R&AC Lecture 38 - Lesson 38 Design Of Air...

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Lesson 38 Design Of Air Conditioning Ducts Version 1 ME, IIT Kharagpur 1
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The specific objectives of this chapter are to: 1. Important requirements of an air conditioning duct ( Section 38.1 ) 2. General rules for duct design ( Section 38.2 ) 3. Classification of duct systems ( Section 38.3 ) 4. Commonly used duct design methods ( Section 38.4 ) 5. Principle of velocity method ( Section 38.4.1 ) 6. Principle of equal friction method ( Section 38.4.2 ) 7. Principle of static regain method ( Section 38.4.3 ) 8. Performance of duct systems ( Section 38.5 ) 9. System balancing and optimization ( Section 38.6 ) 10. Introduction to fans and fan laws ( Section 38.7 ) 11. Interaction between fan and duct system ( Section 38.8 ) At the end of the chapter, the student should be able to: 1. State the important requirements of an air conditioning duct and the general rules to be followed in the design of ducts 2. Classify air conditioning ducts based on air velocity and static pressure 3. Design air conditioning ducts using velocity method, equal friction method or static regain method 4. Explain typical performance characteristics of a duct system 5. Explain the importance of system balancing and optimization 6. State and explain the importance of fan laws, and use the performance of fans under off-design conditions 7. Describe interaction between fan and duct and the concept of balance point Version 1 ME, IIT Kharagpur 2
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38.1. Introduction: The chief requirements of an air conditioning duct system are: 1. It should convey specified rates of air flow to prescribed locations 2. It should be economical in combined initial cost, fan operating cost and cost of building space 3. It should not transmit or generate objectionable noise Generally at the time of designing an air conditioning duct system, the required airflow rates are known from load calculations. The location of fans and air outlets are fixed initially. The duct layout is then made taking into account the space available and ease of construction. In principle, required amount of air can be conveyed through the air conditioning ducts by a number of combinations. However, for a given system, only one set results in the optimum design. Hence, it is essential to identify the relevant design parameters and then optimize the design. 38.2. General rules for duct design: 1. Air should be conveyed as directly as possible to save space, power and material 2. Sudden changes in directions should be avoided. When not possible to avoid sudden changes, turning vanes should be used to reduce pressure loss 3. Diverging sections should be gradual. Angle of divergence 20 o 4. Aspect ratio should be as close to 1.0 as possible. Normally, it should not exceed 4 5. Air velocities should be within permissible limits to reduce noise and vibration 6. Duct material should be as smooth as possible to reduce frictional losses Version 1 ME, IIT Kharagpur 3
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38.3. Classification of duct systems: Ducts are classified based on the load on duct due to air pressure and
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R&AC Lecture 38 - Lesson 38 Design Of Air...

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