Document zoYEvKmGmvqx4o5Er5Joo8BL6

EXHAUST SYSTEMS, GENERAL It COMFORT VENTILATION 250 tons Certainly no small unit heater spotted over this doorway will be able to solve the problem The following discussion of makeup air is taken directly from Industrial Ventila tion--A Manual of Recommended Practice Natural infiltration The need for makeup air should not be based on "air change" figures since there is a great variation between new and old buildings, as well as differences in basic building types A relatively old building with large sash areas is "open" and air leakage will be quite pronounced On the other hand, a modern, windowless plant of masonry construction is practically air tight and the building will be truly "air starved" with appreciable exhaust ventilation Be tween these two extremes it is difficult to apply engineering data to determine mini mum volumes required In general it is best to provide makeup to supply approximately the same volume of air as that exhausted Makeup air is important for the follow ing reasons 1 To ensure that exhaust hoods operate properly--A lack of makeup air creates a "negative pressure" condition which in creases static pressure which the exhaust fans must overcome This causes a re duction m exhaust volume and is particu larly senous with low-pressure fans such as wall fans and roof exhausters 2 To ensure the proper operation of natu ral stacks such as combustion flues and vents 3 To eUmmate high-velocity cross drafts through windows and shipping doors-- These cross drafts not only interfere with proper operation of exhaust ventila tion but may also disperse contaminated air from one section of the building into another This is particularly important in the case of dusty operations where settled material may be dislodged from beams and ledges 4 To eliminate cold drafts on workersDrafts not only cause discomfort and re duced working efficiency but may also result in lower overall plant ambient temperatures from the existing heating equipment 5 To eliminate differential pressure on doors It must be emphasized that makeup air will always enter the building, otherwise the exhaust fans cannot function Further, this air will be heated eventually by ineffi cient mixing, while still producing all the disadvantages in air leakage and inefficient exhaust ventilation It is more economical to introduce the air through makeup air heaters whereby its temperature and distri bution can be controlled Application of makeup air The fresh air intake should be located re mote from any contaminating sources such as exhaust stacks or furnace exhausts It is advisable to filter the fresh air to protect the equipment and provide nunmiim beat exchange efficiency In the case of finnedcod heating units, filtering is necessary to prevent plugging The air supply system must be provided with a fan Otherwise the room will stdl be under a negative pressure since the exhaust fans must pull the air through the resist ances of the filters, coils and ductwork Makeup air sources should be located to provide cross ventilation In this way the air can be "used twice " First, it will pro vide general ventilation of nuisance produc ing areas and finally, makeup air for the exhaust systems This does not apply for spot cooling applications where the air will be introduced directly at the work station and may vary significantly from room tem perature The air distribution pattern must be engineered carefully to provide effective area coverage without excessive cross drafts which will interfere with the workers or the existing systems Preferably, there should be convenient "two-season" control to afford changes m the makeup air pattern for summer and winter without closing off the outlets Makeup air should be introduced into the "living zone" of the plant, Jhat is, below the 8 to 10 ft level In this manner the air is used first by the people and best results nf "American Conference of Governmental Industrial Hygienists, Lansing, Mich In dustrial Ventilation--A Manual of Recom mended Practice, 7th ed pp 7-1 and 7-2 1982 28-84