PNEUMATIC AND HYDRAULIC EQUIPMENT

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MODULE TITLE : APPLICATIONS OF PNEUMATICS AND

HYDRAULICS

TOPIC TITLE : PNEUMATIC AND HYDRAULIC EQUIPMENT

LESSON 3 : ANCILLARY EQUIPMENT

APH - 2 - 3

Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

Tees Valley, UK

TS1 3BA

+44 (0)1642 342740

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________________________________________________________________________________________

INTRODUCTION ________________________________________________________________________________________

In the previous lesson we dealt with types of compressor and their control. In

this lesson we continue with the compressor installation and the ancillary

equipment required to ensure that the compressed air produced is of an

acceptable quality.

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YOUR AIMS ________________________________________________________________________________________

On completion of this lesson you should be able to:

• explain the meaning of the term Free Air Delivered (FAD)

• estimate the amount of compressed air that a machine should deliver

for a given set of circumstances

• sketch a typical compressor plant installation in block diagram form

• calculate the amount of condensate delivered from a compressor for a

known set of circumstances

• state and describe three methods of compressed-air drying.

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________________________________________________________________________________________

FREE AIR DELIVERED (FAD) ________________________________________________________________________________________

Compressors and air-tools are rated in terms of the volume flow-rate of air at

standard atmospheric conditions taken in and delivered, rather than the actual

volume of compressed air delivered, which will depend on the delivery

pressure employed. This flow rating is known as Free Air Delivered (FAD)

and is usually expressed in units of m3 min–1 or l s–1.

Local atmospheric conditions, in terms of pressure, temperature and humidity,

will vary with weather, altitude, and geographic location: e.g. at high altitude

the air pressure and temperature will be lower than at sea level. Because

compressors are used in locations where these atmospheric variations occur,

and also to deliver air at differing pressures it is important that a reference

standard is used so that compressor capacities may be properly compared.

There are two standards commonly used to define free air conditions; these

are:

• S.T.P. (standard temperature and pressure) where the values used are:

Temperature: 0°C

Pressure: 101.3 kPa or 1.013 bar.

• N.T.P. (normal temperature and pressure) where the values used are:

Temperature: 15°C

Pressure: 101.3 kPa or 1.013 bar.

FIGURE 1 illustrates the difference between free air and compressed air; the

ratio which connects the two is approximately the compression ratio of the

machine.

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FIG. 1 Free Air and Compressed Air

With reference to FIGURE 1, we can calculate the amount of compressed air

produced if we know various values. These are:

• the pressure at the compressor inlet p1 • the volume taken in at the inlet V1 • the pressure at the compressor outlet p2.

If we presume isothermal compression for simplicity, then T1 = T2

therefore

We will now consider the effect of various values for p1 and p2.

p V p V

V p V

1 1 2 2

2 1 1

F.A.D.

Compressed air

Compressor

p1 V1 T1 p2 V2 T2

Atmospheric free air

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Example 1

The compressor in FIGURE 1 is rated at producing 5 m3 min–1 FAD (NTP).

Calculate the volume of compressed air produced per minute, if the

temperature at the outlet is at ambient conditions, the outlet pressure is 6 bar

absolute, and the inlet pressure is 1.013 bar abs.

Solution

Using

Therefore

Note that this is a simplification of FAD to illustrate the effect that atmospheric

pressure and system pressure have on output volume.

The compressor in FIGURE 1 has its usage changed and now produces air at a

pressure of 7.5 bar absolute. Calculate the volume of compressed air produced per

minute if all other conditions remain the same and comment on the effect of increasing

the working pressure of a system with respect to the volume of air delivered.

...................................................................................................................................................

________________________________________________________________________________________

p V p V

V p V

1 1 2 2

2 1 1

1 013 5 6

0 844

= m mi3

nn–1

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It can be seen that as system pressure increases the flow-rate of compressed air reduces.

Show by calculation the effect on compressed air flow-rate if the pressure at the inlet

were to drop to 0.85 bar absolute, all other values being the same as above. Comment

on your results.

...................................................................................................................................................

________________________________________________________________________________________

It can be seen that as atmospheric pressure reduces so too does the flowrate of compressed

air into the system.

p V p V

V p V

1 1 2 2

2 1 1

0 85 5 7 5

0 567

= m m3

. .

iin–1

p V p V

V p V

1 1 2 2

2 1 1

1 013 5 7 5

0 675

= m 3

. .

mmin–1

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HUMIDITY

Humidity is the degree of dampness of the air and it may be defined as "the

mass of water vapour present in a given unit volume of air".

Air is said to be saturated when it is holding the maximum amount of moisture

as vapour. The amount of moisture that a given volume of air can support is

dependent upon its temperature. As the temperature of air increases, it can

hold more moisture before saturation occurs.

FIGURE 2 shows a chart illustrating the amount of moisture saturated air can

support at 1.013 bar. To read the chart, select the temperature of the air; draw

a line vertically upwards to a point where it intersects the curve, then

horizontally along until the vertical axis is reached; then read off the water

content.

Using the chart (FIGURE 2) opposite find the amount of moisture present in saturated

air at the following temperatures:

(a) 10°C

(b) 20°C

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FIG. 2 Dew-point Chart (at 1.013 bar)

– 40 –30 –20 –10 0 10 20 30 40 50 60 70 80 90 100

233 253 273 293 313 333 353 373 K

°C

W a te

r co

n te

n t

Temperature

0.1

0.2

0.3

0.4 0.5

40 50

100

200

500

g m–3

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Considering 1 m3 of air at 20°C, from the chart we found it could support

16 grams of water vapour. If that 1 m3 of air is now cooled to 10°C it can only

support 9 grams, hence there are (16 – 9) = 7 grams of water condensed out as

liquid. This is one of the basic principles that we apply for the removal of

moisture from compressed air systems – cooling the air.

We will now consider the effect of compressing 1 m3 of saturated air from

1 bar abs to 2 bar abs at a constant temperature of 20°C.

From the gas laws we know that to compress air to twice its pressure we must

halve its volume: therefore we are left with 0.5 m3 of air at 20°C. From the

chart we know that air at 20°C can support 16 g m–3. Therefore 0.5 m3 can

only support 8 grams: the other 8 grams is given up as condensate. From this

we can see that compressing saturated air via a volume reduction results in

moisture being given up. FIGURE 3 illustrates this process.

FIG. 3 Moisture Generated by Compression

A B

1 m3 saturated air at 1 bar abs

supporting 16 grams

of moisture

0.5 m3 saturated air at 2 bar abs

supporting 8 grams of moisture

8 grams of moisture given

(a) 9 g m

(b) 16 g m

–3

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Therefore, we can see that the amount of water which can be supported by a

given volume of air, is dependent on its pressure.

FIGURE 4 shows the amount of moisture saturated air can hold for different

pressures. To read the chart, select the temperature of the air, draw a line

vertically upwards to a point where it intersects the line for the pressure being

considered, then horizontally to read off the water content.

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FIG. 4 Pressure Dew Point Chart

0 10 20 30 40 °C

W a te

r co

n te

n t

Temperature

g m–3

1.4 bar

2 bar

2.6 bar

5 bar

3.5 bar

7 bar

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Dew Point

If we take a sample of air, which possesses moisture but is not saturated, and

then cool it to a point where saturation occurs and liquid starts to form, the

temperature at which this occurs is known as the dew point.

Saturation Quantity

This is the maximum amount of vapour by mass which a sample of air can

support.

Absolute Humidity

This is the amount of water vapour by mass which a sample of air, which is not

fully saturated, is supporting.

Relative Humidity

This is the ratio of absolute humidity compared to the saturation quantity, i.e.

A sample of air, 1 m3 at 20°C, is supporting 10 grams of moisture. Calculate the

relative humidity of the sample.

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________________________________________________________________________________________

RH = absolute humidity saturation quantity

expressed as a percentage( )

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From the dew point chart the saturation quantity at 20°C is 16 grams and from the question

the absolute humidity is 10 grams. Therefore:

This means that the air is supporting only 62.5% of the moisture that it can

actually support before saturation occurs.

From this we can say that, if the air taken into a compressor is of a low relative

humidity, the problems associated with water in the system will be reduced.

RH = 10 16

RH =

× 100

62 5

. %

RH = absolute humidity saturation quantity

expressed as a percentage( )

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________________________________________________________________________________________

COMPRESSOR HOUSE INSTALLATION AND ANCILLARY EQUIPMENT ________________________________________________________________________________________

The correct siting of the compressor and its ancillary equipment is very important

to its operation. If the compressor is installed in an unsuitable position, this can

adversely affect the operational performance of the compressor, with regard to

quality and quantity of air delivered into the system, as well as exposing the

compressor to the potentially harmful effects of a dusty or damp environment.

FIGURE 5 shows the arrangement of a typical compressed air plant.

Wherever possible it should be sited to operate with the greatest efficiency:

this would normally suggest that it is sited somewhere central in the plant or

factory, so as to avoid large pressure drops over long pipe runs to the more

remote parts of the plant.

Ideally the compressor should be located in its own purpose-built building

whose flooring and walls should be sealed to reduce dust entering the

compressor intake, if the air is being drawn from inside the building. If the air

is being drawn from outside the compressor house, then the building should be

sited away from steam boilers, cooling towers or other high humidity

equipment. Where possible it should also be sited on the coolest side of the

building, and away from any plant which generates large amounts of dust.

This precaution will ensure that the air being drawn into the compressor is cool

and dry and reasonably dust-free.

Failure to heed these precautions will result in reduced compressor throughput,

increased moisture content and premature blocking of intake filters.

The compressor house should also be adequately sized and well ventilated, to

allow unrestricted access for servicing and maintenance, and also to allow

unrestricted entry of ambient air to flow around the compressor to assist in its

cooling.

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FIG. 5 Typical Compressed-air Plant

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S to

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W a ll

M o is

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P re

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A u to

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Tw o s

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d o u b le

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F o u n d a ti

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lo ck

B el

t d ri

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COMPRESSOR FOUNDATIONS

Foundation requirements for compressor plant will vary from machine to

machine depending upon its size, type and how it is being dr iven.

Reciprocating piston machines are notorious 'bouncers' and the bigger they are

the worse the problem seems to get. The foundation requirements for this type

of machine are often quite substantial and normally require isolation from the

structure of the building so that noise and vibration are not transmitted into the

fabric of the building or into other machines causing a detrimental effect on

their performance or reduction in their working life.

Rotary machines do not usually provide the same problems. Many machines,

including the prime mover and control equipment, come in a packaged form,

which only requires to be fastened to the floor, and isolated from generated or

transmitted vibration.

COMPRESSOR ANCILLARY EQUIPMENT

Although the compressor and its prime mover are the most important parts of a

compressed air production plant, other items of equipment are necessary to

allow the compressor to fulfil its function as efficiently and effectively as

possible.

The most commonly found ancillary units are listed below.

Air Intake Filter/Silencer

Atmospheric air, especially in industrial areas, is extremely dirty. To prevent

harmful matter from entering the internal parts of the machine, filters are fitted

to the suction or intake line. They can be either oil-bath or paper-type element

filters and, needless to say, will require to be cleaned and serviced at regular

intervals, as stipulated by the compressor supplier.

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An air silencer is sometimes necessary, to reduce the noise generated by the

rushing of air into the compressor: this can be fitted before or after the filter,

depending upon the silencing effect required. FIGURES 6 and 7 show

examples of wet and dry type filters. However, it should be noted that, for

compressors fitted with carbon rings etc., filters must always be of the dry

type.

FIG. 6 Oil Bath Filter FIG. 7 Intake Filter with Silencer

Care should be taken with the placing of air intake filters, to ensure that the air

drawn into the compressor is as cool as possible and free from any potentially

harmful fumes and vapours from factory or plant processes.

Vibration of walls and windows can also occur if the intake filter is sited in

contact with them or in close proximity to them, particularly if no attempt is

made to dampen transmitted vibrations.

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What would be the effect on the quality of air produced should the compressor intake

be sited incorrectly, i.e. in a warm and moist position?

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________________________________________________________________________________________

The air taken into the compressor would have a high relative humidity and excessive

amounts of moisture would be carried into the system.

Intercooler

An intercooler is a heat exchanger placed between each stage of compression

to cool the air and reduce its volume, prior to it being further compressed in

the next stage. This greatly improves compressor efficiency.

The cooler will also reduce the air's moisture-carrying capacity and cause

moisture to be condensed in the intercooler. If this moisture was carried over

into the next compression stage and condensed on the cylinder walls, it would

result in damage to the compressor's internal components. The moisture

collected in the intercooler must be removed at regular intervals, to prevent

build-up and moisture being carried over. This is usually achieved using some

form of automatic drain trap, which will sense the presence of water and

actuate to remove it from the intercooler.

Intercoolers may be air-blast or water-cooled, but must be regularly serviced

and cleaned to ensure that the air-cooling mechanism is working efficiently.

FIGURE 8 shows a compressor with an intercooler fitted.

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FIG. 8 Two-stage Compressor with Intercooler

Aftercooler

When a compressor operates, it takes in large volumes of atmospheric air,

which always contains some moisture in the form of water vapour. The

compression process reduces the volume of the air, but cannot reduce the

volume of water. For this reason, it must be noted that the compressed air

emerging from a compressor, at a temperature slightly higher than ambient

intake temperature, is always 100% saturated. This means that it is holding the

maximum amount of moisture that the sample of air can hold at that particular

temperature and pressure. Therefore any reduction in temperature or pressure

of the air will cause moisture to precipitate.

The following calculation, using the gas laws and the dew point graph shown

in FIGURE 2 and the pressure dew point graph in FIGURE 4, indicates the

amount of condensate generated during compression.

Stage 1 Stage 2

Cooling water out Cooling water in

Intercooler

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Example

A compressor is delivering 5 m3 min–1 FAD at 6 bar gauge pressure. The air is

being drawn into the compressor at an ambient temperature of 23°C. The

relative humidity of the air being drawn in is 68% and its pressure is

1.01 bar abs. Calculate the amount of condensate per hour that would drop out

at a point after the compressor, where the air has been cooled to a temperature

of 32°C.

Solution

During 1 hour at 5 m3 min–1 FAD:

Conditions at the inlet port of the compressor are:

Conditions at a point after the compressor are:

Using the gas laws:

p V

V p V T

T p

1 1

2 2

2 1 1 2

1 2

=∴

p T2 26 1 01 7 01 32 273 305= = bar abs., = = K+ +. .

p V T1 1 11 01 300 23 273 296= bar, = m = = K 3. , +

volume delivered = = m FAD35 60 300×

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Therefore, air volume after compression:

From the dew point graph (FIGURE 2):

1 m3 of air at 23°C can hold 20 g of moisture at 100% relative humidity (RH).

At 68% RH this mass would fall to:

Therefore, moisture taken in at the compressor inlet every hour

Referring to the dew point graph (FIGURE 4) for a pressure of 7 bar (approx.

7.01), 1 m3 of air at 32°C can hold 4 g of moisture at 100% RH.

Therefore the moisture suspended by the air after compression is:

4 = g× 44 53 178.

13.6 = g× 300 4080

20 68 100

13 6 ×

= g.

1 01 300 305 296 7 01

92 415 2074 96

44 5

. .

× × ×

33 m 3

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Considering that in the unit time:

Then the moisture the compressed air cannot hold in one hour is:

Since 1000 g water occupies 1 litre, the air will deposit 3.9 litres of water

every hour.

The aftercooler is the device used to cool the air immediately after

compression, to encourage the air to drop as much condensate as possible,

before it reaches the air receiver.

The most efficient aftercoolers are usually water-cooled: the best are those that

can produce the lowest air temperatures. However, as a general guide, the air

leaving the compressor installation, prior to being used, should be no more

than 15 degrees above the air intake temperature.

FIGURE 9 shows an illustration of a water-cooled aftercooler with an integral

moisture separator. Note particularly the safety valve, which protects the shell

of the heat exchanger from excessive pressure build-up, and also the automatic

drain trap which is used to periodically remove the precipitated moisture from

the bottom of the cooler.

4080 = g– 178 3902

condensate drop out = moisture taken

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FIG. 9 Water Cooled Aftercooler

In situations where cooling water is not available or is too expensive an option

to use, then the air-blast aftercooler becomes the only viable alternative.

In this device ambient air is blown by an electric motor and fan over a bank of

fins through which the compressed air flows. It does not usually provide the

same degree of cooling as a water-cooled heat exchanger. The compressed air

discharge temperature is likely to be about 6 degrees higher than for a

Safety valve

Incoming hot compressed air

Outgoing cooling water

Throttle valve

Water separator Automatic condensate

draining

Outgoing compressed air

Thermometer for outgoing compressed air

Incoming cooling water

Cooling element

Cooler body

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watercooled device. However, it will still precipitate moisture from the air,

and needs to be fitted with a valve to allow for the automatic removal of this

moisture from the unit.

In both types of aftercooler it is of prime importance that all heat exchanger

surfaces be kept as clean as possible for the unit to function at its maximum

efficiency.

FIGURE 10 shows an illustration of an air-blast aftercooler with an automatic

drain trap fitted.

FIG. 10 Air Blast Aftercooler

Strainer

CA air trap

Air

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MOISTURE SEPARATOR

After the air has been cooled in the aftercooler and water has been condensed,

it is necessary to separate the water droplets from the air supply, to prevent

them from being carried over into the system. Since the water droplets are a

lot heavier than the air, many separator types rely on velocity changes or

centrifugal force to bring about separation.

In one type of separator, the air is made to change its direction several times on

its way through a screen or an arrangement of baffles. The heavier droplets of

moisture impinge on the surface of the screen or baffle plate, and run down to

the base of the separator, where they are collected and removed by an

automatic drain trap.

Another type uses centrifugal force to remove the water droplets, by causing

the air to spin so that the drops are thrown to the sides of the outer casing,

where they fall to the bottom of the separator and are once again collected and

removed.

Sometimes, as in FIGURE 9, rather than use a separate device, the separator is

incorporated into the design of the aftercooler. In this type, a baffle and pipe

arrangement, in the separator portion of the equipment, causes the air that

enters to change both its direction and velocity. This results in the moisture

droplets being separated, allowing them to be collected and automatically

drained from the bottom of the separator.

Explain the consequence of an accumulation of dirt on the heat-exchanger surfaces of

an aftercooler.

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The air would pass through without adequate cooling and therefore carry extra moisture into

the system.

AIR RECEIVERS

The air receiver is used as a storage reservoir for the overall compressed air

system. However, it will also serve to balance out pulsations in air flow from

the compressor, so providing a continuous pulsation-free supply to the

pneumatic equipment receiving the air. In addition, a certain amount of

cooling will take place in the receiver, causing even more moisture to be

removed from the air supply before it can enter the system.

The size of the air receiver is determined by the rate at which the air is

consumed, and the production capacity of the compressor. There are of course

other factors controlling the size of the receiver, such as the type of capacity

control provided for the compressor and maximum cycling rate; but the air

storage and the ability to supply all system demands at a relatively constant

pressure are the principal determinants.

A certain storage capacity is essential for the fail-safe operation of the items of

equipment connected to the system, in the event of a power failure or an

interruption in supply.

As a general guide to receiver sizing, it can be stated that the volumetric

capacity of the receiver in m3 should be equal to the delivery capacity of the

compressor in m3 min–1.

However, when sizing a suitable receiver, all of the relevant factors should be

considered: if in doubt the compressor manufacturers should be consulted.

Financially it is cheaper to use a receiver that is too large than one that is too

small.

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Air receivers should be installed outdoors wherever possible and on the

shaded, cooler side of a building. This will promote better cooling, result in

more moisture being separated, and give drier air. If receivers are sited indoors

then good ventilation of the area is advised.

FIGURE 11 shows a typical air receiver.

FIG. 11 Typical Air Receiver

Pressure gauge

Compressed air inlet

Inspection manhole

Supports Drain cock

Compressed air outlet

Pressure relief valve

Receiver shell

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AIR DRYERS

In many of today's manufacturing processes and in industrial automation, the

presence of any moisture in the compressed air cannot be tolerated. To allow

the air supply to meet such stringent standards, it must undergo a far more

sophisticated process than the normal aftercooling and moisture separation.

The aim of compressed air drying is to lower the pressure dew point (PDP) of

the pressurised air to a value less than that which is likely to occur in the

distribution system or point of use.

The pressure dew point of the compressed air is that temperature at which

water vapour contained in compressed air at a specified pressure (other than

atmospheric pressure) will condense to form liquid water. The dew point at

atmospheric pressure is referred to as atmospheric dew point (ADP).

There are three methods of compressed air drying:

Regenerative Adsorption Drying

The unit will usually consist of two pressure vessels which are filled with some

water adsorbing chemical. Wet compressed air will be passed through one

chamber until the chemical is saturated. The flow of wet air is then diverted

automatically to the second chamber, whilst the saturated chamber is

regenerated, by a coil heater inside the vessel drying the adsorbent chemical or

by passing a charge of ultra dry air through the vessel, to remove the moisture

from the chemical inside. This cycle repeats continually with one vessel

operational and the other regenerating. Adsorption dryers can achieve pressure

dew points down to –20°C for air at a pressure of 7 bar, some will achieve

–50°C dependant upon the type of desiccant used. Common desiccant

materials for adsorption dryers are silica gel or activated alumina.

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FIGURE 12 shows a typical adsorption dryer system.

FIG. 12 Regenerative Adsorption Dryer

Dry air exit

Purge air

Wet air entry

Vessel being regenerated

Two-way valve

Adsorbing chemical

Vessel drying

air

Coil heater

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Chemical Absorption Drying

In this type of drier, the moisture in the air, on coming into contact with the

absorbent desiccant, will react chemically and liquidise as opposed to being

adsorbed onto the surface of the desiccant as in the adsorption dryer.

The solution formed by the water and the absorbing chemical falls to the

bottom of the chamber, where it is drained off by an automatic drain trap.

Typical desiccant materials used in absorption dryers are urea lithium and

calcium chloride which must be regularly replenished, normally at monthly

intervals, to compensate for the desiccant used in the drying process.

FIGURE 13 shows a typical chemical absorption dryer system.

FIG. 13 Chemical Absorption Dryer

Access opening for adding desiccant

Dry air exit

Drain outlet

Wet air entry

Desiccant pellets

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Refrigeration or Chiller Dryers

These units are heat exchangers which will cool the air down to a theoretical

dew point of 1 to 3°C and thus precipitate out the moisture. The system is a

straight mechanical refrigeration unit with one extra facility included: this is a

second heat exchanger in which the outgoing cold air is used to pre-cool the

incoming air supply. This has the effect of warming the outgoing air slightly

to about ambient temperature.

FIGURE 14 shows a typical refrigeration dryer system.

FIG. 14 Refrigeration Dryer System

Wet air entry

Expansion valve

Water separator and drain

Air-to-air heat exchanger

Water separator and drain

Dry air exit

Condenser

Air-to- refrigerant

heat exchanger

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

...................................................................................................................................................

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________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. Produce a block diagram of a typical compressor plant installation

showing all essential components in the correct order.

2. With reference to FIGURE 14, simply explain the refrigeration drying

process.

3. List three functions of an air receiver.

4. Describe an ideal location for an air receiver.

5. Explain the main advantage a water-cooled aftercooler has over an air-

blast type.

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

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________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. The answer is shown in FIGURE 15

FIG. 15

2. Wet air enters the air-to-air heat exchanger and gives up heat to the cool

air passing out into the system. Condensing moisture is removed and

drained. Air then enters the refrigeration unit where it is cooled, further

condensing moisture is again removed and drained. Finally the air passes

back through the air-to-air heat exchanger, gaining heat from the

incoming air, and passes into the system.

3. Three functions of an air receiver are:

• to store the air

• to remove moisture

• to dampen pulsations.

4. The air receiver should be sited in a cool, shaded location with good

access for maintenance purposes.

5. The main advantage is being able to cool the air to below ambient

temperature and hence remove more moisture.

SilencerFilter Comp. Stage 1 Intercooler

After- cooler

Dryer ReceiverSystem

Atmosphere Comp. Stage 2

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________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

At the end of this lesson you should now appreciate:

• the reason why compressors are rated in free-air delivered (FAD)

rather than compressed air, and appreciate that the relationship which

exists between the two is the compression ratio of the machine

concerned

• the relationship between pressure, temperature and the moisture-

carrying capacity of air

• the fact that siting a compressor in a humid location will cause

problems with moisture carry-over, and the methods used to

minimise this problem

• that different types of air dryers are available, and their basic

constructional details

• the order and arrangement of compressor plant components.

Teesside University Open Learning (Engineering)