2017-11-01
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Unit 3 Ventilation standards
1. Introduction
1.1.Read the text title and hypothesize what the text is about. Write down your hypothesis. …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..
1.2What do you know concerning this issue? List your ideas in the table left column "I know".
| I know | I have learnt |
1.3Do 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 is the required outdoor air fraction? |
| What is the best way to determine outdoor air flow? | |
| What are ventilation standards for existing buildings? | |
| Are there different outdoor air fractions in multiple zone? | |
| How can intermittent occupancy be measured? | |
| What are main thermal comfort requirements? | |
| How is humidity and microbial growth controlled? |
1.4Circle 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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| fraction | pressure | ||||
| airflow | capacity | ||||
| occupants | inadequate | ||||
| supply | intermittent | ||||
| determine | prevent | ||||
| in response to | occupancy | ||||
| lack | humidity | ||||
| manufacturers | requirements |
The required outdoor air fraction is the fraction of outdoor air required so that the total outdoor airflow in the supply air is sufficient to provide the amount of outdoor air per occupant required in the Standard. However, the outdoor air fraction in the supply air is not equivalent to the outdoor air requirements of the Standard. That is, if the Standard requires 20 cfm of outdoor air per occupant, that does not mean that the outdoor air fraction should be 20%. The best way to determine outdoor air flow is to measure it.
For VAV systems, the outdoor air fraction will change as the supply air volume changes in response to changing loads. In the case of control systems that provide a constant outdoor air fraction and meet outdoor air requirements at design (peak) loads, outdoor airflow into the building at part-load will reduce the outdoor air to between one-half to two-thirds the design flow. This may be a cause of indoor air quality complaints. Manufacturers offer controls for VAV systems that can vary the outdoor air fraction to satisfy the Standard under all load conditions.
Existing Buildings
For existing buildings, the HVAC system should be operated to meet, at a minimum, operating parameters for providing thermal comfort and outdoor air ventilation flow as specified in design documents. However, provided that capacity is available in older buildings, it is a good idea to go beyond design requirements where feasible, and program the operating controls to satisfy the outdoor air ventilation requirements.
Should the outdoor air flow rates exceed the system’s design flow rates, a careful load analysis at these elevated flow rates should be undertaken to insure that the system has sufficient capacity for the added load at peak load conditions. Failure to perform such an analysis could result in deterioration of IAQ and/or coil freezing during extreme weather conditions.
Multiple Space Systems
In multiple zone systems, different spaces within a system will call for different outdoor air fractions. This is because loads (and therefore supply air requirement) are different, and/or occupant densities (and therefore outdoor air requirements) are different.
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For multiple space systems, even when the total outdoor air volume equals the sum of the requirements of individual spaces, many of the spaces may be under-ventilated most of the time. For example, even with uniform occupant densities, systems servicing both the perimeter and core zones will leave the core zone with only a third to a half of the outdoor air required throughout the year, while
Unfortunately, both the critical zone and the outdoor air fractions will be different at full load and at part-load. Some manufactures do offer DDC/VAV control systems that dynamically calculate the correct outdoor air fraction at the air handler as the space load requirement changes.
