"When I start with a development concept or the design of a high-pressure system for a machine tool, the first question I ask myself is how I can fulfil the customer's requirements in such a way that I generate the lowest possible heat input into the system in order to avoid active cooling if possible. After all, why generate energy and heat for expensive money that I don't need in the first place and then cool it down again for expensive money?" (Jürgen Müller).

Coolant systems for machining are available in many designs. Many manufacturers try to achieve a modular system with a high degree of combinability of components, which is often required by the industry.

However, if you want to take a balanced approach to the issue of cooling a cooling lubricant system, there are only two ways to go if you do things right:

1. Optimise the energy efficiency of the system so that active cooling is not required

2. Design a balanced system with active cooling

To avoid active cooling, the motor pump output can be selected from the outset so that there is no overdimensioning, which then has to be cooled down again, or appropriate control technologies can be used, such as self-regulating pumps or the use of frequency inverters. This makes it possible to develop system concepts across a broader spectrum and at the same time prevent oversizing.

If a system is designed without active cooling and purely from the point of view of energy avoidance, a stable machining process can be achieved in most cases. But you can neither regulate nor control the temperature, you can only keep it relatively stable within a certain range.

If it is clear from the outset that a machine tool with an integrated high-pressure or coolant system must run under very specific temperature conditions, this can only be achieved through active cooling.

A building is equipped with a central cooling water system and can provide a permanently cooled medium for closed-circuit cooling. Machines can then either be connected directly to the cold water network via a built-on plate heat exchanger or via a built-on heat exchanger station with integrated filtration (e.g. heat exchanger station (WTS) from Müller) or you can have peripheral systems, such as a coolant system with fine filtration, on which the plate heat exchanger can also be installed (e.g. CL3/CL4/CL5 from Müller)

Centralised cooling

Advantages:

• The waste heat from the central cooling system can either be used to heat the building and water or is channelled outside
• The factory building is not heated by the exhaust air

Disadvantages:

• If the centralised system fails, the entire production process can be disrupted
• High investment costs due to corresponding failure protection

A production hall does not have a centralised cooling water supply and so-called stand-alone cooling solutions are used, i.e. active cooling of a single machine tool. If no peripheral systems with fine filtration are available, the following options are available:

• Heat exchanger station + cooling water recooler in combination attached to the machine tool
• Active cooler with filter circuit attached to the machine tool

With existing peripheral devices, such as a cooling lubricant system or high-pressure system, it is possible to install a plate heat exchanger in the filter circuit and then supply it via a cooling water recooler. Alternatively, the coolant system can be equipped directly with active cooling. This can be solved in different ways, e.g. via immersion coolers in the additional tank (also possible without filtration, as it is insensitive to dirt, but this can lead to the formation of condensation in the additional tank) or via active coolers mounted on the system with an integrated plate heat exchanger in the filter circuit.

Immersion cooler as active cooler in the dirty tank area

Advantages:

• Does not require filtration
• Low maintenance

Disadvantages:

• Forms condensation in the cooling lubricant, which can lead to problems
• Requires a lot of space

Active cooler with integrated plate heat exchanger for the clean area (filter circuit)

Advantages:

• Requires little space
• Can be easily retrofitted
• High efficiency in relation to space requirement

Disadvantages:

• Can only operate in clean area
• Requires more regular maintenance compared to immersion coolers

Decentralised cooling

Advantages:

• High system availability (no pure dependence on a centralised solution)

Disadvantages:

• Waste heat from the individual workstation solution can hardly be collected. Waste heat massively increases the ambient temperature
• Quite a high investment, as there are many small stand-alone solutions

Müller's guiding principle is to initially design all systems in such a way that active cooling can be avoided or that the lowest possible heat input into the system is always achieved. Active cooling is only used if the customer requires temperature stability in the cooling lubricant or in the process on the machine tool. Even then, the advantage is that the cooling capacity required is lower thanks to the balanced design of the technology. All in the interests of energy efficiency - and all in the interests of the environment.

It cannot be denied that there are many coolant and high-pressure systems equipped with pseudo-cooling systems on the market. Two constellations stand out in particular:

• Oil-air coolers installed on the system
• Small active coolers installed on the system

From a technological point of view, these two configuration variants work, but we completely reject them. The efficiency of the cooling capacity of oil-air coolers (case 1) depends crucially on the ambient temperature. If the cooler is defined large enough and the ambient temperature is low, such an air cooler can actually reduce the temperature of the supplied cooling lubricant. But when is this the case? Rarely, depending on the time of year and the nature of the workshop. If the workshop is air-conditioned, such a system can make perfect sense. However, it is important to remember that the exhaust air from the oil-air cooler immediately increases the ambient temperature again.

If small active coolers are installed on a high-pressure system (case 2), the small clean tank of the high-pressure coolant system is usually cooled, but not the machine tank directly, unless the overall concept has bypass cooling. This leads to two conclusions: either only the clean tank of the coolant system is cooled, which means that cooled high-pressure medium reaches the cutting edge during the machining process and then encounters warm medium from the flooding cooling system, which is sucked in from the uncooled machine tool tank. This constellation can lead to major process problems under certain circumstances. Temperature stability of the overall system cannot be achieved in this way. The situation is similar with an installed bypass cooling system. The cooler, which is too small, is not able to keep the overall system consisting of the machine tool tank and the additional tank of the coolant system at a stable temperature. Cooling does take place here and mixing is achieved via the bypass. However, this results in a random temperature that cannot be controlled at any time.