Record pressing requires rapid and precise heat exchange across a wide thermal range — typically from nearly 200 °C down to about 50 °C. To meet these demands, all vinyl record manufacturing facilities are equipped with centralized thermal systems that supply energy to the pressing machines.
Heating and cooling of the records are controlled by opening or closing valves, which regulate the flow of thermal vectors to the presses. This setup allows for precise modulation of the thermal exchange process, drawing from a central "reservoir" that maintains a constant supply of thermal energy.
While various heat sources exist, the one universally adopted in record factories — even today — is steam. According to the three classical methods of heat transfer outlined in thermodynamics (conduction, convection, and radiation), steam heats primarily through convection.
In this process, thermal energy is transferred from a moving gas (steam) to a solid surface (specifically, the channels within the moulds that hold the stampers). This is an example of forced convection, as the steam is pressurized and its flow direction is driven by a pressure differential. The steam circulates in an adiabatic system, meaning it is largely isolated from the external environment, preventing energy loss. Pressurization not only ensures controlled directionality but also enhances heat transfer efficiency — since in convection, the rate of heat exchange is influenced by the velocity of the fluid.
Although steam might seem like an outdated thermal vector, it remains the most efficient choice in terms of the ratio between thermal capacity and cost per unit. Water, the base of steam, has a high specific heat and can store large amounts of thermal energy. When steam condenses on a cooler surface, it releases this energy in the form of latent heat — specifically, 2.26 kJ/g — which is significantly greater than what could be transferred through conduction alone.
Steam generators
Steam is produced by a generator, which can be:
- firetube
- watertube
Firetube generators consist of a large cylindrical tank in which the furnace is located, at the end of which is the burner (for methane, LPG, diesel). Inside the firebox, there is a series of tubes through which the normal flow of gases occurs. During their passage through the tubes, the gases transfer heat to the water in the tank; hence the name "firetube".
In the category of watertube generators, we find instant vapour generators which, unlike the previous ones, are much smaller in size and consist of two concentric rings formed by a spirally wound tube, through which water is introduced at high pressure by a pump. Here, the flame of the burner is enclosed within the smaller ring and the water is heated as it travels through the serpentine until it turns into steam. This type of generator can actually meet rather limited instantaneous steam demands and is rarely used for records pressing, except in the case of plants with only 1 or 2 presses.
In any case, the steam produced flows into a series of pipes that can be considered an "adiabatic environment", meaning isolated from the external environment and therefore not exchanging heat with it. The output of the generator is connected to the main piping of the steam line, and along this, the various branches extend to the valves of the individual presses.
At the end of the line, an automatic condensate drain is usually installed, which allows only the portion of steam that is condensing due to inertia to pass into the condensate return line.
To prevent the cooling of the pipes, a large part of them is thermally insulated with rock wool coverings.
The return pipe of the condensed steam, which is nothing more than extremely hot water, flows into the feed tank of the steam generator, greatly benefiting energy savings, as once at full operation, the generator will require little fuel to bring the fluid back to boiling. A small amount of steam and condensed steam is still lost in the process; therefore, the tank may have a float with a corresponding solenoid valve to replenish the level with softened water.
As mentioned earlier, the steam is constantly kept under pressure, regulated and controlled in the steam generator via a pressure sensor, connected to the burner, and control and safety pressure switches.
As long as it is in operation, the generator continues to produce steam, which continues to accumulate in the piping, increasing its pressure until the maximum set pressure is reached.
Clearly, if the steam valve of a press is opened, for a few seconds a portion of the steam present in the line will flow into the moulds of that press, heating their surface and consequently cooling and condensing. For this reason, as seen in the table shown in the introduction, normally when the steam valve is open, the condensate steam outlet valve is also open.
This partial release of steam from the main steam pipe, which is connected to the head of the generator, will slightly decrease the pressure on the line; the subsequent action of the steam generator will therefore be to increase combustion until the produced steam has restored the pressure to the predetermined value. Obviously, if multiple presses are working simultaneously, more complicated situations can arise, but the final result is always the same.
There are various systems to modulate the intensity of the burner flame, and consequently the amount of steam produced per unit of time. For example, there are two-stage burners, or burners with a modulation ramp. The aim is still to modulate the combustion, and therefore the production of steam, based on the detected pressure.
When the steam pressure reaches the established maximum, the burner (which is presumed to be a common ventilated burner) is stopped by the "stop" pressure switch or by the generator‘s PLC.
At that point, before the burner supplies current to the electrode to reignite the flame, the fan must blow a certain amount of pressurised air, which is necessary for the combustion of the gas. This is an operation that cannot be overlooked and normally takes several tens of seconds. Therefore, to avoid excessive fluctuations in steam pressure, a pressurised steam accumulation tank is often used, or — alternatively — one may choose to work with a slightly oversized generator relative to actual needs. This is because larger generators have more pipes, thus more steam accumulation. If, however, an instantaneous vapour generator is used, the accumulation tank is essential.
Pressure and temperature of steam are related by a proportionality relationship, although it is not linear. The reason that pressurised steam is hotter is always explained by thermodynamics.
If at a pressure of 14,5 PSI the water molecules vaporise at 100°C, at higher pressures — being more compressed — they must overcome a greater cohesive force to transition to the vapour state. This means that higher temperatures are required to provide them with sufficient kinetic energy to overcome their mutual attraction.
The operating steam pressure commonly used for records pressing is 175 PSI, which corresponds to a temperature of 188°C. On average, the amount of steam required to press a record is 1.5 Kg.
Since the thermal vector used for heating is steam, it is quite natural that the one used for cooling is water, pressurised approximately like steam. Thanks to its high thermal capacity, in fact, water:
- can absorb and store large amounts of heat before significantly increasing its temperature. This makes it ideal for transferring heat away from a hot source;
- compared to other common liquids, it transfers heat more quickly;
- when water evaporates, it removes a large amount of heat from the surrounding environment, making evaporative cooling systems (see below) very efficient.
It has been observed how the steam loop (outgoing) — condensed steam (return) is practically a closed circuit, in the sense that the condensed steam feeds the generator again and is therefore "recycled" in the form of new steam.
The cooling circuit is a similarly closed circuit, meaning it always uses the same water, except for the integration of the evaporated part, which is also done with softened water.
As mentioned before, the water is pumped at a pressure more or less equal to that of the steam‘s operating pressure (which is necessary to eliminate any residual steam that has not yet condensed, at the beginning of the cooling cycle).
When the water valve (inlet and outlet) of the press opens, a portion of the water from the main pipeline enters the mould of the press and begins to exchange heat with its surface. Consequently, when it exits, the water goes into the return pipeline at a higher temperature than that with which it entered. To prevent the water temperature from becoming too high over time, it is essential to incorporate a component into the system that cools the water and ideally maintains it at a constant temperature. The two commonly used solutions are:
Cooling towers
The open circuit cooling towers directly exchange the heat of the water with the surrounding air. The reference parameter, in this case, is not only the air temperature but also its relative humidity.
As can be derived from the psychrometric tables, when considering relative humidity, the effective air temperature is approximately 5°C lower than that measured by a standard thermometer.
The cooling process using a cooling tower works like this:
- the cooling water enters through the upper entrance of the tower and is then atomised through a series of nozzles;
- the atomised water then falls downwards, passing through the bundle of heat exchangers located inside the tower;
- a large fan placed at the top of the tower, spinning continuously, creates a flow of air that moves from the inside to the outside, further facilitating the thermal exchange;
- as the water falls from the top to the bottom, a small part of it evaporates, leaving the rest of the mass of water, which in the meantime collects in the lower basin, slightly cooler than it was at the moment of its entry into the tower (about 10°C less, although much depends on the size of the tower and the atmospheric conditions).
The capacity of a cooling tower mainly depends on its size. Unlike a chiller, there is no surgical control of the water temperature; however — if the tower is appropriately sized for the thermal capacity of the system — it rarely causes problems of insufficient cooling. Furthermore, the tower is much more economical than a chiller, as it only requires the operation of two motors: that of the fan and that of the water recirculation pump.
Chillers
The chiller performs a more precise and faster cooling process, allowing for an even greater temperature drop. The process works as follows:
- the water to be cooled is introduced into a heat exchanger containing a coolant;
- the coolant absorbs heat from the water and is transferred to a compressor, where it is gasified, increasing its pressure and temperature;
- the heated gas is then condensed by a fan that introduces fresh air from outside;
- this liquid then passes through an expansion valve, where it loses pressure, expands its volume and releases additional heat;
- finally, it returns to the exchanger for the start of a new cycle.
The chiller certainly has a superior performance compared to a cooling tower; however, at equal thermal capacity, it requires more electric power.
In the figure below, a somewhat simplified diagram of a thermal system serving a press is shown.
Thermal vectors