2
Facto ry 0 rgan i sat io n
PLANT LOCATION
The location of the dehydration factory must be considered very
carefully, because, whereas at one time proximity to the source of raw
materials was of paramount importance, availability of an adequate labour
force and the provision for effluent disposal will, perhaps, constitute a more
pressing requirement.
A rural environment is obviously better than a location in a highly
industrialised area, because the operation involves the utilisation of a
considerable amount of open space for the reception and storage of raw
materials, particularly where vegetable dehydration is being undertaken.
This chapter, in dealing with factory planning, proceeds on the premise that
vegetables are the principal raw material to be processed. Fruit dehydration
is dealt with in Chapter 7.
On the assumption that the labour availability factor has been
resolved, and co-operation from the Local Authority on effluent disposal has
been secured, it is important that the location should be a focal point for the
intake of indigenous raw material within a radius of some 80 kilome-. In
the case of highly perishable vegetables, such as vined peas and green beans,
availability should be within half that radius.
These figures are relevant where the scale of operation presupposes an
average intake of 70-80 tons of produce per day which is, in effect, a viable
level upon which to work. This would be regarded as a medium size plant.
Production on a larger scale would call for a wider area of raw material
21
availability but, in the ensuing chapters, it is intended to concentrate on the
problems of the medium size operator, coming into the industry for the first
time, or developing into dehydration from some other food processing
activity. It is, perhaps, more likely, at this stage of the growth of dehydration,
that development in Europe will be at this sort of level, rather than at that
applying in the US, where plants with an intake capability of upwards of 500
tons a day are not uncommon.
RAW MATERIAL
Once the vegetable programme has been established, an efficient
procurement system is an absolute necessity, as it is vital that the plant be fed
with a round-the-clock supply of raw material for many months of the year,
with no hiatus or short-fall due to weather conditions, or any other
circumstances. Binding contracts must be made with reliable growers to
ensure this and also to ensure that suitable varieties of vegetable - ones that
lend themselves favourably to the process -are grown. Random purchasing
on the open market is a system not to be relied upon by the dehydrator,
because wherever the price is right the variety will be wrong, and vice versa.
The one exception, when this procedure is varied, is in the case of
potatoes, and spot purchases are often made as the season proceeds. Whilst
there is an element of risk in this particularly in a Season of low solids and a
short crop - potato prices tend to be more stable than those for some other
crops, and availability is spread over a much longer period.
Some particular potato products, however, do call for special varieties
as a first choice. Potato granules and flakes, for example, should be made
from high starch content tubers and, if these types am not grown in the
traditional growing areas, special plantings under contract may have to be
made adjacent to the plant location.
On the other hand, some processors find that, to meet a cost problem
with some potato products, they have to process culls from ware (table
grade) potatoes, and, in this case, contracting is not usually possible. Grading
stations can usually supply large quantities of this material, which is
satisfactory for the manufacture of some grades of dehydrated pmduct, and
long term arrangements can always be made for intake from such sources,
provided certain basic quality requirements are covered.
Contracts
The procurement contract with the grower can take one of two forms,
either: (I) A contract wherein the purchaser undertakes to buy from the
vendor a specific tonnage of vegetables at a fixed price per ton, delivered to
the factory over a specified period, or (2) a contract wherein the purchaser
22
undertakes to buy the produce of a specific acreage laid down by the vendor,
at an agreed price per acre.
In the first instance, the grower must cover all contingencies arising
from total or partial crop failure, and carry a not inconsiderable element of
risk of the purchaser buying in against him in the event of a crop failure.
The second form of contract implies that the processor takes over
some of this risk but, in return, has a reasonable chance of buying his product
cheaper, if he has an efficient fieldsman service to supervise the grower and
ensure a high level of good husbandry.
The contract, in whichever form, will specify varieties and, in some
cases, the processor will elect to supply the seed at an agreed cost to the
grower. Further stipulations will cover the quality standard, the rate of intake
per day and the period over which intake will be accepted. The contract will
also state whether the produce is to be delivered in bulk, or in containers and
how accepted weights will be established. There is invariably provision for
arbitration in the event of a dispute.
It is important that the quality clause in the contract should be most
specific, so that the grower is made aware of the standard of grading or
dressing required. The processor will also underline his right to oversee,
through his fieldsmen, the sowing, planting, cultivation and harvesting of
the crop. Supervision must also be exercised with types of fertiliser,
weedicide and pesticide used in connection with the cultivation, as
dangerous residuals can sometimes affect the pduce at maturity. All these
factors must be fully considered and covered in the document but, as this
procurement procedure has been adopted for many years by canners and
freezers, and has equally been accepted by growers, either individually or by
their Associations, where group negotiations arise, the dehydrator should
not really have any difficulty in getting all the above clauses implemented.
LABOUR REQUIREMENTS
In selecting a site for the factory, the consideration of suitable labour
availability will have been made. On the assumption that the location is
rural, certain advantages as to quality of labour are usually apparent, in that
both male and female operatives who have agricultural backgrounds and
traditions often integrate very satisfactorily into vegetable processing as this
is, in effect, an extension of their way of life.
The labour content of the putative 70-80 ton per day plant will vary
according to the type of drying plant selected and the range of products to be
handled but some guide can be given if certain assumptions are made at this
point.
Let it be supposed that the operation is mounted on a ten months’
23
programme over the following range of vegetables: peas, green beans,
potatoes (granules and cubes), cabbage, leeks, camts, celery, beetroot and
turnips.
The process staff will require to operate on a three shift system, usually
six days per week, with a plant cleaning shift and a maintenance shift in
addition at the weekend. Local custom may require that this system be
covered by four teams to avoid excessive overtime at weekends but it is by
no means uncommon in some factories for staff to accept this overtime, the
three eight hour shifts being covered by three teams changing their shifts by
rota every week.
The many types of dehydration plant will be considered in detail in
the following chapters but, for the purpose of assessing the process labour
content of a typical medium size plant, it could be assumed that the following
dryers would be required to handle the above ten month programme:
For Granules: an Air-Lift dryer with a capacity of 1.5 tons of raw
potatoes per hour;
For peas, green beans, cabbage, celery and leeks one Hot Air Through
Conveyor Band Dryer, with a capacity of up to 3 tons of prepared m
vegetables per hour- the capacity will vary according to the type of vegetable
and size of cut;
For mot vegetables and potato cubes: one Hot Air Through Conveyor
Band Dryer with the same capacity as mentioned for other vegetables.
These dryers would not be operating concurrently throughout the
whole of the season but would come into operation in accordance with the
availability of the crops. It is vitally important that at certain times some
excess drying capacity is available to cope with gluts, or a dryer breakdown,
hence the advisability of duplicating the band dryer. Some important crops,
which have a short harvest period, such as peas and French beans, can
overlap and, in these circumstances, shortage of drying capacity can have
dire consequences.
The process labour to man this scale of operation, per shift, would be
approximately as follows:
Shift Superintendent: 1
Foreman: 1
Forewomen: 1
Male Plant Operators: 10
Fitters: 2 (maintenance duties only)
Electrician: 1 (maintenance duties only)
Boilerman: 1
Quality Control: 2
24
Women: 30-50 (trimming and selection)
Permanent night men 15-25 (Trimming and selection)
The female labour content will vary according to the types of vegetable
being pmcessed. The above numbers in any case assume that, at the selection
and packing end of the pmcess, the products e being bulk packed. Where
the product is intended for retail distribution in small units, a very much
larger female packing staff would be required to operate filling and packing
machinery. Such considerations, in this instance, however, have not been
taken into account. Male operators will normally substitute for women on
the lOpm to 6am shift.
It is stressed that the figures given can only be taken as a mugh guide,
and they are based on the manning of two particular types of dryer, the
methods of operating which are efficient and particularly economical in
labour content generally.
The conveyor band dryer, either single or multi pass, is probably more
versatile than most in the types of vegetable it will handle, and it is
particularly suitable for long sustained runs of one product. Its merits, along
with those of other dryers, will be described in later chapters but in the
context of the present plant under review, it requires a minimum of staff to
operate it.
As in the canning and freezing industries, the greatest concentration
of female labour is in the trimming and selection departments, and unless
strict control is exercised in these areas, as to the numbers and efficiency,
costs can easily overtake profits.
Technology has made great strides, however, in reducing the tedium
and unrewarding work of product selection, and there are now very efficient
colour sorting devices on the market, which effectively select and reject
blemished material in the raw state, or after drying. This reduces the labour
requirement for trimming and selection, and the development of these
machines to the present peak of efficiency has cut the cost of these operations
very considerably.
As the particle size of dried vegetables rarely exceeds lOmm by lOmm
by lOmm, the electronic colour sorter can scan these particles in their
trajectory through an optical box against a predetermined coloured
background. Any particle, whose colour, by reason of blemish, differs from
that which is acceptable, is pushed into a different trajectory by a jet of
compressed air and diverted through a waste spout. These machines have a
high efficiency and perform the task of selection at a fraction of the cost of
hand labour.
Some visual inspection is still advisable after the product has passed
25
through a colour sorter but the labour content at this point is minimal.
Intake Staff
Very considerable labour economies can be effected in the vegetable
intake department by the use of bulk loaders for potatoes and root vegetables,
500kg tanks for peas and large crates or tote boxes for cabbage, leeks, celery,
etc. Green beans are perhaps the only vegetable for which bags or nets need
to be
Also, the use of storage silos for potatoes and roots reduces the intake
staff to one, or two at most, per shift, and this staff can be used for fluming
the raw material into the plant. This presupposes that an adequate supply of
cheap water is available from an adjacent river, waterway or borehole, fmm
which the processor has a prescribed right to abstract water. This need not be
potable water, as it is only used as a means of moving the vegetables fmm
the silos through flumes into a primary washer, and thence into the
preparation plant. Such water is normally discharged from the washer into
settling tanks, to remove silt and stones washed off the vegetables, before
being returned to waste or recycled. In some instances, it may have to go
through an effluent treatment plant before it can be discharged but the Local
Authority will have to be consulted on this matter. A destoning machine is a
necessary adjunct to this part of the system.
The shift quality controller has an important part to play at this point,
as it is his responsibility to ensure that the quality of the intake measures up
Typical box tipper
26
to the contractual requirements of the buying department.
Potatoes must be examined for total solids, reducing sugar content,
excess soil, blemish and grading. Vined peas are examined for quality and
tested for temperature and tenderometer reading. Camts, cabbage, celery,
leeks, etc, are checked for the standard of dressing which is called for in the
procurement contract. Extraneous matter in vegetables, apart from the waste
factor, can also do untold damage to preparation plant later in the process,
and intake staff must be very vigilant on this score.
The period of intake will normally extend over twelve hours of each
day, except during the season when highly perishable vegetables, such as
peas and beans, are being handled, then intake and quality control staff will
be required round the clock on three shifts. Adequate staffing at this point is,
therefore, imperative.
Warehousing Staff
The staffing of the warehousing and despatch departments will
depend, as to numbers, on the nature and size of the packs being produced,
and whether a bulk or retail consumer trade is being carried on. If the factory
is concerned mainly with bulk packs, two men per shift for stock movement
will be adequate, with a daytime superintendent in overall charge.
Maintenance Staff
Apart from the fitting staff detailed to cover the shift maintenance
duties, it is necessary to provide for an adequate day shift maintenance staff
in the field of both mechanical and electrical engineering. Dehydration plant,
operating round the clock for many months of the year, demands a high
standard of maintenance,' and there is invariably a heavy day shift
programme of servicing equipment, both mechanical and electrical,
temporarily taken out of service for a major overhaul.
A minimum of six day shift fitters and three electricians would be
required for the size of undertaking under review. These are supplementary
to maintenance personnel working on the three shift rota.
A carpenter, painter and bricklayer can also be usefully employed all
the year round on the day shift maintenance staff because continuous shift
operation levies a heavy toll on the buildings, particularly at the wet
preparation end of the factory.
Laboratory and Horticultural Staff
Laboratory staff quired, excluding the shift quality controllers, will
be three as a minimum, depending on the range of products being handled.
The main duties in the laboratory are the routine analytical tests on the dried
27
product, and these are supervised by a chief technician, assisted by two or
more academically unqualified assistants. Some research and development
will also be carried out by the senior staff. A horticultural officer and two
fieldsmen are also required.
WATER
For the size of plant under review, the requirements for water washing,
process use, boilers, etc, but excluding any fluming operations, will be of the
order of 8 million litres per week of eight hour shifts.
The quality of the water is important, and it is necessary to know the
temporary and permanent hardness characteristics. It may be that the local
supply is not entirely suitable for every kind of vegetable to be processed
and some treatment may be necessary.
For example, peas require soft water for washing and blanching, as a
high calcium content in the supply has a toughening effect on the skins. In
these circumstances a water softening plant will need to be installed to treat
the water which is in contact with the peas in process. Conversely, potatoes,
particularly when used for cubing or slicing, require a higher degree of
hardness in the processing water, and it is usually necessary to treat this with
calcium chloride in the blanching process.
Ideal water conditions are, therefore, unlikely to be found in many
locations, and perhaps a supply with 8 degrees of hardness might be
regarded as a fair average, requiring less treatment overall than most
samples. Consideration of the water analysis should, therefore, be treated as
of some importance when choosing the factory site.
Boiler water should be reasonably soft, to avoid scale formation but,
where hard water conditions apply, the supply will have to be treated
chemically, or a base exchange softening plant installed. With the latter,
some further organic or chemical treatment will be necessary to correct excess
alkalinity which is a feature of the base exchange method.
It is good boilerhouse practice to =turn all the available condensate
from the processing plant, ie, from dryer steam batteries, blancher and lye
peeler steam coils and space heating batteries, back to the hot well in the
boilerhouse, so that the feed water supply is available at a high temperature.
There are several points in the factory where mains water can be
conserved by recycling, particularly the washing plant. Care must be
exercised here, however, to ensure that recycled water is not contaminated,
and some in-plant chlorination willbe necessary to implement this recycling
system.
28
Fluming
Fluming water, as referred to earlier, must come from a free or, at least,
a cheap source. It is not viable to use mains water for this purpose, as the
volume required for fluming, say 70-80 tons of root vegetables or potatoes
per twentyfour hour day, will be of the order of 1.2 million lihes. It is,
thefore, desirable that the factory be situated near a river, or waterway, and
that the River Authority can be persuaded to give the necessary licence to
abstract water for this particular purpose. An alternative to this sourre of
supply is, of course, a borehole, or a series of boreholes capable of that sort of
capacity.
The sugar beet factories use this system of fluming beet into their
plants, some thousands of tons per day being moved from silo to process by
high pressure water jets, with a minimum amount of manual effort. It is a
system well worth emulating in the dehydration factory, where large
quantities of roots and potatoes have to be handled.
/.--.--
AI7 r’ I, --___. I.,//;*/, /
-v7Ab.- ‘rr I
End elevation of root vegetable silo with centrefluming channel
Silos
The construction of the silos is quite a simple matter and a typical sue
for a 100 ton unit would be 30m by 4m. The side walls are lm in height and
constructed of 23cm thick masonry. Both ends of the silo are left open to
allow incoming bulk loaders or tipper lorries to discharge their loads. The
concrete bottom of the silo slopes from either side wall to the fluming channel
in the middle. This runs longitudinally down the whole length of the silo
with a fall of 50cm in the 30m length. The width of the channel is 30cm, and
this is rebated to take 7.5cm thick timber cover boards which are fitted flush
with the sloping silo bottom. The shallowest point of the flume channel
must be not less than 15cm deep, and it is from here that the water supply is
introduced from powerful centrifugal pumps with a capacity of 91,000 to
136,000 litres per hour. The cover boards are removed in that part of the silo
which requires to be emptied, and the vegetables feed in a steady stream
into the fluming channel, whence they are carried along with the flow of
29
water. Beyond the end of the silo, the fluming channel continues,
maintaining the same rate of fall, to the prewasher and destoner. At this
point the fluming water is discharged, and the vegetables are elevated into
the processing plant.
In setting out this system, levels have to be carefully studied, as it may
not be possible to arrange for a natural fall for the whole length of the flume,
and some elevation may be necessary prior to the washer. This can only be
decided by the processor in the light of local circumstances.
Adequate protection from bad weather must be provided, and the
silos should be under cover in suitable outbuildings wherever possible.
Alternatively, 50 ton storage bins may be erected over the silos to discharge
directly into the flume channels. A mobile elevator will be needed to fill
these from bulk transport.
POWER
Electricity requirements for the putative plant will be in the range 300-
400KW. Whilst this will normally be taken from the mains, it is a wise
precaution to have some internal generating facilities, even if only to keep
essential plant running in the event of a mains power failure. Downtime in a
continuous process is very expensive indeed and, apart fmm raw material in
mid-process being at risk, heavy standing charges have also to be met from
reserves. Also, when highly perishable vegetables, such as vined peas, are
being handled, every hour lost by breakdown is irrecoverable, as the vining
programme on the farms is geared accurately to the plant’s optimum
processing throughput, and any hold-up at the factory can result in the loss
of many tons of raw material, which could be the processor’s responsibility.
This situation is very obvious when it is realised that the whole season‘s
pack of fresh peas has to be processed in six weeks, therefore every hour of
electrical or mechanical breakdown in the working week can precipitate
disastrous losses for both the grower and the dehydrator. The economics of
dehydration are elaborated in a later chapter but it cannot be stressed too
strongly at this point that any steps, which can be taken by way of ensuring
complete reliability and continuity of the power supply, should be taken, in
the full knowledge of the consequences of breakdown.
It is sound practice to install electric motors rated somewhat in excess
of their actual duty, and to ensure that they are adequately protected against
water, steam and conditions of high humidity, remembering also that they
will probably be running continuously for many hours. It is possible to
spray windings, starters and switchgear with a water repellent compound,
and this precaution should be taken by the shift maintenance staff as a
regular procedure.
30
A spaR motor unit should be kept in the stores for all key plant and,
where drives and power requirements can be standardised for several
machines, this should be arranged in order to cut down the number of
spares carried.
Most electrical supply authorities impose a maximum demand tariff
on their consumers, and as the dehydrator is involved with a considerable
power Ilequirement, and the use of several high powered motors, it is very
necessary to install power factor comxtion equipment in the factory to
reduce the start-up load on the meters. It is usually found that the cost of
such equipment can be recovered within twelve months of normal operation
and this will, of course, depend on the Local Authority’s tariff, and to what
extent the maximum demand rate is levied on the commercial consumer.
For a stand-by supply, a 2OOKVA alternator, driven by a diesel engine
or, if a steam supply is available, by a steam turbine, should be adequate to
keep the principal preparation plant and the main dryers operational in the
event of a mains power failure, and this supply should be arranged so that it
can be fed into the mains immediately such a failure occurs. Some factories
keep their steam turbine ‘stand-by’ plants continually operating, integrating
the supply with that from the mains, and utilising the back p~ssure steam
for process heating in the factory This is an efficient and economical method
of producing, simultaneously, electricity and low pressure steam at IO-15psi
from one power source, ie, the boiler. At the same time the essential plant can
be kept operational in the event of an external power cut or supply failure.
BOILERS
Boiler plant for this scale of operation should be rated at GOO to 7000kg
per hour, at 17.5atm, with a stand-by boiler of similar capacity This boiler
rating is based on the assumption that the whole of the heat requirement for
drying is not expected to be taken from the boilerhouse. This point is
considered later in the chapter when fuels for drying operations are
specifically reviewed.
Some dryers incorporate steam batteries and air fans for convection
drying and, if this type is used, then the rating of the boiler plant may have to
be increased but with the advent of natural gas, and the various forms of
Liquid Petroleum Gas, a very much more efficient heat source for drying is
now available to the dehydrator, and steam batteries are figuring less
prominently in the many modern drying plants.
The boiler rating stated above, therefore, takes into account the steam
requirements for peeling, blanching, scalding, jacketed or coil heated vessels,
steam batteries for conditioning bins (see Bin Finishing), steam hosing for
sterilisation of plant and equipment, and space heating. Dryers relying upon
31
Propane combustion chamber showing burner arrangement and air control
damper developed by Aeromatic Ltd.
steam heating may require a further 5000kg per hour of steam at 17.5atm.
Steam usage is related to evaporation of water from the product, and it
requires 1.9kg steam per kg of evaporation.
HEAT RAISING PLANT
Heating systems used in air drying are either direct or indirect.
Direct System
Here the fuel is burned in the air stream and has direct impact on the
product. This system calls for a high degree of purity and non toxic
constituents in the fuel. The development of natural gas propane and butane
for commercial applications has provided the dehydrator with an ideal and
economical fuel for vegetable drying and, in the author’s opinion, is to be
against all other fuels, wheever the dryer lends itself to direct preferred
firing.
LPG in the form of propane is particularly suitable on account of its
very low sulphur content (0.02 percent), its consistency of composition and
freedom from unsaturated compounds. It is clean and easily controlled,
adjustable to special requirements and, moreover, it is relatively competitive
and widely available from the major oil companies as a by-product of
The combustion equipment, comprising a series of burners in an
insulated chamber, thermostatically controlled and with air and flame failure
safety devices built in, is relatively cheap to install and maintain.
Natural gas has similar favourable combustion characteristics but may
not be so readily available in rural areas, and a special run of mains from the
nearest pipeline may involve considerable installation outlay
AThrough Conveyor Band Dryer with a raw material throughput of 3
tonnes per hour would require a combustion unit for propane with a
maximum rating of 15 million Btu’s per hour. This would call for a seven
burner chamber, five burners firing continuously with two on thermostatic
control. Satisfactory flame conditions can be maintained with propane at a
regulated pressure of 0.95atm and, whilst the fuel consumption will
obviously vary in accordance with the evaporative duty demanded, this
could be roughly taken at 140kg of LPG per hour, or 36cu m of natural gas.
The oil industry, in collaboration with the UK Factory Inspectorate
and other experts, has laid down a Code of Practice in relation to the storage
and commercial usage of liquid petroleum gas, and the processor intending
to use this type of fuel should familiarise himself with the regulations,
particularly as to storage tanks and pipelines. Where LPG storage tanks
cannot be sited at least 15m away from the factory buildings, and 15m away
from the nearest roadway, they wiU almost certainly have to be installed
underground, and such installation is relatively expensive compared with
ground level tankage. Whilst propane will vaporise naturally from an
overground installation, such is not the case from an underground tank.
Here the liquid propane has to pass through a steam heated vaporiser to
convert it to a gas. Very low temperatures prevail in an underground tank, as
vaporisation takes place; these are not relieved by a datively high ambient
air temperature as in the case of an overground installation, hence the
necessity for assisting vaporisation by supplementary steam heat. Tank
pressure will be 4.5 to 6.8atm and this is reduced to 1.36atm by regulators on
the pipe main to the combustion plant, where the gas pxessure may be further
reduced to suit required combustion conditions and drying requirements.
refining.
Indirect Systems
The indirect system can be applied in several ways, and with some
types of dryer there is no alternative method of heat transfer to the product.
The Through Conveyor Band Dryer can derive its hot air supply from
a heat exchanger used in connection with an oil or coal-fired furnace. The
heat exchanger comprises a set of cast iron or steel elements or plates, usually
33
gilled to provide the optimum surface area for heat transfer, and the gases of
combustion from the oil or coal fired furnace pass across one side of the
plates, transferring heat to the clean air pulled across the xeverse side by the
dryer fans. The fans draw the heated air into the dryer proper, either through
or across the product which is being continuously conveyed on the dryer
belt.
Heat exchangers can also be constructed with a series of tubes, instead
of gilled plates; this is very similar to the design of an economizer for a boiler
plant.
The obvious weakness of this system is that there are considerable
thermal losses, in spite of the facility, and indeed practice, of recycling the
gases of combustion before discharge to atmosphexe. Then there is usually a
heavy maintenance factor in cleaning the heat exchanger elements or tubes,
to keep the apertures through which the gases flow free from a build up of
fly-ash (where solid fuel is used) and carbon and sulphurous deposits from
oil. Also, at the cool end of the exchanger, where temperatures are often
below the dew point, corrosion can be a pmblem. Another indinxt heat
system, which can be used with band, cabinet, tunnel and bin dryers,
incorporates the use of steam batteries of gilled tubes through which the
drying air passes, either by induction or pressure. Here again there are some
thermal losses, and the batteries require constant and careful maintenance to
ensure that the air passages are kept free and that they are efficiently trapped
on the steam side. Dust is not an uncommon problem in a dehydration
factory, and this can be a source of trouble on the air side of this type of steam
battery.
Combined Systems
With air lift and thermal venturi dryers, it is possible to use a
combination of the indirect system, with steam batteries, and the direct
system of LPG or natural gas - the one system supplementing the other. The
use of batteries is often economical when the factory has a surplus of low
pressure steam or exhaust steam from a turbine or some other part of the
plant. No heat should go to waste, and every conceivable use should be
made of it. Gas heat, as a supplementary source, can then be used to obtain
the desired processing temperature.
The drum dryer is another example of the indirect system. Here, high
pressure steam passes through either one or two large rotating steel drums,
and the product to be dried is fed by spraying or by feed rollers on to the
outer surface of the drums in a thin layer, and heat transfer from the inside to
the outside surface of each drum evaporates the water content of the product
during the course of about 300" of the drum's revolution. Owing to the
34
thinness of the surface coating, evaporation is extremely rapid, the moisture
being flashed off, literally, in seconds. Steam requirements are heavy for this
drying method, as a pressureof at least 5.4atm has to be maintained, and the
volumetric capacity of the drum is high, some being 4.6m long by 1.8m in
diameter.
FUELS
Suitable fuels, therefore, are coal and oil for the indirect systems, and
propane, butane or natural gas for the direct systems. In no circumstances
should the gases of combustion of coal or o il be used in the direct airstream
when dehydrating food for human consumption. The risks of contamination
by sulphur and carbon compounds are very considerable in these
circumstances and, whilst this method has been used in the past for
dehydrating certain food products, it should certainly be avoided in the
context of modern practice.
EFFLUENT
The subject of waste and industrial effluent disposal is too wide and
complicated tobe dealt with in depth in this volume. It has become, in recent
times, a problem of national significance and importance, and the dehydrator
1
Screening solids from liquid effluent in Italy
35
must have this subject very high on his list of priorities when contemplating
the pros and cons of a factory site, and he must obviously discuss all the
implications with the Local Authority or River Board before any location is
determined.
It has already been established that for a medium size vegetable
processing plant, some 1,200,OOO litres of fluming water, where applicable,
and an equal volume of processing water are going to be discharged to waste
each day. The authorities will need to assess the nature and quality of this
discharge before any permission will be given for it to pass into any sewer or
waterway. The days when effluent could be flushed down the nearest drain
regardless of its ultimate destination, are gone in the Western World and
disposal today can be a very laborious and expensive business, and expert
advice on the subject must be sought. It is not unknown for an effluent
treatment plant to cost one third of the value of the pmcess plant, because in
certain conditions very intensive treatment is required by the authorities,
particularly where the discharge is made into an inland waterway
First, the fluming water is likely to have a high percentage of silt and
vegetable matter in it, and, in the processing water effluent, there is likely to
be starch, spent lye, solid vegetable matter and various chemical compounds
from the blanching water.
The scale of treatment required will depend on the level of
contamination, and the Biochemical Oxygen Demand of the discharge, and
the processor will possibly have to decide between steam and lye, or abrasive
peeling of vegetables in the context of the diffenmt effects these methods will
have on his effluent pmblems. Lye peeling, involving concentrated solutions
of sodium hydroxide, will create a high BOD in the effluent, and the
treatment cost on this account may be considerably higher than if steam
peeling was used. Steam peelers involve a higher capital plant investment
but this may be offset by a lesser involvement in effluent treatment, therefore
no hard and fast rules can be laid down for any specific case.
Abrasive peeling of potatoes releases large quantities of free starch
from potatoes, and this again creates a serious treatment problem. Some
large potato processors, who have been committed to abrasive peeling, have
had to install expensive starch manufacturing plants to dry the free starch
collected in settling tanks used as the first stage of effluent treatment,
although the financial recovery from this by-product is so low that it cannot
cover the capital investment or the operating costs, but the exercise has to be
done as a progressive stage in the overall effluent treatment system.
The method of disposal, therefore, must be considered in the light of
local circumstances, and the following options may be open to the
dehydrator:
36
(1) Simple screening out of solids - this is a prerequisite to all
subsequent means of disposal;
(2) Filtration through gravel beds:
(3) Pumping into settling tanks, followed by treatment with
(4) Aeration and lagooning;
(5) Spray irrigation;
(6) Discharge into a public sewer, after one or other of the above
biological agents;
treatments - this will invariably involve a charge for the volume
accepted by the Local Authority;
(7) Discharge into a tidal or other waterway, after whatever treatment
is prescribed by the River Board.
Solid wastes can sometimes be dried for animal foods but the
operating cost of this procedure must be carefully examined to ensure that it
is a viable operation.
The only satisfactory approach to this major problem is to engage the
services of an effluent consultant at the outset of the project, and the expert is
likely to fulfil the local requirements very much mo- economically than the
dehydrator would do, as the latter invariably enters this field with less than
expert knowledge.
Contour irrigutiw
Central Turkey
- - rc ,;- Wasteful control but low capital cost
37
Unlimited potential
from Rio Francisco in
Pernambuco, Brazil
Onion field trials
on contour
irrigated land in
Pernambuco
Desert irrigation
Khorramshaar, Iran
b
