2017-11-01
445
Unit 2 Ventilation systems
1. Introduction
1.1 Read the text title and hypothesize what the text is about. Write down your hypothesis.
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1.2 What do you know concerning this issue? List your ideas in the table left column "I know".
| I know | I have learnt |
1.3 Do you know answers to these questions? Write down your short answers in the space given after each question.
| Are people standards of living the same in different countries? What are some of the rules about changing business cards? Where is it unusual to shake hands whea meeting? Why? Do you know any rules about clothes? Is it OK to discuss business when eating? What does this saying mean: "When in Rome, do as the Romans do"? | What two major types of HVAC systems are mentioned in the text? |
| Are CV systems less energy efficient than VAV systems? | |
| Do VAV systems reduce HVAC energy cost? | |
| Can VAV systems also complicate pressure relationships in the building? | |
| What is an economizer? | |
| Why is it important periodically monitoring water quality? | |
| What is the function of fossil fuel combustion boilers? | |
1.4 Circle in the list the words and expressions you know. Write down their translation in table and calculate the percentage of your lexical competence.
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| spatial | pressure | ||||
| volume | enthalpy | ||||
| occupants | inadequate | ||||
| adequate | economizer | ||||
| features | prevent | ||||
| recirculate | fossil fuel | ||||
| lack | combustion | ||||
| fraction | exhaust |
There are significant spatial and seasonal variations in the volume of air delivered by most HVAC systems. HVAC Operators must understand the variations to know how to provide occupants with adequate outdoor air in all spaces throughout the year. The ventilation features most important to IAQ are the way in which supply air volume is controlled, and the way in which outdoor air delivery is controlled.
In most HVAC systems a portion of ventilation air supplied to occupied spaces is outdoor air and a portion is recirculated air. The total volume of air is important for two reasons:
Air movement contributes to thermal comfort. The lack of air movement can create a sensation of hot/stuffy air.
In many VAV systems outdoor air is a constant fraction of the total supply air. Thus, the total volume of outdoor air depends on both the outdoor air fraction, and the supply air volume.
There are two major types of HVAC systems based upon the use of airflow to control temperature — the Constant Volume (CV) system, and the Variable Air Volume (VAV) system.
Constant Volume (CV) Systems
In a Constant Volume (CV) ventilation system, variations in the thermal requirements of a space are satisfied by varying the temperature of a constant volume of air delivered to the space. A constant fraction of outdoor air will mean that a constant volume of outdoor air will be delivered to occupied spaces. This volume can be set to satisfy applicable ventilation standards. CV systems are less energy efficient than VAV systems, but controls for outdoor air delivery are simpler to manage.
Variable Air Volume (VAV) Systems
In a Variable Air Volume (VAV) ventilation system, variations in the thermal requirements of a space are satisfied by varying the volume of air that is delivered to the space at a constant temperature. VAV systems reduce HVAC energy cost by 10-20% over CV systems but complicate the delivery of outdoor air. If the fraction of outdoor air is constant, the total volume of outdoor air will be reduced as the supply air volume is reduced. An inadequate outdoor air fraction, combined with an inadequate VAV box minimum setting, may result in inadequate outdoor air flow to occupant spaces. This would occur during part-load conditions.
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VAV systems also complicate pressure relationships in the building and make testing, adjusting and balancing more difficult.
