Ventilation And a Decay Equation

The need for ventilation

The minimum amount of fresh air required for breathing purposes is really quite small; about 0.2 litres s“1 per person (see section 4.10). For comfort conditioning, however, it is insufficient to supply this small amount of fresh air; other factors enter into consideration and enough fresh air must be delivered to achieve the following:

(1) Meeting the oxygen needs of the occupants.

(2) The dilution of the odours present to a socially acceptable level.

(3) The dilution of the concentration of carbon dioxide to a satisfactorily low level.

(4) Minimising the rise in air temperature in the presence of excessive sensible heat gains.

(5) Pressurising escape routes in order to prevent the spread of smoke in the event of a fire.

(6) Dealing with condensation.

Odours may be diluted to a socially acceptable level by the introduction of odourless air from outside or by the use of activated carbon filters (see section 17.10). The original work on the use of fresh air for this purpose was done by Yaglou et al. in 1936. This was based on research in American schools and it was found that the quantity of outside air needed to give a satisfactory reduction of odours depended on the number of people present and their standards of personal hygiene. It was also found that odours disappeared more rapidly, with a given ventilation rate, when there was more volume of the room for each person present. (This is still inexplicable: it may be due to a chemical breakdown of the constituents of the odours or to their adsorption at the room surfaces.) It was further evident that increasing the air change rate gave diminishing returns, the efficiency of odour decay reducing with an increase in the rate because of a failure to scour the whole room. Some fresh air leaves without mixing with the odours at all and so gives no dilution effect. Yaglou was able to show that school children of average socio-economic background required from 5.5 to 13.5 litres s-1 each as the room volume per person decreased from 14 m3 to 3 m3, to produce acceptable odour control. Sedentary adults of a similar back­ground needed rather less fresh air, namely, 3 to 12 litres s_1 each.

Smoking in a room plays a very significant part in determining the necessary fresh air allowance and Brundrett (1975) has produced a survey on this topic. Although the unpleasant odours and the reduced visibility are obvious consequences of atmospheric pollution from smoking, the three main short-term hazards to health seem to arise from the amount of carbon monoxide (CO) generated, the particulate matter liberated and the quantity of acrolein produced. There are longer-term risks of course and it is to be noted that non­smokers in a smoking environment are subject to all these dangers to a smaller but still significant extent. Hence the need for proper ventilation.

Acrolein is a toxic lachrymator with an instantaneous effect of eye irritation and a further, similar effect on throats. Its threshold limit value for an eight hour exposure is 0.1 parts per million (abbreviated ppm), which requires a fresh air dilution rate of at least 3 m3 per cigarette although eye irritation can still be experienced with a rate as high as 16 m3 per cigarette. Note that the threshold limit value (abbreviated TLV) is defined as the airborne concentration of a substance that represents acceptable conditions to most working people, without adverse effect.

The particulate matter is mostly smaller in size than 0.7 |jm and is respirable, according to Hoegg (1972). Depending on the production rate from the cigarette, Bridge and Com (1972) say that a fresh air rate of from 3.5 to 5.5 m3 per cigarette is necessary to achieve a TLV of 10 mg m-3.

The most dangerous pollutant from smoking is CO. Although the TLV for an eight hour exposure is 55 ppm, much less than this is highly desirable to cater for variations in the composition of any general population, which is likely to contain a proportion of very young persons, old people and some who are sick. This minority would be more adversely affected by the presence of CO so the American recommendation for the TLV is 9 ppm and the Russians apparently require as little as 1 ppm according to Brundrett (1975). A dilution rate of 9 m3 per cigarette is needed to limit the concentration to 9 ppm in the view of Hoegg (1972).

The effect of cigarette smoke on visibility is another issue and Leopold (1945) considered that 1 m3 per cigarette was required as a minimum dilution rate in a sports stadium.

The conclusion to be drawn is that although a dilution rate of 20 m3 per cigarette is enough to give satisfactory conditions for the average person this should be increased to 40 m3 per cigarette to cover 98 per cent of the population. A typical smoker in the UK smokes an average of 1.3 cigarettes per hour but the figure is more in the United States. There are differences in the composition of the tobacco, the weight of the cigarette and the length of the stub remaining but, nevertheless, it is possible to estimate the supply of fresh air necessary to deal with a population of mixed smokers and non-smokers.

EXAMPLE 16.1

Calculate the fresh air ventilation rate needed for an office in the UK, (a) if everyone smokes and (b) if only 50 per cent of those present smoke. Take the necessary dilution rate as 40 m3 per cigarette to satisfy the comfort and health of 98 per cent of the population.

Answer

(1.3 cigarettes per h per person) x (40 m3 per cigarette) x 1000

(a) 3600 = 14.4 litres s“1 per person.

(b) If only half the people smoke this reduces to 7.2 litres s_1 per person.

For private offices the assumption might be that everyone could be smoking and so the higher rate would be appropriate. For boardrooms and the like, where very heavy smoking may sometimes still occur, the rate of cigarette consumption could be double that taken as an average for the population. In such cases 29 litres s-1 per person would be the choice. In open plan office areas not everyone will smoke. Taking the view of Brundrett (1975) that 50 per cent of a population are likely to smoke, on an average in the UK, 7.2 litres s-1 per person would then be a reasonable fresh air allowance. It is worth noting that if a small number of offices in a sizeable building contain a high number of smokers, recirculating air from them for mixing with air brought in from outside will not significantly reduce the dilution ability of the mixed air, provided the heavily polluted air is only a small proportion of the total amount of the recirculated air.

Note also that smoking is a simple contaminant that is not influenced by the volume of space per person, unlike body odour which is so influenced. Equation (16.9) describes the way in which the concentration of a contaminant is dependent on the ventilation rate. We can see that the only place in the equation where the volume of the room appears is in exponent n, which equals Qt/V, V being the volume of the room, Q the fresh air supply rate and t the elapsed time. In the steady state n is very large and terms in e“n become vanishingly small, the expression degenerating to the form of equation (16.10), in which the volume of the room is absent.

Although a fall in the oxygen content of the atmosphere to as little as 13 per cent escapes notice, depth and frequency of breathing being unchanged, alterations in the concentration of carbon dioxide (C02) are more significant in their effect. An increase in the C02 content is not immediately apparent but when it has risen from the normal value of about 0.032 per cent (in fresh air) to 2 per cent there is an increase of 30 per cent in the depth of respiration according to the Health and Safety Executive (1979) and at 3 per cent this has gone up to 60 per cent. The same reference concludes that up to 2 per cent C02 causes little discomfort but another view from the BRE (1977) is that it is tolerable up to 4 per cent. A conclusion by Brundrett (1975) is that for concentrations of C02 exceeding 3 per cent to 5 per cent there is a conscious and objectionable need felt for an increased respiratory effort. The TLV for an eight hour exposure is 0.5 per cent, say the ACGIH (1971), but it is generally thought that an upper acceptable level is 0.1 per cent. If we take the average rate of production of C02 by a human being as 4.72 x 10-3 litres s-1 the steady state concentration of C02 approaches 0.1 per cent in an office of the statutory minimum volume of 11.33 m3 (in the UK), as example 16.3 shows.

Refer to Figure 16.1. Formerly, ANSI/ASHRAE (1989) recommended that a minimum of 7.5 litres s-1 of uncontaminated outside air should be supplied for each person in an occupied space. This was based on research by Berg-Munch et al. (1986) showing that fewer than 20 per cent of people first entering an occupied space would notice undesirable body odour if 7.5 litres s“1 per person of uncontaminated outside air were provided. Supporting evidence by Brundage et al. (1988) suggested that supplying about 7.5 litre s_1 per person significantly reduced respiratory infections in occupied spaces.

More fresh air than this may be required in many instances to dilute the concentration of other emissions to acceptable levels. Typical of such contaminants are the volatiles emitted from wall finishes, floor coverings, curtains, furniture, cleaning materials, plastics, office equipment etc. Such contaminants, together with the presence of housemites and their droppings, and other small particulate matter, have been associated with what is termed the Sick Building Syndrome, according to WHO (1983). This is the group of symptoms (such as irritation of the mucous membranes and eyes, headaches, lethargy, tight chests, the perception of stuffy and stale air, etc.), complained of by some of the occupants in some buildings and related to poor indoor air quality. See section 4.11.

It is to be noted further (see example 16.6) that supplying about 7.5 litre s“1 of fresh air

36

35

подпись: 36
35
CIBSE 1999

_

Off. ces heavy smoking

CIBSE (1999)

Offices with smoking

German DIN (1946), Part 2 (1983)

Offices, some smoking

CIBSE (1999)

Offices, no smokina

German DIN (1946), Part 2 (1983)

ASHRAE A2 (1999)

Offices, no smoking, CIBSE (1999)

ASHRAE 62 (1989)

CD

Ol

25

24

<0

<n

<D

1 20

2 19.4

CL

A.

* 16 ‘« 15

8 139 ■a

Yaglou s (1936)

10

30

 

Ventilation And a Decay Equation

Per person reduces the carbon dioxide content in an occupied space to approximately 0.1 per cent.

ASHRAE Standard 62 (1989) did not distinguish between places where smoking occurred and those where it did not (see Grimstrud and Teichman (1989)) and neither does ASHRAE Standard 62 (1999). The latter argues that indoor air quality depends on many factors and offers alternative procedures for choosing a rate of supply of outdoor air. The first is the Ventilation Rate Procedure which prescribes outdoor air supply rates, related to density of occupancy, for a range of commercial and other applications, in litres s“1 per person or in litres s’1 m-2, as appropriate. For offices, 10 litres s-1 are proposed, based on a maximum population density of 14.3 m2 per person. Among the caveats is the reasonable one that compliance will not necessarily give acceptable indoor air quality. The alternative is the indoor air quality procedure. This does not specify outdoor air supply rates. Instead it

Proposes restricting all known, relevant, indoor air contaminants and offers guidelines for achieving acceptable indoor air quality. Design documentation must be kept and made available for operating the system.

Fanger (1988) proposed the use of a pollution balance in an occupied space to establish an appropriate ventilation rate and suggested the following equation.

Qo = 10[G/(Cia — Coa)ev] (16.1)

Where

Q0 = outdoor air supply rate in litres s-1 G = sensory pollution load in the space in olf cia = perceived indoor air quality in decipol coa = perceived outdoor air quality in decipol ev = ventilation effectiveness factor

Fanger (1988) introduced the concepts of the olf (to express pollution) and the decipol (to express the perception of air quality). They are defined as follows:

One olf is the pollution generated by a standard, sedentary, non-smoking person in a state of thermal neutrality (comfort).

One decipol is the perceived quality of air in a space wherein the pollution source strength is one olf and the ventilation rate with clean outdoor air is 10 litres s-1 (i. e. 1 decipol = 0.1 olf/litres s~’). The pollution generated by a smoker is 6 olf and interpolation is allowed for mixed populations of smokers and non-smokers.

The equation applies to the steady state and if the fresh air supplied mixes completely with the air in the room then the value of the ventilation effectiveness factor, ev, is 1.0. If some

Of the air supplied short circuits and does not mix with the room air then the value of ev is

Defined by European Concerted Action (1992) as

Ev = cjcz (16.2)

Where

Ce = pollution concentration in the exhaust air

Cr = pollution concentration in the occupied zone in a room

Values suggested for ey are given in Table 16.1.

According to Fanger (1988) and European Concerted Action (1992) an exponential relationship exists between the percentage of dissatisfied people and the ventilation rate for a pollution of one olf, produced by a standard person. This is given by

P = 395 exp(-1.83<7°’25) (16.3)

Where

P = percentage of people dissatisfied q = ventilation rate in litres s-1 olf"1 The equation applies when q > 0.32 litres s“1 olf“1.

EXAMPLE 16.2

Determine the percentage of people dissatisfied by a pollution of one olf from a standard person when the ventilation rate is (a) 7.5 litres s“1 and (b) 0.32 litres s_1.

Supply-exhaust

Relationship

Temperature difference between the supply air (ts) and the room air in the occupied zone (tr)

Ventilation

Effectiveness factor, ev

Supply and exhaust

Ch-t

R)<0

0.9 to 1.0

Points above the

(h-t

R) = 0 to 2

0.9

Occupied zone

(h-t

R) = 2 to 5

0.8

(fs — t

R) > 5

0.4 to 0.7

Supply point above the

(h-t

R)<-5

0.9

Occupied zone, exhaust

Ih-t

R) = -5 to 0

0.9 to 1.0

Point at low level in the

(ts -1

R)>0

1.0

Occupied zone

Supply point at low level

(h-t

,)<o

1.2 to 1.4

In the occupied zone,

Ch-t

R) = 0 to 2

0.7 to 0.9

Exhaust point above the

(fs — t

R) > 2

0.2 to 0.7

Occupied zone

Reproduced by kind permission of the CIBSE from Guide A, Environmental Design (1999).

Answer

By equation (16.3):

(a) P = 395 exp(-1.83 x 7.50 25) = 19%

(b) P = 395 exp(-1.83 x 0.320’25) = 100%

The relationship between the percentage of people dissatisfied and the perceived air quality in decipol is given by European Concerted Action (1992) as:

Cia= 112[ln(P)-5.98]^ (16.4)

Where

Cia = perceived indoor air quality in decipol P = percentage people dissatisfied

EXAMPLE 16.3

Determine the indoor air quality in decipol if the percentage of people dissatisfied is 19 per cent.

Answer

By equation (16.4):

Cia = 112[ln(19) — 5.98]^ = 1.3 decipol

If, as European Concerted Action (1992) propose, the concentration of carbon dioxide, cCq2, in an occupied space is used as an indicator of the pollution caused by bioeffluents from people, equation (16.3) can be modified to read

This refers to sedentary, non-smoking people and excludes emissions from room furnishings etc.

The use of equations (16.3) and (16.4) makes it possible to determine values for the percentage of people dissatisfied, the perceived quality of the indoor air and the ventilation rate needed to achieve it. This procedure has yielded fresh air supply rates that are unacceptably high, because of the absence of reliable information on the emission of bioeffluents from people and volatiles from furnishings and the like. The method has not found acceptance.

EXAMPLE 16.4

Determine the percentage of people dissatisfied in an occupied space if the concentration of carbon dioxide is 0.1 per cent (1000 ppm).

Answer

By equation (16.5)

P = 395 exp(— 15.5 x lOOO-0’25) = 25%

This appears to be inconsistent with the generally held view that 0.1 per cent C02 is acceptable. See example 16.6.

The German standard, given in DIN 1946, Part 2 (1983), requires 13.9 litres s-1 per person in an open plan office without smoking (corresponding to 1.54 litres s-1 m-2) but is increased to 19.4 litres s_1 per person (corresponding to 2.16 litres s-1 m-2), when there is smoking. See Figure 16.1.

One other aspect of ventilation is that a minimum rate is needed to dilute the radioactive gas emission (radon) from some building materials used for construction according to Swedjermark (1978). The small radioactive constituent in substances such as granite may need a minimum rate of about half an air change per hour to produce an acceptably low concentration.

The three methods of expressing ventilation rate, air changes per hour, litres s-1 per person and litres s_1 m-2, must be applied in the proper way. The use of litres s-1 per person is appropriate where the number of people can be established with some assurance, as in a theatre or restaurant. For offices or other areas where a precise occupancy is not known but a typical population density (e. g. 9 m2 per person) can be adopted, the use of litres s“1 nT2 of floor area is suitable. Values commonly used in the UK have been 1.3 litres s"1 rrf2 in the past, which has since risen to 1.4 litres s-1 m~2. It would be uneconomical to use air changes per hour for rooms with large floor-to-ceiling heights but appropriate for lavatories where heights are conventional. Lavatories are rather a particular case, especially when there are no openable windows, and it is recommended that at least 12 air changes per hour of mechanical extract ventilation be provided, desirably with a mechanical supply at a lesser rate to ensure a slightly negative pressure. A commonly used rate of 6 air changes per hour is inadequate, in the author’s opinion.

The CIBSE (1999) recommends outdoor air supply rates for sedentary occupants that are related to the production of body odour and the extent to which smoking occurs, as given in Table 16.2.

Smoking intensity is considered to be: some—when 25 per cent of the population are smokers; heavy—when 45 per cent are smokers; very heavy—when 70 per cent are smokers.

Uncooled outdoor air can also be introduced by mechanical means to mitigate the rise in room air temperature in warm weather. The air suffers a rise in temperature as it passes

16.1 The decay equation 475 Table 16.2 Recommended outdoor air supply rates for sedentary occupants

Condition

подпись: conditionRecommended outdoor air supply rate litres s_1 per person

8

16

24

36

With no smoking with some smoking with heavy smoking with very heavy smoking

 

Ventilation And a Decay Equation

Reproduced by kind permission of the CIBSE from Guide A, Environmental Design (1999).

Through the supply fan (see section 6.5) and it follows that, ignoring any possible beneficial effects from the thermal inertia of a heavy building structure, the room air temperature can never be less than the outside air temperature. It seems likely that providing more than about eight or nine air changes per hour of uncooled mechanical ventilation gives diminishing returns and is not worth while. It is essential that openable windows are provided in buildings that are mechanically ventilated, to give relief for the occupants in warm weather.

Escape corridors and staircases are often slightly pressurised, by supplying more air than is extracted, in order to discourage the entry of smoke and so facilitate the escape of the occupants in the event of a fire. In the case of escape staircases more than thirty air changes per hour of outside air may be needed but it must be remembered that the system only operates in an emergency when a staircase free of smoke is the aim, not comfort.

The provision of sufficient air from outside can be used to deal with condensation problems. See section 6.3 and equation (6.8).

Posted in Air Conditioning Engineering


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