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By
Rob Moult
Director, Southeast Asia
Johnson Controls
Rob has been visiting India for more than a decade and making presentations on energy management topics. Part of his current responsibilities include input to worldwide BAS product development.
Previous articles addressed the design issues with a VAV system, the control of the VAV AHU and control of the VAV box. This article the last in the series, addresses an issue of increasing concern in India - Indoor Air Quality Control.
Variable air volume systems have become the air conditioning system of choice for office environments throughout Southeast Asia because they are energy efficient and allow zone control of temperature. To date, little attention has been paid to the issue of indoor air quality control with VAV AHUs. The issue of indoor air quality is gaining increasing attention in Southeast Asia and Singapore has already issued indoor air quality guidelines.
ASHRAE has done considerable research into indoor air quality and published a standard (ASHRAE Standard 62 - 1989, Ventilation for Acceptable Indoor Air Quality). One of the key; parameters contained in the ASHRAE standard is the amount of fresh air required per person for various types of building environments. For an office environment, the minimum amount of fresh air is specified air is specified to be 20 CFM per person.
Designs of air conditioning systems in Southeast Asia generally follow ASHRAE standards. In Southeast Asia, it is not common to modulate the outdoor air dampers. If a designer wishes to meet the ASHRAE Standard 62 with a VAV AHU using fixed fresh air dampers, it will be necessary to size the fresh air inlet to allow the required CFM of fresh air per person at minimum fan speed. When the fan is running above the minimum speed the amount of fresh air being introduced will be more than the specified requirement. Cooling this additional fresh air wastes energy.
This article shows a cost-effective way to measure the amount of fresh air being introduced and a way to modulate fresh air dampers to ensure that the required amount of fresh air is being introduced, even at part loads on the VAV AHU. In others words, the approach described in this paper will ensure compliance with the ASHRAE Standard 62 ventilation rates while using the minimum possible energy.
The key to this approach is the calculation of the percentage of fresh air in the supply air stream (%FA). The %FA can then be multiplied by the measured airflow through a VAV box to calculate the amount of fresh air being delivered to a zone. The amount of fresh air being delivered by the AHU can be calculated by totaling the airflow from all of the VAV boxes and then multiplying this total by %FA. In North America, an air flow monitoring station is often installed in the supply air to read the total airflow for the AHU directly, but this is not done in Southeast Asia.
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The supply air is a mixture of fresh air and return air. If we assume that
all of the airstreams have a measurable property, X, then the percentage of
fresh air in the supply air is as follows:
Where:
XRA = Measured value for return airstream
XSA = Measured value for supply airstream
XFA = Measured value for fresh airstream
The most obvious and least costly airstream measurement is temperature. Using temperatures to calculate the percentages of fresh air is called the "energy balance method". In this case a mixed air sensor would have to be used rather than a supply air sensor. Unfortunately, the energy balance method does not give accurate results for the following reasons:
• Stratification of airstreams makes it difficult to get an accurate temperature reading. This is particularly true for the mixed air where an averaging sensor must be used. This means that the measurement error for each value can be significant
• Placement of the mixed air sensor (not provided with AHUs in Southeast Asia) is critical. The sensor must be mounted such that the reading is not affected by the radiant cooling from the coil. Radiant cooling from the cooling coil introduces error into the mixed air temperature sensor reading.
• In a tropical climate such as Southeast Asia, the temperature difference between the return airstream and the mixed airstream is quite small. Similarly the temperature difference between the return airstream and the fresh return airstream is also quite small.
To illustrate the problem considers nominal values for the various temperatures with reasonable measurement errors:
XRA = Return air temperature = 28ºC ± 0.2ºC
XSA = Mixed air temperature = 28ºC ± 1ºC
XFA = Fresh air temperature = 32ºC ± 0.2ºC
Using the nominal values, we calculate %FA as follows using Equation 1:
Consider the numerator of the equation (XRA - XFA). It has a nominal value of 0.8ºC, but the error in this value1, ± 1.02ºC, is even greater than the nominal value. With the error in the numerator greater than the nominal value, it is obvious that temperature readings cannot provide an accurate result for the calculated percentage of fresh air in the supply air.
Using CO2 to calculate the percentages of fresh air has a number of advantages:
• Because CO2 is a gas and mixes completely with the airstream, the location of sensors is not critical and there is no issue of stratification
• The CO2 sensor in the supply air does not haves to be mounted in the limited space before the cooling coil; the CO2 sensor can be mounted in the supply air duct after the coil and fan. The coil and fan do not affect the CO2 reading.
Using CO2 to measures the percentages of fresh air is called the "mass balance method". CO2 is a naturally occurring gas and is generated by people in the space as they breathe. Fresh air has a CO2 concentration of 350 ppm to 420 ppm. A typical indoor office environment will have a CO2 concentration of 500 ppm to 800 ppm, and an office with poor ventilation may have a CO2 concentration of up to 1200 ppm.
On first inspection, it appears as though the mass balance method will also not be able to provides accurate results. This is because even good quality commercial grade CO2 sensors have an accuracy of ± 100 ppm. Substituting nominal CO2 values into equation 1 shows:
Where:
XRA = Return air CO2 concentration = 700 ppm ± 100 ppm
XSA = Supply air CO2 concentration = 640 ppm ± 100 ppm
XFA = Fresh air CO2 concentration = 400 ppm ± 100 ppm.
As with the energy balance method the mass balance method has an error in the numerator (± 141 ppm) that is greater than the nominal value (60 ppm)
1 The error is calculated as [(1)2 + (0.2)2]1/2
With the energy balance method, much of the measurement error comes from the location of the sensors. With the mass balance method, the sensor location is not critical as the error comes from the CO2 sensing technology.
Two common CO2 sensing technologies are "non-dispersive infrared" and "photoacoustic". Non-dispersive infrared technology is preferred over photoacoustic for two reasons:
• Photoacoustic sensors have increased error at low humidities (i.e. less than 25% RH). This is not an issue for Southeast Asia
• Photoacoustic sensor generate a relatively "noisy" output signal
Both non-dispersive infrared and photacoustic CO2 sensor technologies are subject to drift (± 100 ppm / year ) and inaccuracy (± 100 ppm). Both types of sensing technologies require annual calibration to maintain accuracy.
Field experience has shown that the signal from a CO2 sensor should be smoothed using a first order differential filter algorithm before being used as part of control algorithms. Filtering uses the following formula to eliminate spikes:
(Equation 2)
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Figure 1 shows how a single sensor can be used to collect air samples from multiple locations. The two solenoid air valves are controlled to select which of the airstreams (fresh air, return air or supply air) is drawn across a single CO2 sensor. Figure 2 shows how he controller activates the solenoid air valves and when the CO2 reading is sampled. Test results have shown a sampling time of five seconds on a one-minute interval to be effective. The limitations of CO2 sensing technology in calculating the percentage of fresh air can be overcome using a multipoint approach. The calculation of the percentage of fresh air depends on the difference between CO2 readings rather than the raw CO2 reading itself. By using a single sensor to measure all CO2 readings, the inherent inaccuracy of the sensor is "cancelled" when the difference reading is taken. As mentioned above, the error in the numerator of Equation 1 when using two independent CO2 level is about 141 ppm. This is an unacceptable degree of error when the nominal value is 60 ppm. Experiments have shown that the multipoint sensing approach has an error in the numerator of Equation 1 of less than ± 5 ppm, which is quite acceptable if the nominal value is 60 ppm.
It is important to note that the reading of any one CO2 value (Fresh Air CO2, Return Air CO2 or Supply Air CO2 ) will still have an error of ± 100 ppm when using the multipoint approach. The advantage of the multipoint approach is that the error in differences in CO2 levels between airstreams become small. For example, if the CO2 sensor is reading high by 50 ppm because of the limitations of the sensing technology, the error between two readings taken with the same sensor is not affected.
The error of ± 5 ppm is quite acceptable when the nominal value is 60 ppm, but becomes excessive when nominal value for the numerator of Equation 1 drops below 30 ppm. The numerator of Equation 1 will reduce when the CO2 level in the Return Air is only a little bit higher than the CO2 level in the Supply Air. This would only occur during very low occupancy in the space. Indoor air quality is typically not a problem under these conditions and can best be assured by establishing a minimum fresh air damper position.
We have not discussed the denominator of Equation 1 because the error in the denominator will be the same as in the numerator but the nominal value will always he higher than the numerator.
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So far, this article has described the use of CO2 as part of mass balance approach to calculate the percentages of fresh air in the supply air stream (%FA). When using the mass balance approach, CO2 is used as a "tracer gas" and it is the differences in CO2 level between airstreams that is important rather than the absolute value of the CO2 level.
In ASHRAE Standard 62 - 1989, the absolute value of CO2 level is an important parameter for demand controlled ventilation and for ventilation alarms. An important benefit of the multipoint CO2 sensing approach is that the same sensor can be used for ventilation measurement and control (using mass balance approach), demand controlled ventilation (using Return Air CO2 value) and ventilation alarms (using Return Air CO2 value).
Outdoor air CO2 levels range upwards from 350 ppm and are produced by natural processes such as combustion CO2 in the indoor air is generated by human respiration. Other sources of CO2 such as cigarette smoke are typically negligible in an office environment. The CO2 generation rate is a function of diet and levels of activity. A typical person generates 0.0106 CFM of CO2. People emit a large variety of bioeffluents from sweat, respiration, etc. These bioeffluents are a major factor influencing Indoor Air Quality, but their concentration cannot be cost effectively measured. The indoor air CO2 concentration is the best indicator of occupancy and the corresponding bioeffluent level. Many experiments have been performed to establish the relationship between bodily emissions, CO2 level and occupant comfort and these experiments show that a 20% dissatisfaction criteria corresponds to a CO2 level of 1000 ppm. In other words, when the CO2 level is above 1000 ppm, 20% of the people will find the air quality unacceptable; this is not because of the CO2 (CO2 levels below 5000 ppm are do not affect people), but rather because of the level of bioeffluents and other pollutants.
The following equation can be used to determine the amount of fresh air per person to maintain the CO2 level below 1000 ppm when the outdoor air CO2 level is 350 ppm:
Where:
Vo = Fresh air flow rate per person
N = CO2 generation rate per person = 0.0106 CFM
CSP = Space CO2 concentration = 1000 ppm
CFA = Fresh air CO2 concentration = 350 ppm
In this example, the CO2 concentration in occupied space will not exceed 1000 ppm as long as 16.3 CFM per person of outdoor air (with CO2 concentration of 350 ppm) is continuously being added to the space to dilute the CO2 generated by people's respiration. More importantly, the outdoor air dilutes the other air pollutants so that an acceptable indoor air quality is maintained. This calculation illustrates the approach taken by the ASHRAE Standard 62 - 1989 in determining the outdoor air flow requirements for ventilation. The ASHRAE Standard requires fresh air flow rates of 20 CFM (i.e. higher than 16.3 CFM) for the following reasons:
• If the fresh air CO2 level is higher than 350 ppm, more fresh air is required (if a fresh air CO2 concentration of 420 ppm is put into Equation 3, the CFM increases to 18.3)
• The return air CO2 represents an average level and individual zones may be higher
• There may be other air pollutants in the space in addition to those generated by people
• 1000 ppm is the high limit and CO2 concentrations should be kept below this value.
As shown in Figure 3, the output signal to the fresh air damper is the maximum of three values:
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• A control signal to achieve a return air CO2 level of 800 ppm. This is called a demand strategy (DCVCS).
• A control signal to achieve the required fresh air CFM based on occupancy level. This is called occupancy based ventilation control strategy (OBVCS).
• A minimum damper position.
Because there is some interdependence between the three inputs, the "Select Maximum" function should add a time delay before switching. In other words, an input must be the maximum of the three for a duration of time before the input takes control of the Fresh Air Damper.
In addition to the control strategy shown in Figure 3, alarm limits should be placed on the Return Air CO2 level. A Return Air CO2 level in excess of 1000 ppm may indicate failure with the fresh air damper actuator or out-of-control conditions. A Return Air CO2 level of less than 300 ppm indicates a faulty CO2 sensor.
The ASHRAE Standard requires an occupancy based ventilation control strategy (OBVCS). However, a demand controlled ventilation control strategy (DCVCS) can be used to supplement the OBVCS to save energy.
A typical office has fixed hours with minimal occupancy outside of those hours. The air conditioning system is operated for an extended period before and after typical occupancy to accommodate workers who come in early or leave late. For example, an office may be in operation from 09.00-17.00 while the air conditioning system operates from 08.00 - 18.30. Introducing extra fresh air ventilation between 08.00 - 09.00 and 17.00 - 18.30 will waste energy. During this period, the DCVCS can be used to provides the minimal level of fresh air ventilation based on the actual requirements at that time. If DCVCS is not used, the OBVCS must assume an occupancy level. Since this assumption must be based on worst-case conditions, energy will be wasted.
The DCVCS can provide additional energy savings during the day. If the DCVCS is not used, the OBVCS must assume a worst case occupancy level. If the DCVCS is used, the OBVCS can assume a typical occupancy level and rely on the DCVCS to provide and rely on the DCVCS to provide extra ventilation during periods of extraordinary occupancy.
Implementing the DCVCS does not require any additional hardware if an OBVCS is implemented. The actual return air CO2 level can be read from the multipoint CO2 sensor. If a DCVCS is used, the maintenance and calibration of the multipoint CO2 sensor becomes an important issue. The DCVCS also requires the fixing of a high level setpoint for CO2 level. Because the Return Air CO2 represents an average of zone conditions, it is recommended that a setpoint of 800 ppm be used to ensure that the CO2 level in any one zone does not exceed 1000 ppm. If there are critical zones for which an additional CO2 sensor is cost-justified, a room mounted sensor can be used and the DCVCS can be controlled based on the maximum of Return Air CO2 and the room mounted CO2 level.
The ASHRAE Standard 62 - 1989 requires an occupancy based ventilation control strategy (OBVCS) as a "Ventilation Measurement and Control" procedure. The dotted lines in Figure 1 shows the extent of the OBVCS. The OBVCS compares a desired Fresh Air CFM (setpoint) with the actual Fresh Air CFM (controlled variable) and calculates an adjustment tot the fresh air damper. The Desired Fresh Air CFM is calculated by multiplying the CFM / person Requirement by the occupancy level. The CFM / person Requirement is typically a constant such as 20, based on the usage of the space (20 is the value prescribed by ASHRAE for office areas).The occupancy level can be manually entered or automatically changed based on a time schedule.
The Actual Fresh Air CFM is calculated by multiplying the %FA by the Supply Air CFM. The %FA is calculated using inputs from the Multipoint CO2 sensor using Equation 1. The Supply Air CFM can be calculated by summing the individual CFM readings from each of the VAV boxes. In North America, an air flow monitoring station is often installed to read the Supply Air CFM for the AHU directly, but this is not done in Southeast Asia.
Figure 4 shows how an OBVCS can be applied for specific zones. The "Select Maximum" function that decides the command to be sent to the Fresh Air Damper has more inputs. The Actual Fresh Air CFM for a specific zone is calculated by multiplying the actual CFM for the VAV box times %FA. The Desired Fresh Air CFM for a specific zone calculated by multiplying the CFM / Person Requirement (i.e. 20) times the occupancy level. As with the more generals OBVCS, the occupancy level may be fixed by a manual entry or a time schedule. An OBVCS on a zone basis ensures that critical zones have sufficient ventilation but adds to the number of variables (occupancy level for each zone) that must be managed
Much of the material on this paper was extracted from a Project Report prepared
by George J. Janu of Johnson Controls, Milwaukee Wisconsin Johnson Controls
has patented the multipoint CO2 mass balance method
of calculating the percentage of fresh air in the supply air stream.
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