Encyclopedia of Environmental Science and Engineering, Volume I and II

INDUSTRIAL HYGIENE ENGINEERING 523

whenever possible, some extracted air is recirculated and

only that amount is discarded which is sufficient to balance

the amount of fresh air brought in. The ratio of fresh air to be

recirculated can be fixed the same all the year round or vari-

able to take advantage of outside conditions. Programmed

outside thermostats controlling motorized dampers achieve

this. Some partial air conditioning systems use 100% recir-

culation and rely on natural infiltration of air to provide the

necessary refreshment.

Processes The air conditioning processes for which

items of plant are required are as follows:

1) sensible heating, requiring a heater battery sup-

plied with either hot water, steam or refrigerant or

electrically powered,

2) humidification requiring either a water spray or a

controlled jet of steam,

3) cooling and dehumidification requiring a cooler

battery supplied with either chilled water or refrig-

erant, the process involving cooling the air to

below its dew point so that water condenses out.

Plant items are assembled in the correct combination to

suit the particular design problem. The systems divide them-

selves into three basic types:

—The air systems where the required supply conditions

are generated centrally, the air being ducted to the rooms. Hot

water is supplied from a central boiler plant and air cooling is

provided directly from the evaporator of a refrigeration plant or

by means of chilled water from a center refrigeration plant.

—The air and water systems in which only sufficient

air required for ventilation purposes is handled centrally

and ducted to the rooms, the remaining heat transfer being

achieved in room units containing finned coils supplied with

heated or chilled water over which some room air is continu-

ally being passed. Heated and chilled water is provided from

a central boiler and water chilling plant.

—The unitary of self-contained systems where the com-

plete air handling and heat transfer plant is sited in the room,

the necessary ventilation air being drawn from outside local

to the room usually via a hole in the wall or window.

There are many variations of these basic types of systems.

Possible Problems Faults in air conditioning systems

usually result in discomfort for the room occupants and give

rise to vociferous complaints, which take one or more of

the following forms: too hot, too cold, cold draughts, dry

throats, too humid, a feeling of stuffiness, unpleasant odours,

too noisy. Most of these can be measured or sensed by the

investigator. Faults can be divided into: inherent design

faults, installation deficiencies, maintenance deficiencies

and malfunction of components.

Inherent design faults include:

1) badly sited air inlets and outlets

2) undersized heat transfer components

3) omission of humidification or de-humidification

facilities

4) bad siting of thermostats

5) insufficient provision of fresh air supply.

Installation deficiencies include:

1) badly or wrongly connected ducts

2) air leakage

3) badly sited thermostats

4) control dampers and valves functioning incorrectly.

Maintenance deficiencies include:

1) blocked filters

2) no filters

3) dirty heat transfer surfaces and fan blades

4) control dampers and valves out of adjustment

5) thermostats and their associated controlling

mechanisms and circuits out of adjustment

6) heating or cooling plant in poor condition.

The breakdown or malfunction of any item of plant or

control systems will lead to the creation of an unsatisfactory

environment.

An air-conditioning system operates as an integrated

design, an alteration of any one item will unbalance the

others thus plant should not be tampered with by inexperi-

enced or unqualified personnel. In most cases the identifica-

tion of faults and the provision of remedial action should

be left to a qualified and experienced environmental/heating

and ventilation/air-conditioning engineer.

SOFTWARE CONSIDERATIONS

The range of options available for the control of the work-

ing environment is diverse and it is all too easy to make

a choice based upon what is the easiest to install and

implement, but the control must be capable of providing

protection over the operational life of the process. Thus

monitoring and maintenance must be built in to the opera-

tional side of the system. Continued effectiveness depends

upon the awareness of the workforce and their supervisors

and managers. Regular checks on the working environment

and the performance of the control measure must be made

and a procedure laid down and adhered to to record the

results of the checks and to implement remedial measures

if the system is failing in any way. This can be made easier

if the control system is seen as an integral part of the pro-

duction process warranting the same attention as a produc-

tion machine.

A planned education and training programme may be

required to introduce the operators and management to the

importance of the control measure and to explain how it

works and what could go wrong and what the consequences

are of a malfunction.

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