Which filter class for supply air is required in a typical HVAC system?

Transcription

1 Which filter class for supply air is required in a typical HVAC system? Thomas Carlsson, Technical Director, Magnus Johnsson, Vokes Air Table of Contents: Background 2 Energy Demand 2 Air Quality and PM Measurements in Europe 3 Energy Demand caused by Filter Pressure Drop 3 Air Quality Improvement with Different Filter Classes 4 Calculation of Energy Demand for Different Filter Classes 6 Conclusion 7

2 Background Air Quality In 1555, the Danish bishop Olaus Magnus said that the farmers should not thresh hay against the wind because the dust is so fine that one cannot notice its inhalation and accumulation in the back of the throat. The effect of particles on human health had been recognised as early as the sixteenth century. The present levels of airborne particles are causing severe damage to human health in Europe. The World Health Organisation (WHO) has estimated that 100,000 premature deaths in Europe every year can be attributed to exposure to particulate matter (PM) [1]. Furthermore, it is believed that the incidence and severity of respiratory diseases such as asthma and bronchitis are aggravated by exposure to PM. Levels of PM must be reduced if we are to reach the objective of avoiding significant negative impact on human health. The distribution of costs on the different categories depends on which filter class is used, fan efficiency and service time. The dominating part is the energy cost (for a filter with filter class F7 the energy cost is 80 % of the total cost [1] ). Using a filter with lower pressure drop (more filter bags) will reduce the cost for energy use, but the filter cost will increase. To get a cost-effective installation, the ventilation system has to be optimised regarding the number of pockets on the filter. Energy Demand The Energy Performance of Buildings Directive (EPBD) [2] came into force at the beginning of 2003, and the EU member states were obliged to incorporate the directive into national law by The EPBD will be an important tool in helping the EU members to fulfil their commitments according to the Kyoto protocol. A major part of EU energy demand originates from the building sector. The EU has estimated that it will be able to reduce its total energy demand by 11% by targeting measures in the building sector. The directive allows member states considerable freedom in implementation, but one area that will receive a great deal of attention are measures to reduce the power demand in HVAC installations. Fans in office buildings are one of the largest consumers of power in buildings (15-20% in Sweden) [3]. A very cost efficient way to reduce the power demand of this type of application is to use filters that have the correct design and dimensions in an air conditioning system. The EPBD also states that the requirements on the energy demand of a building must be strongly correlated to the requirements on the indoor climate. This means that we must consider both the energy demand of the filter and the minimum filter class required in order to maintain a good indoor air climate. The correct design of filter class for supply air ensures that the installation will fulfil the threshold values for PM concentration in the indoor air. Such a design will also ensure that the requirement for low energy demand imposed by the EPBD will be fulfilled. 2

3 Air Quality and PM Measurements in Europe Air quality measurements are becoming more common in Europe. Many European municipalities have continuous measurements of sulphur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), particulate matter 10 (PM 10 ), particulate matter 2.5 (PM 2.5 ), lead (Pb), carbon monoxide (CO), benzene (C 6 H 6 ) and ozone (O 3 ). Twenty-six EU member states submit their results from air quality measurements to the EIONET network [5]. The measurement stations are located both in city centres and in rural areas, and the purpose of the measurements is to check whether the air quality requirements defined by the European Commission are achieved. Results from air quality measurements can be found on the internet [5]. The following threshold values have been laid down for particles [6] : Table 1: Threshold values for PM, according to EU directive 1999/30 [6] Pollutant Concentration Averaging period Legal nature Percentile PM µg/m 3 24 hours PM µg/m 3 24 hours Threshold value enters into force Threshold value enters into force The threshold values from the EU are intended to be applied to outdoor air. There are, however, no recommendations for indoor air quality, and thus these threshold values can be considered to be maximum values for indoor air [7]. Energy Demand caused by Filter Pressure Drop The pressure drop across a full size filter (592*592 mm) at a nominal air flow rate of 1 m 3 /s (which is a typical air flow for a full size filter) can lie between 60 and 350 Pa. The value depends on the filter class, the filter design and how much dust the filter contains. The energy demand that the pressure drop creates can be calculated by the following equation: Q = Δp x q v x h 1000 x η where: Q = energy demand (kwh) Δp = average pressure drop across the filter during the service time (Pa) h = service time (hours) q v = flow rate (m 3 /s) η = total fan efficiency (%) 3

4 Example: [4] Air flow rate = 1 m 3 /s Average pressure drop = 200 Pa (typical for a filter of class F7/MERV 13) Service time = 8760 h Total fan efficiency = 50% Q = 1 x 200 x x kWh/year The annual energy demand for the filtration of a 1 m3/s air flow per year is thus 3500 kwh when a filter of filter class F7/MERV 13 is used. The annual energy demand for a filter of filter class F5/MERV 10 is approximately 1100 kwh/year. This value is lower since the pressure drop is lower. The energy bill for air filtration is huge. There is a great potential for energy saving by decreasing pressure drops! Air Quality Improvement with Different Filter Classes The first question facing a designer of filter classes is the quality of air desired. The air quality of supply air must comply with the European Directive 1999/30 [6], namely 50 µg/m 3, 98 percentile values for PM 10 (by 2010). The concentration of PM 10 in the air entering a building (supply air) can be found using data from existing measurement stations in Europe. The filter data (particle removal efficiency) can be found from a test report of the filter [9]. The distribution for the particle sizes can be estimated using a Junge distribution. This information enables us to calculate the PM 10 value of supply air for different filter classes. PM 10 concentrations in Stockholm in 2006 were 98 percentile, 109 µg/m 3. Calculating the PM 10 removal for different filter classes gives the following results. 109 µg/m³ F5=71 µg/m³ F6=48 µg/m³ F7=22 µg/m³ PM10 Gauge PM10 Gauge Figure 2. The particle removal efficiency differs for different filters in the same filter class. The calculation gives results for a typical filter from each filter class. 4

5 F5/MERV10 F6/MERV12 F7/MERV13 Filter Class Filter class F6/MERV 12 is sufficient to meet the required threshold value of 50 µg/m 3. The calculation gives the particle concentration in the duct downstream of the filter. Previous work [8], however, has shown that the air quality in the building will not be the same as in the duct. Not all supply air to a building will pass the supply air filter. Figure 3: Not all supply air will pass the filter before entering the building. It has been shown that 20-30% of the supply air does not pass the filter (in the case in which the air exchange rate is one exchange per hour)the infiltration is caused by extra exhaust air which is needed to create an under pressure in the building. Moreover, wind and the temperature differences also contribute to the infiltration. Exfiltration can occur in special cases. Infiltration and exfiltration must be included in the calculation of indoor air quality. This gives the following values for Stockholm. 109 µg/m³ F5=81 µg/m³ F6=67 µg/m³ F7=48 µg/m³ Air Leakage Figure 4: The particle removal efficiency differs for different filters in the same filter class. The calculation gives results for a typical filter from each filter class. Including infiltration and exfiltration leads us to conclude that a filter of filter class F7/MERV 13 is needed to meet the requirement for indoor air quality. The F6/MERV 12 is not sufficient. 5

6 Calculation of Energy Demand for Different Filter Classes The pressure drop across a filter will depend on the filter design and the filter class. The equation that gives the energy demand for a filter is Q = Δp x q v x h 1000 x η If we take the following typical values: Air flow rate = 1 m 3 /s Service time = 8760 h Fan efficiency = 50% and use the following values of average pressure drop for filters of different classes: F5/MERV 10=65 Pa, F6/MERV 12=100 Pa, F7/MERV 13=200 Pa, we can predict the energy demands: 3500 Energy Consumption For Different Filter Classes at a air flow rate of 1m 3 /s F5/MERV10 F6/MERV12 F7/MERV13 Filter Class 6

7 Conclusions This article has described a design tool for determining the filter class required to achieve a given air quality with the lowest possible energy demand. The EU s EPBD [2] has set a target for electricity demand reduction in buildings of 11%. Statistics of filter sales and energy demand for fans in Sweden allow us to calculate that it is possible to save 2-3% of the electricity demand in office buildings by using the correct design of the filter class. This saving can be achieved simply by changing the filter; there is no need to rebuild the system. For calculation of heating and cooling systems climate data is used, today it is possible to use particle data for designing filter classes. References [1] World Health Report 2002, WHO [2] Energy Performance of Building Directives 2002/91/EC [3] Förbättrad energistatistik för lokaler Stegvis STIL Rapport för år 1, Energimyndigheten, 2006 (in Swedish) [4] Recommendation Concerning Calculating of Life Cycle Cost for Air Filters, Eurovent [5] 6] EU Directive 1999/30/EC [7] The Swedish Building Code, 2006:12 [8] Healthy Buildings 1997, System Effects on Filtration Efficiency, Thomas Carlsson [9] EN779:EN 2002, Particulate Filters for General Ventilation Determination of the Filter Performance 7