R&AC Lecture 40 - Lesson 40 Ventilation For Cooling...

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Lesson 40 Ventilation For Cooling Version 1 ME, IIT Kharagpur 1
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The specific objectives of this chapter are to: 1. Discuss the use of ventilated air for cooling of buildings and cooling of occupants ( Section 40.1 ) 2. Make a comparison between natural ventilation and mechanical ventilation ( Section 40.2 ) 3. Discuss characteristics of natural ventilation and estimation of airflow rate due to wind and stack effects ( Section 40.3 ) 4. List the general guidelines for natural ventilation ( Section 40.4 ) 5. Discuss briefly forced ventilation using electric fans ( Section 40.5 ) 6. Discuss interior air movement using interior fans, unit ventilators, whole house fans and solar chimneys ( Section 40.6 ) At the end of the lecture, the student should be able to: 1. Discuss the effectiveness of ventilated air for cooling of buildings and occupants 2. Compare natural ventilation with mechanical ventilation 3. Estimate airflow rates due to wind effect and stack effect and combined wind and stack effects 4. List the general guidelines for natural ventilation 5. Discuss the benefits of mechanical ventilation using fans 6. Discuss the benefits of interior air movement and ways and means of achieving interior air movement such as the use of fans, ventilators, solar chimneys etc. 40.1. Introduction: In a previous chapter, ventilation has been defined as “ supply of fresh air to the conditioned space either by natural or by mechanical means for the purpose of maintaining acceptable indoor air quality”. However, when outdoor conditions are suitable, the ventilation can also be used for cooling of the buildings, for cooling of the occupants or both. 40.1.1. Ventilation for cooling of buildings: When the ambient dry bulb temperature is lower than the building temperature, then the outdoor air can be used for cooling the building. Normally due to solar and internal heat gains, buildings can become hotter than the ambient air. This provides an opportunity for cooling the building at least partly, by using the Version 1 ME, IIT Kharagpur 2
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freely available outdoor air. This can significantly reduce the load on air conditioning plants. Though the cooling of buildings during daytime may not be possible on all days, in an year there are many days during which outdoor air can act as a heat sink for the building. Greater opportunities exist for cooling the buildings especially during the night, when the outdoor air is considerably cooler. This is especially effective for hot and dry climates where the diurnal temperature variation is quite large. 40.1.2. Ventilation for cooling of occupants: Under certain circumstances, outdoor air can also be used very effectively for cooling the occupants of a building directly. By allowing the outdoor air to flow over the body at a higher velocity, it is possible to enhance the heat and mass transfer rates from the body, thus leading to a greater feeling of comfort. As a thumb rule, studies show that each increase in air velocity by 0.15 m/s will allow the conditioned space temperature to be increased by 1 o C. As mentioned before, maintaining the
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This note was uploaded on 02/29/2012 for the course CHEMICAL 302 taught by Professor Nptel during the Spring '12 term at Birla Institute of Technology & Science.

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R&AC Lecture 40 - Lesson 40 Ventilation For Cooling...

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