GB2394043A

GB2394043A - Air sampling system

Info

Publication number: GB2394043A
Application number: GB0223269A
Authority: GB
Inventors: Stephen Henry Ellwood; David Appleby
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-10-08
Filing date: 2002-10-08
Publication date: 2004-04-14
Anticipated expiration: 2022-10-08
Also published as: GB0223269D0; GB2394043B

Abstract

The system has a plurality of air sampling units 1 - 4 into which air is sampled via a sampling tube and then passed through a filter into a sensor (not shown). The air is drawn into each unit 1 - 4 by a fan or pump in a unit and the sampling units are connected to each other and to a control 10 by an electrical circuit comprising a two core cable 5- 9 which carries digital communications and power to operate the sensors and fans or pumps. The fans or pumps of the units are operated intermittently and at different times to minimise the current carried by the electrical circuit. Typically air is sampled from rooms 11, 12, 13 and the sensors are smoke or aerosol sensitive and used to activate a fire alarm. The fans/pumps may be activated by a signal from air velocity sensors disposed in the units 1 - 4.

Description

DISTRIBUTED AIR SAMPLING SYSTEM

This invention relates to an improvement in the method of air sampling for the detection of airborne smoke and other aerosols, and gases. Air sampling systems are well known in which air is sampled by being drawn through a tube or a system of tubes. It is then passed through a filter into the chamber of a sensor which can detect and measure smoke, other aerosols and gases. For smoke detection the sensors generally operate on the principle of optical scatter, and the application is the detection of fires or overheating which leads to the early stages of fires. The air is driven through the sampling systems by the relative pressure generated by an electrically powered fan or pump Air sampling systems generally comprise large centralised units which include the filter, sensor, fan or pump together with associated power supplies and control electronics. The tubing in such centralised systems is generally a long length, which can be used to monitor the air in a large area of a building or other such installation, through several apertures in the wall of the tubing. In some situations the tubing can be several hundred meters long, and a single system of tubing can be branched if required to terminate at several locations.

Such systems are used in a range of applications. One advantage is that the detection apparatus can be largely hidden from view without the need for visible smoke detectors on the ceiling of a room. A further advantage of such a sampling system over the more common passive sampling techniques is the fact that the air is filtered before it enters the sensor. This can enable it to operate, for example, in conditions where airborne dust is present and more conventional smoke detectors would generate false alarms.

There are a number of disadvantages with centralised air sampling systems particularly those installed with long lengths of tubing with several sampling apertures. The first disadvantage is that the precise location of a change in the parameter being detected cannot be sensed. A further disadvantage is that a failure of any of the sampling apertures, such as a blockage, may not be sensed which could result in a lack of detection in part of a building or other such installation. Further in the case of a fire sensor, the sensitivity has to be set very high in order to provide adequate detection as, for example, smoke entering one aperture may be diluted by clean air entering all of the other apertures.

It is generally regarded as good design practice that the return airflow from a sampling system should be resumed into the volume of air being sampled, otherwise pressure differences which may exist throughout a building or other such installation may prevent or reverse the required airflow though the tubing into the sensor. This is not always possible or convenient to carry out, particularly if the sampling apertures are in several separate volumes, or the centralised unit is remote from the volume being sampled.

It is known that in some installations that several of the units used for the centralised systems are interconnected by electrical cable to form a larger distributed sampling system.

However they are generally large and expensive units and each one will generally sample from more than one aperture and provide detection over a wide area of a building or other such installation. Further the cable used to interconnect these units is generally only used for communications, and the units are generally separately mains powered and each contain batteries to provide stand-by power in the event of a mains failure. If they are not separately mains powered more than too cores of cable are generally required to provide power in addition to communications. The use of more than two cores of cable means that the systems are expensive to install and would it would be difficult to provide protection

against all combinations of short and open circuit cable failures which is a requirement for certain applications such as fire detection and alarm. It is the main object of the present invention to provide for an improved distributed air sampling system. This system will include individual sampling units which are distributed on a system and which are interconnected by tvs core electrical cables. Such a distributed system will Overcome the disadvantages of a centralised system as each of the sampling units on the distributed system will have a sensing aperture with its own associated filter, sensor, fan and monitoring means, as well as the ability to transmit the location of an event such as a fire or a fault using the cable to transmit a signal to a control and indicating panel. It will also be more convenient to return air directly into the volume being sampled. Further the use of two core cable will reduce installation costs of a systems and it is practical to provide protection against all combinations of short and open circuit cable failure.

According to the present invention there is provided a distributed air sampling system for detection of airborne smoke and other aerosols and gases which comprises more than one individual sampling unit in each of which air is sampled by being drawn into a tube and then passed through a filter into a sensor or sensors, and wherein the air sampling is driven by the relative pressure created by a fan or pump, characterized in that a plurality of said sampling units are interconnected on an electrical circuit using two core cable which carries both two way digital communications and the current to operate the sensors and said fans or pumps and wherein said fans or pumps operate intermittently at different times such as to minimise the current carried on said circuit.

The invention will now be described by way of example with reference to figures 1 to 5 of the accompanying drawing. As shown schematically in figure 1, a number of individual sampling units 1, 2, 3, 4 form a distributed air sampling system. They are interconnected by lengths of two core cable 5, 6, 7 and they are connected to a control and indicating panel 10 by lengths of two core cable 8 and 9. The control and indicating panel 10 is mains powered and provides power along the lengths of two core cable 8, 5, 6, 7, 9 to the distributed air sampling units 1, 2, 3, 4. The control and indicating panel may also include a user interface, batteries to provide power in the event of mains failure and other control means as required. The lengths of two core cable 8, 5, 6, 7, 9 also carry communications signals between the sampling units 1, 2, 3, 4 and the control and indicating panel 10. The lengths of two core cable 8, 5, 6, 7, 9 may also carry communications signals between individual sampling units 1, 2, 3, 4. The sampling units 1, 2, 3, 4 sample air from rooms 11, 12, 13 of a building which comprises walls 14, 15, 16 ceilings 17, 18, 19 and floors 20, 21 and 22. Sampling units 1 and 2 both sample air from room 11. Sampling units 3 and 4 are the only sampling units in rooms 12 and 13. In the system described herein each individual detector may indicate the location of a measurement event or a fault by signalling it's unique address together with the relevant message along the said two core cable.

The electrical current drawn during the activation of the fan or pump is generally much greater than that drawn by the sampling units when the fan or pump is not being activated.

Shown in figure 2 are a set of five diagrams representing changes of current against time.

The current peaks 23 and 24 represent the current drawn during intermittent operation of the fan or pump in sampling unit 1. Current peaks 25, 26, 27, 28, 29 and 30 represent the current drawn from the fans or pumps in sampling units 2, 3 and 4 respectively. It is a feature of the invention that the activation of the fans or pumps within all of the sampling units on a system are controlled in such a manner that the peak current drawn by the compete air sampling system is as close as possible to the average current drawn by the

system. This can be achieved by arranging that individual sampling units are activated sequentially as shown in figure 2, and not all at the same time. The current peaks 31 and 32 represent the current drawn from the fans or pumps in sampling units 1, 2, 3 and 4 operated intermittently and in sequence. On a large system groups of sampling units may be activated at the same time such that different groups of sampling units are activated in sequence. The control of the activation of sampling units on a system may be carried out by commands transmitted from the control and indicating panel, or by using signals transmitted between individual sampling units. The control of the intermittent operation of the fans or pumps is an essential feature of the invention which makes it possible in practice to design a two wire distributed sampling system. This is because the capacity of a system to provide a high current supply to devices connected on the system cable is limited by the resistance of long lengths of cable around a building, and is further limited if the two core cable is designed to supply both power and digital communications.

Shown schematically in figure 3 are details of a first embodiment of one of the individual air sampling units. Air is drawn through apertures 33, 34, 35, 36 in the tube 37 into a volume 38 bounded by walls 39. The air is passed through a filter 40 into a sensor volume 41 bounded by walls 42. The air is driven by relative pressure created by a suitable fan or pump 43. The passage of air through the unit is measured by an air velocity sensor element 44, which is connected by electrical wires 45 to a circuit board 46. The circuit board 46 contains the electronic circuitry required to take readings from the air velocity sensor 44. The circuit board may also include connections 47 to the interconnecting two core cable as well as circuitry for the power and communications system. Within the sensor volume 41 there is a sensor or sensors for the measurement of smoke, other aerosols or gases. The circuit board 46 may also include circuitry for the sensor or sensors, or the circuitry for such sensor or sensors may be provided on separate circuit boards. The circuit board 46 may also include circuitry for driving an electrically powered fan or pump 43. According to a further feature of the invention the fan or pump 43 is periodically powered or activated for a duration of time which allows a complete change of air within the tube 37, the volume 38, the filter 40 and the sensor volume 41. A measurement is made from the air velocity sensor during each activation of the fan or pump to measure the air velocity which is multiplied by the duration of the fan activation to obtain a measure of the volume of air sampled. It will be understood that a calibration constant and a knowledge of the volume required to be sampled will be known by the unit, in the form of data held in the memory of a microcomputer within each unit or within the control and indicating panel. The measured volume flow of air is compared with the required volume flow of air in order to control the duration of the fan or pump activation for the time required to ensure the required volume flow, thereby making the power requirements of each activation optimal. By minimising the power requirements of the air sampling units it is made more practical to connect the air sampling units on a two wire power and communications system.

If the tube 37 fitted to the sampling unit has a small inside diameter then it will make a large contribution to the resistance of airflow through the sampling unit, but only a small contribution to the volume of airflow required. The resistance to airflow will tend to increase more rapidly than the proportional increase in volume required as the length of the tube is increased. In this case it is a further feature of the invention that the period of activation of the fan or pump will be adjusted to the minimum time required to sample air through different lengths of tube, as with longer tubes fitted the measured air velocity will be reduced because of the increase in resistance to the flow of air. This will tend to require

a longer activation of the fan or pump to sample a measured air volume equal to the air volume required for a complete change of air within the tube 37, the volume 38, the filter 40 and the sensor volume 41. In service the filter will become partially blocked with dust or other such contamination and this will also cause the resistance to airflow through the system to increase. It is a further feature of the invention that as this contamination will tend to reduce the measured air velocity within the sampling unit the duration of the fan or pump activation will be automatically increased to compensate for this in an optimal manner. Following each activation of the fan or pump measurements are taken from the sensor or sensors in order to carry out the detection required of the air sampling system. In this way the response time of each sampling unit to events or changes in properties of the air measured by the sensor or sensors is defined by the interval between the activations of the fan or pump as during each activation there is a complete change in the air within the sampling unit between the sampling apertures and the sensor volume The response time can be adjusted by changing the time period between the activations of the fan or pump. If the time period between the activations is decreased the response time will be decreased and the average current drawn by the sampling unit will be increased, but only to a level required to sample the air at the required rate. It is a further feature of the invention that an optimal trade-off can be made between response time of an air sampling unit and the operating power required. If required different individual sampling units on a distributed sampling system can be configured to have different response times as required for a particular application, and the current drawn by the total system will remain at the optimal minimum level.

In figure 3 the fan or pump 43 is shown as sited downstream of the sampling tube 37, the volume 38, the filter 40 and the sensor volume 41. With this preferred arrangement the volume of the fan is not included in the volume of air required to be changed during each activation of the fan or pump. This ensures that each activation of the fan or pump is required to change air only in the essential parts of the air sampling unit, thereby minimising the average power requirement for each individual sampling unit.

Shown schematically in figure 4 is a second embodiment of one of the individual air sampling units, which also shows more details of the mechanical arrangement. Air is sampled through the single aperture 48 in the tube 49 and passed into volume 38. The air is passed through a filter 40 into a sensor volume 41. The air is driven by relative pressure created by a suitable fan or pump 43. The passage of air through the unit is measured by an air velocity sensor element 44, which is connected by electrical wires 45 to a circuit board 46. The said components are contained within a sealed outer casing bounded by walls 50. The sampled air is resumed to the volume being monitored through aperture 51 in tube 52. The wall of tube 52 is arranged to surround the wall of tube 49. The tubes 49 and 52 pass though a surface 53 which could be a wall or ceiling of the volume to be monitored by the sampling unit. It may be desirable that a mesh or barns of some sort is included between the sensor volume 41 and aperture 51, in order to prevent the ingress of insects. As drawn the outer casing of the sampling unit is hidden from view and only the tubes 49 and 52 are visible within the volume to be monitored. This arrangement ensures that sampling will be efficient because the sampled air is resumed to the volume being monitored, and the unit will not be affected by pressure differentials that may exist within a building or other such installation. Further the monitoring of the air within the volume being

monitored is achieved with an installation which is largely hidden from sight which is a significant advantage if the appearance of the installation is an important requirement.

Shown schematically in figure 5 is a detail of a third embodiment of the sampling tube and the return tube. Air is sampled through aperture 54 in tube 55 which is part of an assembly bounded by the walls of the part 5B. The air is passed into volume S7 and then though filter 58 into passage 59 which ducts the air directly into the sensor volume 41 bounded by walls 42. The sensor volume 41 bounded by the walls 42 is contained within the sealed outer housing 50 which directs the return airflow through the aperture 60 in the part 56.

One advantage of this arrangement is that the sampling unit may be made smaller without the requirement for volume 38 as the filter is fitted between the sampling and return air ducts, and the sampled air will go more directly into the sensor volume. With this arrangement the volume of air required to be sampled in each activation of the fan or pump is less than that required for the previous embodiment in figure 4. This will also contribute to minimise the average current consumption of each individual sampling unit. A further advantage of this invention is that the part 56, the filter 58 and the sampling tube 55 could be designed to be an assembly that could be clipped in position and removed and replaced when maintenance is required. It will be seen that if the installation is fitted through a wall or ceiling 53 of a building the replacement of this module could be made to be much more convenient than having to remove part of the sealed outer case 50 of the sampling unit, which could be sited in a normally inaccessible part of a building or other such installation.

As shown the air velocity sensor 44 is fitted through a sealed aperture in the outer casing 50, which with this arrangement need not be removed or disturbed if the said assembly is being removed or replaced for maintenance. The said assembly is shown fitted within the tube 61 which is shown fitted though the wall or ceiling 53. The tube 61 may be so arranged to locate the said assembly. The tube 61 has a flange 62 at one end which lies on the surface of the wail or ceiling 53. such a flange would be desirable to cover any roughness in the hole drilled through the wall or ceiling 53 during the installation of the sampling unit. Further it would also serve to make the installation more aesthetically pleasing. The duration of the activation of the fan or pump 43 in each individual sampling unit is controlled by the measurement from the air velocity from the sensor 44, and a calculation of the air volume sampled. It will be understood that if there is a change in the resistance to airflow within the sampling unit the duration of the activation of the fan or pump will be automatically adjusted. For example if the aperture 54 in the sampling tube 55, or the filter 58 were to become partially blocked the duration of the activation of the fan or pump would be increased, because the resistance to airflow would have been increased and the measured air velocity would have been decreased. Also if the sampling tube 55 were to be significantly reduced in length the duration of the activation of the fan or pump would be decreased, because the resistance to airflow would have been decreased and the measured air velocity would have been increased. By setting limits on the proportional or absolute change in the duration of the activation of the fan or pump faults in the air sampling capability of an individual air sampling unit can be detected. Further, such a fault could indicate that a unit is still operational, but that maintenance is required to be carried out such as the replacement of the filter. The location of said fault can be signalled on the distributed air sampling system described herein as the address of the air sampling unit which has detected the fault. In the extreme case if there is a complete blockage to airflow within the air sampling unit, or a failure of the fan or pump, the measured air velocity will be zero or close to zero and a complete failure in the method of sampling can be indicated.

The measurement of air velocity as described heretofore is made during the intermittent operation of the fan or pump. According to a further feature of the invention readings of the air velocity may also be made during the period that the fan or pump is not activated.

The value of air velocity used for calculation of the duration of activation, or for fault monitoring, is calculated from the difference in the readings made during activation of the fan or pump and when it is not activated. This has the advantage of checking that the air velocity through the sensor is in the correct direction and is due to the activation of the fan or pump. This a useful check of the operation of each sampling unit, but is particularly important if the return airflow from a sampling unit is not into the same volume as the air being sampled, because pressure differentials which exist between different parts of a building for example due to the effects of external winds could prevent the correct airflow through the sensor when the fan or pump is activated. A further check on the direction of the airflow can be carried out by varying the power applied to the fan or pump. For example if the measured velocity increases when the power is increased the airflow is due to the activation of the fan or pump, and is in the correct direction. However if the measured velocity decreases when the power is increased the airflow is not due to the activation of the fan or pump, and is not in the correct direction. The monitoring of the correct direction of airflow through each sensing point is a feature of the distributed sampling system that is not possible with a centralised system.

If the sensor or sensors are sensitive to smoke in the air being sampled the distributed air sampling system will detect fires and can be used for fire detection for the purposes of sounding an alarm to alert and evacuate building occupants, activation of a fire suppression system or automatically sending a signal to fire fighting services. The distributed air sampling system could also be used for the detection of fires if the sensor or sensors are sensitive to other products of combustion such as carbon monoxide. Air sampling units which include both smoke and sensors sensitive to other products of combustion would be more sensitive to fires, and less prone to false alarms, than units which contain only one sensor sensitive to one of the products of combustion. One form of air velocity sensor that is common uses an element which is heated by the passage of a continuous or pulsed electrical current. Changes in measured temperature of the element are used to calculate air velocity. Because cooling is increased with increased air velocity the temperature rise due the heating will be reduced. Such a sensor can be made sensitive to changes in the ambient temperature as well as the air velocity, and this can be used to detect an increase in air temperature. It is a further feature of the invention that the detection of an increase in air temperature by the air velocity sensor 44 can be used in conjunction with a sensor or sensors sensitive to products of combustion such as smoke or carbon monoxide in the sensor volume 41 of an air sampling unit to detect fires with increased sensitivity and reduced false alarms. The detection of an increase in temperature could be used to decrease the alarm threshold of the other sensor or sensors below that which would normally be the case. In the arrangement shown in figure 2 the air velocity sensor 44 is effectively upstream of the filter 40, and will be more sensitive to changes in air temperature than alternative arrangements with the air velocity sensor downstream of the filter because of the cooling effect of the filter as the air passes through it. The air sampling units which are connected to form the distributed air sampling system could be constructed using plastic and metal components and electronic circuitry of suitable design, such as would be obvious to one skilled in the art of the design of sensing

and detection equipment. A preferred material for the sampling and return tubes is plastic manufactured by means of an injection moulding process. Suitable plastic material is polystyrene or ABS, and it is preferred that those parts which are exposed to view within the volume to be protected are moulded from white or lightly coloured plastic. A preferred material for the filter is a plastic or fibrous matting, and suitable material is widely available and is used for example to prevent dust entering through the cooling fans on personal computers. A suitable specification for the filter would be that 75% of particles above Sum

in diameter are trapped by the filter. A preferred method of sensing the air velocity is the use of a heated element as described herein. A suitable sensor element is a negative temperature coefficient thermistor, and such devices are widely available from a number of manufacturers. The method of design of the electronic circuitry for such an air velocity sensor is well known and is not described here in detail. Preferably the control of the duration of the activation is implemented with the aid of a microcomputer included in the electronics of each individual sampling unit. Preferably the microcomputer would also be used for other functions within the sampling unit such as controlling and monitoring the air velocity sensor, and also the sensor or sensors within the sensor volume. The microcomputer could also control the interface to the two wire cable which interlines the individual sampling units and provides power and communications. Suitable single chip microcomputers are available from a number of manufacturers.

A design of smoke sensor is not described here in detail, but such sensors are well known and an example of a suitable smoke sensor operating on the principle of optical scatter is provided in patent GB2273769B. Suitable other types of sensor such as those suitable for the measurement of the concentration of various gases such as carbon monoxide are widely available in a form suitable for incorporation in an individual air sampling unit.

Preferably the fan or pump is an electrically powered axial or centrifugal fan. Suitable fans are available from a number of manufacturers.

The provision of both power and digital communications along two core cable which interlinks detectors and a control and indicating panel in which each detector is given a unique address is well known.

It will be understood that the embodiments of the individual sampling units which are required to be interlinked to form a distributed air sampling system are given by way of example only. It is not precluded to use sampling tubes which are not circular in cross section. The principle of operation is not fundamentally dependent of the detailed design of the air sampling units or of the techniques used to provide power and communications on two core cable such as is used to interlink the sampling units. These could be realised in a variety of ways by anyone skilled in the art. It will be further realised that elements in addition to those described in the foregoing will be necessary to construct a practical distributed air sampling system, as would be known or obvious to one skilled in the art.

These would include the provision of user interfaces and features to aid the installation and maintenance of the system.

Claims

1 A distributed air sampling system for detection of airborne smoke and other aerosols and gases which comprises more than one individual sampling unit in each of which air is sampled by being drawn into a tube and then passed through a filter into a sensor or sensors, and wherein the air sampling is driven by the relative pressure created by a fan or pump, characterized in that a plurality of said sampling units are interconnected on an electrical circuit using two core cable which carries both two way digital communications and the current to operate the sensors and said fans or pumps, and wherein said fans or pumps operate intermittently at different times such as to minimise the current carried on said circuit.

2 A distributed air sampling system according to claim 1, wherein within each individual sampling unit the fan or pump is periodically activated for a time duration controlled by the signal from an air velocity sensor sited within the unit such that for each activation of the fan or pump a complete change of air is achieved in the sampling tube, in the filter and in the sensor or sensors following the previous activation, and following the activation of the fan or pump signals from the sensor or sensors are automatically examined in order to carry out the said detection.

3 A distributed air sampling system according to claim 2, wherein within each individual sampling unit the average power drawn is optimised by automatic adjustment of the time duration of the activation of the fan or pump andlor the power supplied to the fan or pump such as to compensate for different sizes of sampling tubes installed, and contamination or partial blocking of the tube and filter in service.

4 A distributed air sampling system according to any one of claims 1 to 3, wherein within each individual sampling unit an optimal trade-off between average power consumption and response time can be made by adjusting the period between activations of the fan or pump.

5 A distributed air sampling system according to any one of claims 1 to 4, wherein within each individual sampling unit the sampled air passes through the sensor or sensors before it passes through the fan or pump, so that the air within the fan or pump does not form part of the volume of air which is required to be sampled during each activation of the fan or pump thereby minimising the activation time.

6 A distributed air sampling system according to any one of claims 2 to 5, wherein within each individual sampling unit the sampled air is returned to the volume being sampled.

7 A distributed air sampling system according to claim 6, wherein within each individual sampling unit the said sampling tube has only one aperture at the end of the tube.

8 A distributed air sampling system according to claim 7 wherein the return airflow is carried in a second tube whose wall is arranged to surround the wall of the sampling tube.

9 A distributed air sampling system according to claim 8, wherein within each individual sampling unit the filter is a tubular part which is fitted within a module which includes the sampling tube, and which can be removed and replaced.

10 A distributed air sampling system according to any one of claims 2 to 9, wherein within each individual sampling unit the signal from the air velocity sensor can be used to detect a fault or blockage in an air sampling tube filter fan or pump, or the requirement for maintenance in said items.

11 A distributed air sampling system according to claims 2 to 10, wherein values of air velocity are taken both when the fan or pump is not activated and when it is activated, and wherein the power supplied to the fan or pump is varied such that it may be established that the airflow through the sampling unit is in the expected direction from the volume intended to be sampled.

12 A distributed air sampling system according to any one of claims 1 to 11, wherein the said sampling units are interconnected on a circuit using two core electrical cable, which provides both power and means of digital communications, and wherein the location of smoke and other aerosols and gases, or a fault in the sampling unit, can be indicated by the address of the sampling unit in which the smoke and other aerosols and gases is detected.

13 A distributed air sampling system according to any one of claims 1 to 12, wherein within each individual sampling unit the smoke sensor operates on the principle of the detection of optical scatter and the unit thereby acts as a smoke detector for the detection of fires.

14 A distributed air sampling system according to any one of claims 2 to 13, wherein within each individual sampling unit the said air velocity sensor is sited between the sampling aperture and the filter and comprises a heated temperature sensor which can also be used to detect changes in air temperature and act in combination with the said smoke sensor to improve the detection of fires.

15 An air sampling unit and an air sampling system according to any one of claims 1 to 14 substantially as described herein with reference to figure 1 to figure 5 of the accompanying drawing.