3.1.1. Preparation Device
The preparation device is installed on the body frame and is mainly composed of the filling and recycling tank, the heater, the grouting level sensor, the heating medium level sensor, and a temperature sensor.
The filling and recycling tank makes up the bulk of the preparation device. An NGEL650A nano-based soft felt thermal insulating layer (Shenzhen Furang Energy Saving Technology Co., Ltd., Shenzhen, China) is bonded to the outer wall of the tank and has a thermal conductivity at room temperature of only 0.003–0.012 w/(k·m). Having a good level of hydrophobicity helps to reduce the energy consumption when preparing the grouting material. A polytetrafluoroethylene (PTFE) film with a high temperature resistance and self-cleaning properties is sprayed on the inner wall of the tank to prevent the alloy grouting material from adhering to the wall.
The liquid LMP alloy grouting material is prepared using a hot bath method. Basically, this involves a heater heating a heating medium that is circulated around the grouting material until it has melted. When the melting point of the alloy is below 100 °C, water can be used as the hot bath medium. If the melting point is between 100 and 232 °C, methyl silicone oil can be used as the hot bath medium instead. The heater is used to raise the temperature of the heating medium until it reaches the melting point of the alloy grouting. It is made up of several 304 stainless steel double-ended U-shaped electric heater pipes in parallel. The number of pipes is determined by various constraints, including how much grouting material needs to be produced within a set time, the heat absorbed by the alloys, the nature of the heating medium, the thermal properties of the filling and recycling tank, and the amount of heat dissipated by the nano-based soft felt insulating layer.
The formula for calculating the heat absorbed by the heating medium,
, is shown in Equation (1):
where,
is the specific heat capacity of the heating medium,
is the cross-sectional area of the filling and recycling tank,
is the depth of the heating medium,
is the density of the heating medium,
. is the set temperature (working temperature) of the heating medium, and
is the heating medium’s initial temperature.
The formula for calculating the heat absorbed by the alloy,
, is shown in Equation (2):
where,
is the specific heat capacity of the solid alloy;
is the mass of the alloy;
is the melting temperature of the alloy;
is the initial temperature of the solid alloy, and
;
is the melting heat of the alloy;
is the specific heat capacity of the liquid alloy;
is the set temperature of the liquid alloy, and
; and
.
The formula for calculating the heat absorbed by the filling and recycling tank,
, is shown in Equation (3):
where,
represents the specific heat capacity of the tank material;
represents the mass of the tank;
represents the initial temperature of the tank, and
; and
represents the set temperature of the tank, and
.
The formula for calculating the average heat loss of the nano-based soft felt thermal insulation layer,
, is shown in Equation (4):
where,
represents the surface area of the insulation layer,
represents the rate of heat loss for the insulation layer when the temperature of the heating medium is set at
, and
represents the time taken to heat the alloy from
to
.
The heat loss of the surface of the heating medium in the filling and recycling tank can be neglected if it is considered to be a closed container. The overall designed allowance for the impact of the constraints is 20%. On the basis of the above, the number of heating pipes,
, can be calculated as follows:
where,
is the power of the heating pipe.
A mixture of technological and physical resources are provided to ensure that the grouting material and heating medium can be effectively monitored. An observation window, temperature sensor, and liquid level sensors for the heating medium and grouting material are installed on the side wall of the filling and recycling tank. The observation window enables the working state of the filling and recycling tank to be observed. By means of these various mechanisms, an operator can keep a close eye on how much heating medium and grouting material is currently in the tank and, if necessary, step in to control the filling and heating process. The type, function, and parameters of the three sensors are shown in
Table 1.
As the temperature of the alloy grouting material does not need to go above 232 °C, a highly-sensitive and reliable copper-constantan thermocouple was selected as the temperature sensor. The temperature sensor transmits the overall liquid alloy temperature to the control device in real time. When the temperature of the alloy reaches the set value,
, the control device uses a Proportion-Integral-Differential (PID) algorithm to control the heater [
19] and make sure that the liquid alloy remains at a constant temperature the whole time.
A differential pressure liquid level meter with double flanges was adopted as the grouting material liquid level sensor. This measures the distance between the liquid alloy and the heating medium relative to the axis of a flange at the positive pressure end of the meter. This can be thought of as measuring the level of the interface. The meter is composed of a transmitter, capillary, pressure transfer medium, positive pressure end flange, and negative pressure end flange. The position at which each component is installed is shown in
Figure 6.
As the interface is between two kinds of high-temperature liquids, the pressure transfer medium filling the two capillary tubes at the positive and negative pressure ends of the transmitter was chosen to be silicone oil. This can withstand temperatures of −15–250 °C. The range of the pressure measurement was an important consideration when selecting the differential pressure liquid level meter. It can be obtained by calculating the zero point and the measuring range of the transmitter, which involves adopting the following procedure.
The static pressure of the transmitter’s positive pressure chamber,
, can be calculated as follows:
where,
is the density of the liquid alloy,
is the gravitational acceleration,
is the interface level,
is the distance between the axes of the flanges at the positive and negative pressure ends,
is the distance between the level of the heating medium and axis of the flange at the positive pressure end,
is the atmospheric pressure,
is the density of the silicone oil in the capillary tubes, and
is the vertical distance between the positive pressure chamber and the axis of the flange at the positive pressure end.
The static pressure of the transmitter’s negative pressure chamber,
, can be calculated as follows:
where,
represents the vertical distance between the negative pressure chamber and the axis of the flange at the negative pressure end.
Therefore, the differential pressure value between the positive and negative pressure chambers of the transmitter,
, can be calculated by using Equation (8):
According to Equation (8), when
, the zero point of the transmitter,
, can be obtained as follows:
According to Equation (8), when
, the measuring range of the transmitter,
, can be calculated as follows:
As
and
, the measurement range of the differential pressure liquid level meter with double flanges,
, can be calculated by using Equation (11):
The control device sets the upper and lower limit values for the level of the interface. When the grouting liquid level sensor indicates that the liquid has reached its upper limit, the control device issues an alert, which notifies the operator to stop adding solid alloy through the filling hole in the cover plate. When the measured value hits its lower limit, the control device also sends out an alert, so that more solid alloy can be added. This control method can firmly guarantee the continuous production of liquid alloy in a filling and recycling tank of a limited capacity, so that this does not constrain the pouring of the machine tool foundation.
It is not only the level of the grouting that needs monitoring, but also the level of the heating medium, as this, too, is expended during the filling process. Therefore, this is accomplished in the grouting material preparation subsystem by means of another differential pressure liquid level meter, this time with a single flange, but used in the same way. This assesses the real-time liquid level of the heating medium in the filling and recycling tank. The level of the heating medium is also controlled within upper and lower limits. The solid alloy and normal temperature heating medium are first added to the subsystem from an external source. As soon as the liquid level sensor for the heating medium reaches its lower limit, the control device sets the heater to work. When the level reaches its upper limit, the control device sends a signal to stop the addition of heating medium to the recycling device, so as to prevent it overflowing. During the process of preparing the liquid alloy, the control device ensures that the recycling device continues to fill the tank with the heating medium until the liquid’s level rises to its upper limit. The heating medium in the tank slowly reduces to its lower limit through various losses, at which point, the control device ensures that the filling begins again.