multi rooms, it is important to distribute and maintain

designed air volume for the each room to accomplish design performance.

Commissioning for a duct system might not be necessary for a general office type building to

control room temperature only. However for a manufacturing, research and testing type

facility which constant air volume control and room pressure differential control with adjacent

rooms are required, duct system will affect air changes, cleanness, temperature, humidity and

room pressure will impact the required performance.

Therefore to have a proper method of commissioning for the duct system should be important

for the higher functional facilities.

A functional fault factor on duct systemFor an air distribution system that room pressure control is required, FTA will analyze a

functional fault factor. (fig.1)

Fig1. FTA on Room Pressure Imbalance

A functional fault factor will be classified as the followings.

I. Large amount of air leakage on a duct system

When actual air leakage exceeds estimated leakage caused by some poor duct


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installation, supply fan will not perform properly and it makes room pressure lower.

II. Large pressure loss on duct system

When actual pressure loss exceeds estimated loss, VD and CAV will not be adjusted

against the pressure loss and it makes room pressure lower.

III. CAV characteristic does not match duct system characteristic

When combination of CAV characteristic and maximum/minimum air volume per its

control range does not meet duct pressure loss, it makes room pressure and air flow

reverse.

Simple duct system for model study

Make three biological clean rooms with air volume control and room pressure control to study

model duct system as Fig.2

Fig2. AHU and duct system

Outline of model duct system

This model system is a 100% fresh air system and air pressure in the SA duct after AHU is

controlled as constant pressure by inverter. (Inverter control is not for this model study andassumes a steady state)

SA duct is distributed to three rooms A, B and C through CAV, VD and HEPA filter.

Room pressure is controlled by airflow from Room B to A and B to C to maintain pressure

difference between rooms. (i.e. pressure in B is + +, pressure in A is + and pressure in C is +)

Commissioning process of 3 functional prevention factors above

Step I. Commissioning on air leakage from duct

Compare estimated air leakage that studied by the model with measured leakage calculated by

air volume difference at AHU and diffusers.


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When actual leakage exceeds allowance, adjust and rework the ductwork.

Step II. Duct pressure measurement and model parameter setting

Compare estimated air pressure studied by pressure distribution simulation model under

several air volumes with measured static pressure at AHU (fig. 2) and CAV (fig. 2).

When actual pressure loss exceeds, reset up parameter of simulation model.

Step III. Verification of duct and CAV

Simulate duct pressure loss model by revised parameter with tolerance of CAV and

maximum/minimum air volume per its control range and analyze whether the room pressure

is match to estimated.

Pressure loss in CAV will be analyzed by the other model.

Commissioning on duct system should be possible in a steady state conditions by the three

steps above.

Duct pressure distribution simulation model

Develop a duct pressure distribution simulation model with Bernoulli theorem. (Under

preparation and trial)

Result of duct pressure distribution simulation model

duct model

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