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This module explores the link between fresh air ventilation rate and internal environmental quality
Every building services engineer will appreciate that the supply of ‘fresh’, outdoor, ventilation air into an occupied building is a fundamental requirement, both for occupant comfort and health, and for maintaining a productive living environment. The required amount of fresh air has been set in codes and standards and, for many designers, is a simple – often single – value that is remembered and applied. The increasing use of room units in commercial buildings – to cope with sensible heating and cooling loads – in conjunction with a mechanical ventilation system to deliver fresh air, provides a perfect opportunity for this CPD to look beyond current standards in an attempt to identify future trends, based on recent research work.
The need for ventilation
The primary reason for ventilation is to provide an indoor air quality that contributes towards a safe and effective internal environment by controlling airborne contaminants – both internally and externally derived – as well as temperature and humidity, and by promoting air movement. In general, in commercial buildings it is likely the ventilation will predominantly have to influence the elements listed in Table 1.
The World Health Organisation (WHO) has extensive coverage and information on internal and external pollutants, downloadable from www.who.int/phe/health_topics/outdoorair/en/
Even before ventilation designs are considered and developed, the sources of contaminant should be considered carefully to see if the emission rates might be effectively reduced – or negated – through alternative building materials or application, or by the use of localised extract systems.
Basic modelling of indoor contaminant concentration
When contaminants are being produced in a room, a relatively simple decay equation can provide a basic model to establish the average levels across the space.
CPt = ((CPO+ [106 x P/Q]) x (1-e-nt)) + (CPi x e-nt)
where CP is the concentration of the contaminant (ppm), with suffixes t = after time t, i = initial (at time = 0), O = outdoor air.
P is the rate of release of contaminant (for example, in litres per second), Q is the outdoor air supply rate (in the same units as P – for example, litres per second), t is the time (for example, number of hours), and n is the room air change rate per unit time (for example, room volume in litres/Q x 3,600).
This may be used with any contaminant that is not significantly absorbed by the materials in the room to provide a snapshot of the levels at any time. Using a spreadsheet, this can be developed to consider intermittent – and varying – use. This method is employed by the IAQ section of the spreadsheet that accompanies CIBSE AM10 (available at www.cibse.org/knowledge/design-tool-for-iaq-analysis).
Figure 1: Simplified example of top floor office
So, for example, using the above relationship for the office building shown in Figure 1, the CO2 levels can be modelled for different scenarios to provide the levels as shown in Figure 2. This has assumed a production of 20 l.h-1 CO2 per person. The result indicates that the outdoor air rates of 8.5 and 10 l·s-1 per person can provide an indoor CO2 level of around 1,000 ppm, and illustrates that, as ventilation rates increase beyond this, there are diminishing benefits of CO2 reduction.
Figure 2: Average room CO2 levels at various fresh air supply rates for example office with 20 people
The reality of practical ventilation
Pragmatically, the design ‘ventilation rate’ is often selected so that it meets the requirements of the appropriate local legislation – providing more air will increase fan power and distribution costs. In England and Wales, approved document Part F1 that, for example, stipulates a general (whole building) minimum ventilation rate of 10 l·s-1 per person for offices. That outdoor air supply rate is based on controlling body odours, plus low levels of other pollutants. If there are significant sources of other pollutants, Part F suggests that the rate should be determined in line with the recommendations of CIBSE Guide A.2 This provides recommended rates (Guide A, Table 1.5) for various applications and, in turn, also refers to the wide-ranging set of recommendations in CIBSE Guide B (section 2.3).
The value of 10 l·s-1 per person is broadly the value that is recommended by a number of standards, whether directly – (as in Part F) or through a combination of a fresh air requirement per person, plus an allowance based on the floor area and ventilation effectiveness. For example, applying ASHRAE Standard 62.1 20133 provides a basic combined office fresh air supply rate of 8.5 l·s-1 per person. As shown in the example above, this is the rate that will typically keep internal CO2 levels at around 1,000 ppm.
So no need to look further?
With a reasonable consensus across global standards, is there any reason the outdoor ventilation rate should be higher than 8.5 l·s-1 to 10 l·s-1? CO2 levels have often been used as a proxy to indirectly assess the quality of air in an occupied space, with 1,000 ppm being the traditionally-accepted limit. However, research is increasingly linking such CO2 levels with less than ideal occupant performance. Satish4 reported that in experimental studies at 1,000 ppm CO2, compared with 600 ppm, performance was significantly diminished on two-thirds of the decision-making tasks tested. And at 2,500 ppm CO2, compared with 600 ppm, performance was significantly reduced in more than 75% of the decision-making tasks – with some decreasing to levels associated with dysfunctional performance.
There has been much recent research into the effectiveness of increased fresh air ventilation rates. For example, in 2011 William Fisk5 undertook a comprehensive review of the benefits of altering ventilation rates in US offices. He determined that increasing the rates to significantly above 10 l·s-1 per person would be economically beneficial, while also providing enhanced internal environmental quality. By considering previous studies of absenteeism, occupant performance and energy costs in offices – and allowing for additional capital costs that, in many cases, were likely to be small – he was able to show that by increasing the ventilation rate from the commonly-prevailing 8 l·s-1 per person to 15 l·s-1 per person, the US economy could save US$37.5bn dollars (£24bn) per annum.
Savings of a similar magnitude were predicted if basic ventilation rates were unchanged but heat recovery devices (for example, plate heat exchangers) were added to the systems and the fresh air rate allowed to rise when the heat recovery device enabled it to do so – without increasing heating/cooling costs.
Very recent laboratory studies reported by Maddalena6 – using students in a test cell reminiscent of Fanger’s original lab work that underpins the current international standards for comfort – indicate a statistically significant relationship between the improving decision- making performance of building occupants and increasing (per person) outdoor ventilation rates. Interestingly, the benefit goes beyond what the test subjects were able to record in terms of their perception of air quality (that is, predominantly odours). Although, at the increasingly higher outdoor ventilation rates the occupants could not sense any improvement in air quality, their decision-making powers were improved. Further studies are considering this is in greater detail.
While increasing outdoor airflow rates, the amount of particulate matter externally present – and other outdoor contaminants – will also increase. This may require some careful consideration of the filtration requirements. For example, recent work by Conson7 showed that when outdoor air has extreme particulate concentrations of 200 μg·m-3 (this compares with average daily World Health Organisation (WHO)8 maximum average daily recommendation of 25 μg·m-3), higher-efficiency filters (around 80%) are needed, which will increase the fan- operating energy. Another option would be to accept an increase of internal CO2 by reducing fresh air.
Importantly, the movement of the air itself will affect the quality of the internal environmental space. Pasut9 has recently published a paper that examined the opportunities to improve occupant comfort by increasing the velocity of the air across the body (not necessarily fresh air). Current CIBSE Guide A (Section 1.3) guidance is that, generally, air velocities greater than about 0.3 m·s-1 are unacceptable. However, this research indicated that a velocity between 0.8 and 0.9 m·s-1 has a positive – and statistically significant – effect on users’ thermal comfort and thermal sensation when cooling was needed.
And, in a similar way to the increased fresh air proportions discussed earlier, air quality acceptability was improved by the air movement – even if the amount of air movement is not enough to alter noticeably an individual’s thermal comfort and thermal sensation. In the experiments, ceiling-mounted ‘propellor’ fans were used to determine the effect of the higher velocities, finding that an air velocity of 0.9 m·s-1 directed on a subject’s face did not cause any dry-eye discomfort. So, in times of high outdoor pollution, could there be opportunity to compensate – in comfort terms – for lower fresh air proportions with increased local air velocities?
Delivering the mechanical ventilation
The fresh air supply system (often referred to as ‘DOAS’ in the US) ideally should be able to respond to the varying ventilation demands of the occupied space, to ensure economical operation.
One of the advantages of decoupling the ventilation from being the principal heating/cooling device is that the flowrate may be altered to satisfy the required fresh air rate at all times. This could be modulated by CO2 or mixed-gas sensors, or step-changed with the input of movement or counting sensors, or simply with time clock control; typically, the supply and extract fan speeds are varied by inverter controllers. If a room has a specific contaminant emission, the concentration of this can be used as the control input. To give an idea of how future guidance may move, a proposed addendum10 to the ASHRAE Standard 62.1 is suggesting the outdoor ventilation rate may be reduced (controlled by occupancy sensors) to approximately 20%11, based on a fresh air supply rate determined from the unoccupied floor area, compared with the normal fully-occupied rate.
Considering the office in Figure 1, the winter steady state heat loss may be evaluated readily using the fabric and infiltration heat flow coefficients.12 At an internal temperature of 20°C, the heat loss at an outdoor temperature of -4°C would be (171 W·K-1 + 16 W·K-1) x (20°C – -4°C) = 4,488 W. During periods of occupancy, the heat losses will be offset by the occupant, casual and potential solar gains. In this example, it would not be unreasonable to expect equipment gains of 75 W per person, 60 W per person metabolic sensible heat gain, and lighting gain of 9 W·m-2 and, assuming overcast skies, very little solar gain. For the whole fully-occupied office, this would be 20x(75W+60W)+(192m2 x9W·m-2)= 2,700 + 17,28 = 4,428 W internal gains.
This indicates that, when occupied, the internal space is unlikely to require significant heating, and for most of the year, it will require cooling, even when the outside temperature is below freezing.
However, this does not mean that un- tempered outdoor air should be supplied directly into the space – if it is more than 10 K cooler than the room13, it is likely to create downdraughts. The ventilation system should be able to heat the air to a temperature that prevents downdraughts. A heat recovery device (HRD) with an integral bypass should be able to satisfy the majority of the fresh air heating requirement.
A unit such as that shown in Figure 3 employs a plate heat exchanger for the HRD that allows both sensible and latent heat transfer – this can be particularly useful in winter, when incoming air has a low moisture content. (In some cases, there may be a need for additional humidification.)
Figure 3: A compact ventilation unit with HRD (including bypass), DX cooling, heating and humidification
In mid-season, such a unit can provide ‘free cooling’, by bypassing the HRD and supplying air up to the maximum available flowrate that does not cause excessive noise or air movement. When the outdoor temperature rises above a point where useful cooling is being supplied by the ventilation air, the flowrate would be controlled back to the minimum required to meet IAQ needs, the HRD bypass would close, and the air – cooled and dehumidified as needed – supplied to the room.
Ventilation is essential to wellbeing but, without careful consideration, the functional purpose of the ventilation and the need to properly control the system might be forgotten and lead to excessive energy costs and potentially severe impacts on occupant comfort and productivity.
© Tim Dwyer, 2014.
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Table 1: A sample of the airborne contaminants that will affect the IAQ