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Several Questions About The Use Of Chillers
source: | Author:AUWII | time :2020-12-15 | 174 Browse: | Share:
In fact, a pair of molds is a heat exchanger. The heat is transferred from the molten plastic to the mold, and then the mold is introduced into the circulating cooling medium, ice water, and only a small part of the air enters the molding template of the air and the injection molding machine.

The chiller works well in refrigeration, so how do you choose a chiller? What are the main points of the chiller when it is selected?

First, how to choose a chiller

In fact, a pair of molds is a heat exchanger. The heat is transferred from the molten plastic to the mold, and then the mold is introduced into the circulating cooling medium, ice water, and only a small part of the air enters the molding template of the air and the injection molding machine. It is well known that a relatively large portion of the plastic molding cycle is used for cooling, sometimes accounting for more than 80% of the plastic molding cycle, so it is absolutely necessary to control the cooling time to a minimum. For example, a mold molding cycle is generally 20 seconds. If the water in the original cooling tower is changed to cold water produced by a chiller, it can be shortened to 16 seconds. Although the chiller originally selected is more expensive, it can increase production by 20% and achieve great benefits in long-term production. So how do you choose the energy of ice water? From the above we can know that it is related to the specific heat of the molding material, the temperature at the time of melting, the weight and the temperature at which the product is demolded. The formula for calculating the ice water energy required for a pair of molds is: Q=W×C×△T×S

Where: Q is the required ice water energy kcal/h;

W is the weight of plastic raw materials kg / h;

C is the specific heat of the plastic raw material kcal / kg ° C;

△T is the temperature difference between the temperature of the melt and the release of the product °C (see attached table);

S is the safety factor (generally 1.35-2.0). When the single machine is matched, the small value is generally selected. When a chiller is matched with multiple molds, the value is taken. If the air-cooled chiller is selected, S should also be used. Choose a bigger one.

For example: a pair of molds to produce PP products, the production volume is about 50kg per hour, what is the cooling requirement? What size should the chiller be suitable for?

Q=50×0.48×200×1.35=6480(kcal/h);

It requires 6480kcal/h cooling per hour. It is difficult to obtain more complete data during the PR chiller. According to our previous years of planning and supporting sales experience, △T=200°C, it is an average value of many common products after years of statistics.

If the hot glue path is attached to the mold, the energy of the hot glue path should be added to the calculation of the cold amount. Generally, the hot glue path is in KW. When calculating, the unit should be converted into kcal/h, 1KW=860kcal/h. If the water supplied to the plant is sufficient, the temperature is low, and the cost is low, there is no need to use a chiller at this time, which is generally unrealistic unless the plant can be at the lake with low water temperature; the other is Urban deep well water supplies meet the needs of temperature and flow, but often the cost is too high. This method can be used for experimental devices, but for the factory, this is impractical.



Second, ice water temperature difference

The temperature of the mold cooling fluid (ice water) is generally subject to large changes in the shape of the processed materials and products, such as polyethylene thin wall beakers, the mold requires ice water temperature below 0 ° C; and in most other cases, the mold The required temperature of ice water is above 5 °C. The microcomputer full-function chiller can provide ice water above 5 °C. The low temperature intelligent temperature control chiller can meet the requirements below 5 °C and below 0 °C.

The temperature difference between the ice water at the inlet and outlet of the mold is often set according to the requirements of the product. In many cases, the temperature difference is 3-5 ° C is ideal, but sometimes the temperature difference is 1-2 ° C. The smaller the temperature difference, the greater the flow of ice water required to bring the same amount of heat out, and the smaller the flow required. For example, when the temperature difference is 5 °C, the flow rate needs 60L, and when the temperature difference is 2 °C, the flow rate needs 150L.

Third, ice water flow

The flow of ice water required for a pair of molds is directly related to the heat to be carried away by the mold and the temperature difference between the ice water entering and exiting the mold. For example; to take 6480kcal/h of heat away from the mold, if the temperature difference is 3 °C, what is the minimum flow required? Ice water flow Q = 6480 ÷ 3 ÷ 60 = 36 (L / min).

Fourth, the treatment of ice water quality

The softening of water is also a problem that cannot be ignored in the process of using chillers. The pH value of water needs to be continuously observed. The optimum pH should be equal to 7. The pH value greater than 7 will cause terrible corrosion. If no measures are taken, the scale will be generated in the evaporator and the mold, which will act as a heat insulator. In severe cases, the energy conversion effect will be reduced by 30%. Obviously this requires consideration of softening of hard water. The most efficient method is to configure an electronic hard water softener in the system. The softener is designed by ion exchange principle. Depending on the flow rate, softeners of different specifications can be configured to be directly connected to the circulating water line. Generally, the cost of the water treatment softener is not too high, and a certain proportion of the detergent can be added to the circulation system periodically. .

Five, ice water machine flow, pressure

Generally, the injection molding mold is cooled. The pressure of ice water is 0.1-0.2Mpa, which can meet the requirements. The microcomputer full-function chiller can meet this requirement. When the pressure requirement is higher than 0.2Mpa, it needs to be planned separately to facilitate the corresponding pressure. The pump meets the needs of the system water supply.

Table of relationship between flow and pipe diameter:

Pipe diameter 3/8′′ 1/2′′ 3/4′′ 1′′ 1′/4′′ 1′/2′′ 2′′ 3′′

Flow 12 20 35 60 90 130 230 560

6. Cooling of the hydraulic oil and barrel feeding section

Usually the hydraulic oil and barrel feed section is cooled by the water from the cooling tower, as this is not only the best method, but also economical in terms of production cost, unless there is a specific requirement for its temperature, ice water can be used. It is cooled.

Seven, the insulation of the ice water pipeline

The ice water pipe must be insulated because the pipe insulation not only prevents the serious loss of the cold, but also prevents the dew condensation water formed on the outer wall of the pipe. For example: ice water temperature 10 ° C, ambient temperature 30 ° C, a 25 m long, surface area of 25 m & sup2; metal pipe heat radiation up to 750kcal / h, which is almost 10% of the cooling capacity of 3HP compressor, 5HP The compressor produces about 6% of the cooling capacity.