Guide to know more about cooling towers: "Cooling in the process of molding plastics by injection"

27/07/2017

DESCRIPTION OF PROCESS:

Plastic moulding machines are fitted with a tank for the raw material granules required to manufacture the part.

The granules are heated and melted before being sent under pressure into the mould.

The mould is usually completely automatic, so that the opening and the ejection of the part takes place without intervention by the operator.

Inside the mould there are the runners that serve to cool the mould and, above all, the moulded part: for the cooling  it is generally necessary to use a chiller because the temperature of the water has to be very low to obtain a rapid cooling (to avoid material shrinkage and for a higher throughput).

The injection of the melted plastic into the mould is by means of a hydraulic oil system with pumps that have to be water-cooled.

For this section of the process the evaporation tower can be used since it requires heat dissipation that does not require a particularly low temperature.

PURPOSE OF EVAPORATION TOWER :

As in all cases where it is necessary to dissipate heat produced by hydraulic power plants, also here the evaporation tower is used to full effect.  

In some cases, where the evaporation tower has ample dimensions and a sufficiently large buffer tank is installed, according to the type of plastic material moulded and the size of the part, it is possible to use only the evaporation tower also to cool the mould, with enormous energy saving advantages.

In  any case, making a precise calculation upstream, the use of the chiller can be limited to the hottest months or it can  be used as a support to the evaporation tower only when necessary (see system layout).

The dimensioning of the tower is realised taking into consideration the power installed on the moulding machines (oil pump), usually indicated in kW.

The recommended hydraulic circuit is with two tanks (hot water and cold water), because the users, generally, more than one, need not operate simultaneously, and also because in this way it is possible to dimension an evaporation tower  with a higher capacity in relation to the users circuit to the advantage of the final temperature of the cold water.

The thermal head to be considered in the tower is from  +40°C to  +30°C if it is only necessary to cool the hydraulic power plants, whereas it is necessary to consider the minimum temperature possible for the installation zone (even  26 – 27°C) and a thermal head of  5°C if it is required to also operate on the moulds.

ADDITIONAL NOTE :

As an example, a technical and economic comparison can be made between a system managed with a chiller and the same system in which an evaporation tower is used.

We can use as reference a practical case recently examined, as indicated below.

CHILLER

For a total 223,000 fr/h, consisting of

Ø  2 chillers 44,500 fr/h each, and requiring 11,9.kW (compressor) + 1.1 kW (pump) + 2.2 kW (fans for air condenser) per chiller.

Ø  2 chillers 67,500 fr/h each., and requiring 18 kW (compressor) + 1.5 kW (pump) + 3.3 kW (fans for air condenser) per chiller

Assuming an average external temperature of  25°C, to have water at 15°C the electric power absorbed by the two chillers is equivalent to 74.6 kW/hour

EVAPORATION TOWER

With potential equal to 225,000 kCal/hour, complete with electric motor for 4 kW fan, 2.2 kW centrifugal electric pump to supply tower and 5.5 kW centrifugal electric pump to supply users

The total  electric power used is equivalent to 11.7 kW/hour

The difference in electric power consumed, in favour of the evaporation tower is 74.6 – 11.7 = 62.9 kW/hour

Assuming a work cycle of  8 hours/day, for 22 days/month, for 11 months/year and considering  an  average cost of 0.11 Euro per kW/hour, over a year the saving amounts to Euro 13,884.00; it is to be said that, from this sum the costs for the quantity of make-up water and the chemical treatment  to be carried out to prevent encrustations are to be subtracted, but the difference in management overheads due to the lower electric power consumption copiously covers these two costs.

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