GB2281614A

GB2281614A - Air-conditioning a working enclosure

Info

Publication number: GB2281614A
Application number: GB9417067A
Authority: GB
Inventors: Anthony TAYLOR
Original assignee: Thermac
Current assignee: Thermac
Priority date: 1993-08-27
Filing date: 1994-08-24
Publication date: 1995-03-08
Also published as: GB9417067D0; GB9317845D0

Abstract

To air-condition a sealed work enclosure, or sealed tent 1 surrounding work stripping asbestos from pipe 2, suction unit 4 maintains a negative tent pressure relative to ambient, while blower 10 blows air, possibly cooled, into the tent. When sensor pipe 5, linked to a manometer (6, not shown) detects that the negative tent pressure has fallen to a predetermined threshold, e.g. 5 Pa, it cuts the power supply to blower 10. An isolating valve downstream of blower 10 is also closed. Air may be blown into the tent over individual work positions. <IMAGE>

Description

A METHOD AND APPARATUS FOR AIR-CONDITIONING A WORKING ENCLOSURE The present invention relates to a method and apparatus for air-conditioning a working enclosure such as the containment tents used to enclose workers operating with hazardous particulate substances.

When asbestos stripping operations are carried out it is a requirement that the work is carried out in a sealed enclosure, such as a polythene tent, wherein a negative pressure difference relative to the outside environment is maintained. This ensures that if a problem arises and the seal of the tent is breached, air is sucked inwards rather than blown outwards, thus containing asbestos dust within the tent. Similar considerations also apply when handling other particulate hazardous substances such as fly-ash.

It is common in asbestos stripping operations that they are carried out whilst the plant being worked on remains operational. This means that the temperature within the working enclosure can be very high, which in addition to significantly increasing the risk of the workers in the enclosure suffering heat stress also causes a serious problem in maintaining the integrity of the enclosure as the heat softens the polythene tent and can weaken joints and seams. Problems owing to heat can also arise when a working enclosure is subject to solar gain. Thus hot working conditions of this type are not only particularly hazardous for the individual worker but increase the cost of the whole operation as workers can only work within the enclosure for short periods and require lengthy rest periods in between.

However, it will be appreciated that the control of the environment within such a working enclosure to minimise the effects of heat stress on the worker is difficult as the negative pressure difference within the enclosure must at all times be maintained. Conventional measures adopted to do this include "air management" of the air extracted from the enclosure to maintain the negative pressure difference, and of the make-up air; the use of freestanding fans within the enclosure to provide local air movement at the working position; and the control of sources of humidity, for example steam leaks, within the enclosure as increased humidity increases heat stress. None of these measures is entirely satisfactory and whilst making the worker more comfortable do not necessarily actively reduce the temperature within the enclosure.

The object of the present invention is to obviate or substantially mitigate the aforementioned problem of heat stress by positive control of the air within such a working enclosure.

According to a first aspect of the present invention there is provided a method of air-conditioning a sealed working enclosure comprising the steps of removing air from within the enclosure to maintain a negative pressure difference therein relative to the outside environment, blowing air into the enclosure, monitoring the negative pressure difference within the enclosure relative to the outside environment, and immediately preventing air from being blown into the enclosure should the negative pressure difference within the chamber fall to a predetermined threshold relative to the outside environment.

Preferably, the predetermined threshold comprises a negative pressure difference of 0.5 mm water gauge.

Preferably also, the air blown into the enclosure is cooler than the ambient air within the enclosure.

Preferably also, the air blown into the enclosure is directed over individual working positions within the enclosure.

Preferably also, the air blown into the enclosure is directed at the work face.

According to a second aspect of the present invention there is provided apparatus for air-conditioning a sealed working enclosure comprising a means for removing air from within the enclosure to maintain a negative pressure difference therein relative to the outside environment and a sensor for monitoring the level of negative pressure difference maintained within the enclosure, and characterised in that a means for blowing air into the enclosure is provided which is linked to the sensor, and in that a means is provided for immediately preventing air from the air-blowing means entering the enclosure should the sensor detect that the negative pressure difference within the chamber has fallen to a predetermined threshold relative to the outside environment.

Preferably, said means for blowing air into the enclosure comprises a fan connected to an air inlet into the enclosure via an isolating valve, a power supply for the fan being operationally linked to the isolating valve whereby on cessation of the power supply to the fan the isolating valve operates immediately to obturate the air inlet and prevent any residual movement of the fan from blowing air into the enclosure.

Preferably also, the isolating valve comprises a spring-loaded valve member which is opened by said power supply to the fan against the force of the spring-loading.

Preferably also, the force of the spring loading is such that movement of the valve member from a completely open position to the closed position takes no more than 1.5 seconds.

Preferably also, said valve member comprises a plurality of louvres. Alternatively said valve member comprises a single flap.

Preferably also, the air drawn into the air blowing means by the fan passes over a cooling coil of a refrigeration arrangement.

Preferably also, said means for removing air from the enclosure is set to operate at a rate at least 50% higher than the rate at which air is being blown into the enclosure.

Preferably also, two independent means for removing air from the enclosure are provided, the first such means being adapted to provide at least a minimum level of negative pressure difference within the enclosure and the second such means being adapted to remove from the enclosure at least the same volume of air being blown therein by the air blowing means at at least the same rate.

It will thus be appreciated that the present invention provides a way of actively introducing conditioned air, typically cooled air, into the working enclosure. Until the present invention, no thought had been given to positively blowing air into such enclosures in view of the requirement to maintain a negative pressure difference therein.

However, such an air flow can greatly improve working conditions within the enclosure and thereby improve the safety of the workers as well as assisting in the retention of the integrity of the enclosure itself.

The present invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is a diagram of a working enclosure provided with apparatus for air-conditioning same according to the invention; and Fig. 2 shows schematically the arrangement of a means for blowing air into the enclosure.

With reference to Fig. 1, a sealed working enclosure of the type used, for example, in an operation to strip asbestos from a hot pipe comprises a sealed tent 1, typically made of polythene sheeting, arranged surrounding the length of pipe 2 being worked on. An air-lock (not shown) is provided and has several separate levels of entry chambers, in conventional fashion, to permit entry and exit of the workers 3 and allow them to disrobe and don overalls and breathing equipment which until thoroughly decontaminated do not leave the confines of the tent 1 and air-lock chambers.

A negative pressure environment is maintained inside the tent 1 by means of a unit 4 which withdraws air from inside the tent 1 until the air pressure inside the tent is lower than a predetermined threshold below that of the air pressure outside the tent 1. The air pressure inside the tent 1 is measured via a sensor pipe 5 which is linked to a manometer arrangement 6 (Fig. 2), as will be described.

The negative pressure unit 4 comprises an air outlet pipe 7 through which air is sucked from the tent 1 via a high performance fan 8 covered by a filter (not shown). The fan 8 is located inside the tent 1 and downstream thereof, outside the tent 1, the air passing down the outlet pipe 7 is passed through a high efficiency particulate filter (HEPA filter) located within a housing 9 so that the air is thoroughly decontaminated before being blown into the atmosphere.

It is not possible to specify in detail performance standards for the negative pressure unit 4 as these depend to a large degree on the site conditions for each job.

However, preferably a negative pressure difference of at least 0.5 mm water gauge (5 Pa) is maintained within the sealed tent 1.

Conventionally, make-up air to compensate for a proportion of that removed from the tent by the unit 4 is permitted to enter the tents via inlets located through the air lock arrangement. However, in the present invention whilst such air inlets may still be present, the air pressure within the tent 1 is made up by air which is positively blown into the tent 1 from an air blowing unit 10 via an inlet hose 11.

Referring now particularly to Fig. 2, the air blowing unit 10 comprises a fan 12 connected to the air inlet hose 11 via an isolating valve 13. A power supply (not shown) for the fan 12, which would typically be an electric motor, is operationally linked to the isolating valve 13 so that on cessation of power to the fan 12 the isolating valve 13 operates immediately to obturate the air inlet 14 to the hose 11.

Preferably, the isolating valve 13 comprises a springloaded valve member (not shown) which is opened and maintained open against the force of the spring-loading by the power supply to the fan 12. Thus, when the power supply ceases, the valve member begins closing immediately under the force of the spring-loading. The valve member itself can be of conventional construction, for example a plurality of louvres or a single flap. Preferably, the force of the spring loading is such that movement of the valve member from a completely open position to the closed position takes no more than 1.5 seconds.

Upstream of the fan 12 is a cooling coil 15 of a refrigeration arrangement 16 so that air drawn from the atmosphere by the fan 12 is cooled by a predetermined amount according to the setting of the arrangement 16 before being blown into the tent 1.

The manometer arrangement 6 comprising a manometer and its associated controls is attached to the unit 10 and is operationally linked to the power supply for the fan 12. In this way, if the negative pressure within the tent 1 falls to a predetermined threshold below that of the outside environment, which suggests either a failure of the negative pressure unit 4 or a breach in the seal of the tent 1, the power supply to the fan 12 can be cut off. In these circumstances, as it is important to maintain at least the threshold negative pressure difference within the tent 1, it is critical that any residual movement of the fan 12 does not continue to blow air into the tent 1. Hence the operation of the isolating valve 13 to obturate the air inlet 14.

As previously indicated, the threshold level of negative pressure difference to be maintained within the tent is preferably 0.5 mm water gauge (5 Pa).

As the power supply to the refrigeration arrangement 16 is independent of that of the fan 12 and the arrangement 16 can be left continuously running whether the fan 12 is either on or off, it is convenient for the manometer arrangement 6, fan 12, and isolating valve 13 to be constructed and linked together within an independent housing 17, which can be coupled to the refrigeration arrangement 16 for operation or decoupled and, for example, replaced for servicing.

The air inlet hose 11 preferably ends in a manifold 18 so that air can be blown into several different regions of the tent 1. Preferably, these areas correspond to the various working position within the tent 1 so that each worker 3 is constantly bathed in a downward stream of cooled air. In this way, heat stress can be significantly reduced. In addition7 when hazardous particulate substances are present within the tent 1 the air flow over each worker will tend to blow such particles away from him and the intake filter of his breathing apparatus. The same effect can also be achieved by blowing air directly at the work face so that in, for example, asbestos stripping operations, particles dislodged from the work face are blown away from the worker.

Apart from the cooling of the air by the coil, the cooling process also has a drying effect. This is also advantageous as the relative humidity within the tent 1 is thereby lowered thus additionally reducing the risk of heat stress.

In order to cope with the level of air being blown into the tent 1 by the air blowing unit 10, two or more negative pressure units 4 can be employed. If two are used, one can be set to provide at least a minimum level of negative pressure within the tent 1 and the second set to remove at least the same volume of air being blown therein and at at least the same rate. If necessary, the operation of both negative pressure units 4 can be linked.

Preferably, the air blowing unit 10 will blow into the tent 1 of the order of 500 cubic feet per minute (14.16 cubic metres per minute) per worker 3. Hence, in order to maintain the negative pressure difference at an acceptable level and to compensate for filter blockage, the negative pressure units will be set to remove air from the tent 1 at a rate at least 50% higher and preferably at of the order of 900 cubic feet per minute (25.49 cubic metres per minute).

Heat stress is widely assessed using the Wet Bulb Globe Temperature index (WBGT), as described in ISO standard 7243:1989. WBGTs below 22"C are unlikely to present a serious risk of heat stress. However, above this threshold an increasing heat stress burden has to be borne by the worker. Once, WBGTs in the range 26"C to 35"C are reached then each worker must follow a prescribed work/rest regime and at the higher levels be medically monitored.

Above WBGT 35"C each worker must wear specially cooled protective clothing. The cooling provided by the present invention can significantly reduce the WBGT within a working enclosure thus making each worker more productive and reducing his risk of heat stroke. The level of cooling can also be predetermined as required by appropriate setting of the refrigeration unit 16 and the volume and rate of air passing over the cooling coil 15.

Hence, the method and apparatus of the present invention provides a positive means whereby the atmosphere within a working enclosure can be managed and the risk of heat stress reduced. It will be appreciated that in addition, the cooling effect of the air being blown into the enclosure also reduces the risk of the joints and seals of the tent 1 from failing.

Claims

1. A method of air-conditioning a sealed working enclosure comprising the steps of removing air from within the enclosure to maintain a negative pressure difference therein relative to the outside environment, blowing air into the enclosure, monitoring the negative pressure difference within the enclosure, and immediately preventing air from being blown into the enclosure should the negative pressure difference within the chamber fall to a predetermined threshold relative to the outside environment.

2. A method as claimed in Claim 1, wherein the predetermined threshold comprises a negative pressure difference of 0.5 mm water gauge.

3. A method as claimed in Claim 1 or Claim 2, wherein the air blown into the enclosure is cooler than the ambient air within the enclosure.

4. A method as claimed in any one of Claims 1 to 3, wherein the air blown into the enclosure is directed over individual working positions within the enclosure.

5. A method as claimed in any one of Claims 1 to 4, wherein the air blown into the enclosure is directed at the work face.

6. Apparatus for air-conditioning a sealed working enclosure comprising a means for removing air from within the enclosure to maintain a negative pressure difference therein relative to the outside environment and a sensor for monitoring the level of negative pressure difference maintained within the enclosure, and characterised in that a means for blowing air into the enclosure is provided which is linked to the sensor, and in that a means is provided for immediately preventing air from the airblowing means entering the enclosure should the sensor detect that the negative pressure difference within the chamber has fallen to a predetermined threshold relative to the outside environment.

7. Apparatus as claimed in Claim 6, wherein said means for blowing air into the enclosure comprises a fan connected to an air inlet into the enclosure via an isolating valve, a power supply for the fan being operationally linked to the isolating valve whereby on cessation of the power supply to the fan the isolating valve operates immediately to obturate the air inlet and prevent any residual movement of the fan from blowing air into the enclosure.

8. Apparatus as claimed in Claim 7, wherein the isolating valve comprises a spring-loaded valve member which is opened by said power supply to the fan against the force of the spring-loading.

9. Apparatus as claimed in Claim 8, wherein the force of the spring loading is such that movement of the valve member from a completely open position to the closed position takes no more than 1.5 seconds.

10. Apparatus as claimed in Claim 8 or Claim 9, wherein said valve member comprises a plurality of louvres.

11. Apparatus as claimed in Claim 8 or Claim 9, wherein said valve member comprises a single flap.

12. Apparatus as claimed in any one of claims 7 to 11, wherein the air drawn into the air blowing means by the fan passes over a cooling coil of a refrigeration arrangement.

12. Apparatus as claimed in any one of claims 6 to 12, wherein said means for removing air from the enclosure is set to operate at a rate at least 50% higher than the rate at which air is being blown into the enclosure.

14. Apparatus as claimed any one of claims 6 to 13, wherein said means for blowing air into the enclosure can operate to deliver air into the enclosure substantially at a rate of 14.16 cubic metres per minute per worker.

15. Apparatus as claimed in any one of claims 6 to 14, wherein said means for removing air from within the enclosure can operate to remove air from the enclosure substantially at a rate of 25.49 cubic metres per minute per worker.

16. Apparatus as claimed in any one of claims 6 to 15, wherein two independent means for removing air from the enclosure are provided, the first such means being adapted to provide at least a minimum negative pressure difference within the enclosure and the second such means being adapted to remove from the enclosure at least the same volume of air being blown therein by the air blowing means at at least the same rate.

17. A method of air-conditioning a sealed working enclosure substantially as described herein with reference to the accompanying drawings.

18. Apparatus for air-conditioning a sealed working enclosure substantially as described herein with reference to the accompanying drawings.