Safety

With the passage of time, the filtration medium used in air filters may tend to erode under the influence of airflow and some concern has been expressed11 about this. Glass fibre, rock wool and slag wool, used in filters, shed minute particles into the airstream and thence into the conditioned space, where they may be inhaled, with a potential risk to health.

Fibrous filter material is usually coated during manufacture to reduce significantly the risk of erosion during the life of the filter, and it does not necessarily follow that such filters are dangerous to use in air conditioning systems. However, it is noted in CIBSE (1992a) that the Health and Safety Executive gives a maximum exposure limit of 5 mg m-3 for inhalable micro-glass particles with a limiting fibre count of 2 fibres per minute. The electrostatic charge, naturally present in most filters, also discourages erosion and this is sometimes deliberately increased to enhance the effect. Apparently, according to CIBSE (1992b), it is possible to combine this electrostatic effect with the use of non-respirable fibres.

Fire tests on filters made of glass fibres and of synthetic, non-glass fibres have been carried out and these indicate that filters using glass fibre give a higher risk. It is to be noted that not only will the glass fibre burn but, if the igniting flame is sufficiently large, the dirt itself on the filter constitutes an increased fire hazard.

Notation

Symbol

Description

Unit

A

Arrestance

%

A

Arrestance for an initial increment of injected

Synthetic dust

%

^2

Arrestance for a second increment of injected

Synthetic dust

%

Af

Arrestance for a final increment of injected

Synthetic dust

%

Am

Average arrestance

%

E

Atmospheric dust-spot efficiency

%

Јa

Atmospheric dust-spot efficiency before the initial

Injection of synthetic dust

%

EB

Atmospheric dust-spot efficiency before the second

Injection of synthetic dust

%

Em

Average atmospheric dust-spot efficiency

%

En

Atmospheric dust-spot efficiency before the nth injection

Of synthetic dust

%

Ox

Opacity of a dust-spot on an upstream target

02

Opacity of a dust-spot on a downstream target

Цl

Volume of air sampled upstream

M3

02

Volume of air sampled downstream

M3

7di

Initial light transmission through a downstream target

Td 2

Final light transmission through a downstream target

Tu 1

Initial light transmission through an upstream target

Tu2

Final light transmission through an upstream target

W

Total weight of dust injected

Kg

Wa

Weight of synthetic dust collected by an after-filter

Kg

Wf

Final incremental weight of synthetic dust injected

Kg

Wn

Nth incremental weight of synthetic dust injected

Kg

Wab

Initial incremental weight of synthetic dust injected

Kg

WBC

Second incremental weight of synthetic dust injected

Kg

W(n — 1 )n

(n — l)th incremental weight of synthetic dust injected

Kg

W,

First incremental weight of synthetic dust injected, or

Weight of synthetic dust injected

Kg

W2

Second incremental weight of synthetic dust injected

Kg

[1] (a) Calculate the enthalpy of moist air at 28°C dry-bulb, a vapour pressure of 1.926 kPa and a barometric pressure of 101.325 kPa, using the ideal gas laws as necessary.

(b) Explain how the value would alter if the barometric pressure only were reduced.

[2] The cylinder walls are warmer than the gas leaving the evaporator and hence the gas in the cylinder expands and resists the entering flow through the suction port.

(ii) The density of the gas within the cylinder is less than that of the gas about to enter because of the pressure drop accompanying the flow through the suction valve. Hence the refrigerant used also has an effect on the volumetric efficiency.

The actual refrigeration capacity is defined as:

2ra =T1 (9.24)

Other efficiencies, that influence the power input needed for the process of compression, are:

Isentropic adiabatic efficiency

This is defined as the ratio of the work done for isentropic adiabatic compression of the gas, to the work input to the crankshaft.

Compression efficiency

This refers only to what happens within the cylinder and is the ratio of work done for isentropic compression to the measured work done for the actual compression of the gas.

Mechanical efficiency

This is the ratio of the measured work done for compression of the gas to the measured rate of work input to the crankshaft.

The product of the above three efficiencies, as fractions, would be divided into the ideal power required for compression (as related to the actual refrigeration duty obtained from equation (9.24)), to determine the actual power input to the crankshaft.

EXAMPLE 9.8

Using the results of example 9.6 as necessary, calculate the stroke and bore of an eight cylinder compressor running at 1425 rpm. Assume the dimensions of the stroke and bore are equal and obtain a suitable volumetric efficiency from Table 9.5.

[3] Accuracy. This is expressed as a percentage of the full scale of the instrument, e. g. if a maximum error is 4° for a thermometer having a scale from 0°C to 100°C its accuracy is 4 per cent. Where an instrument includes several components (e. g. a detector and a transmitter) their individual errors combine to give a compound error, expressed by the root of the sum of the squares of the component errors. Thus two components with individual errors of 4° and 1° would give a compound error of Vl7 or 4.12°. Accuracy is the ability of an instrument to indicate the true value of the measured variable.

(ii) Dead band. This is the range of values of the measured variable to which the instrument does not respond.

(iii) Deviation. This is the instantaneous difference between the value of the measured variable and the set point.

(iv) Drift. A gradual change between the set point and successive measurements of the controlled variable, not related to the load.

(v) Hysteresis. This is when the change of response to an increasing signal from a measuring element, with respect to time, is different from the response to a decreasing signal, in a repeating pattern.

(vi) Repeatability (precision). This is the change of the deviation about a mean value. It is an indication of reliability and is the ability of the instrument to reproduce successive measurements in agreement with one another. It is not the same as accuracy.

(vii) Sensitivity. Instruments do not respond instantaneously and the largest variation in measured variable that occurs before an instrument starts to respond is an indication of its sensitivity.

(viii) Stability. This is the independence of the measured property from changes in other properties. Drift is absent.

Assuming a square law relating pressure drop and flow rate in the duct system to which the fan is coupled, determine the quantity of air handled and the fan total pressure if the

Answer

Posted in Air Conditioning Engineering


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