How do industrial hydraulic systems manage heat in continuous operations?

Industrial hydraulic systems control continuously operating heat by using heat exchangers (air-cooled and water-cooled), reservoir sizing and design, appropriate fluid selection, system efficiency optimization, and continuous monitoring of the thermal. Since every inefficiency within the hydraulic system is converted directly to heat, efficient thermal management involves controlling heat production from the point of origin and maximizing the capacity of heat dissipation, as well as keeping the temperature of the fluid within an ideal range of 120°F to 140°F (49°C to 60°C) to maintain component life and performance of the fluid.

The inevitable result of heat is that it occurs during hydraulic operations. Every drop in pressure across an air valve, every obstruction in the line, and each defect in a pump motor is reflected as heat energy in the fluid. When systems run continuously, such as presses or injection molding machines, mobile equipment that run with long duty cycles, or process line power units, this heat isn't allowed the chance to evaporate in a natural way between cycles, the way it would in intermittent applications. If left uncontrolled, it speeds up the process of oxidizing fluids, degrades seals, and gradually reduces the lifespan of each part of the circuit. Knowing how the process of thermal management functions is crucial for anyone who is designing, operating, or maintaining hydraulic equipment.

Why is heat the antagonist of hydraulic systems?

Hydraulic systems are at their heart the power conversion mechanisms. Mechanical or electrical energy is transformed into fluid power, and the fluid power is transformed into mechanical power. There is no way to be 100% effective, and the energy that is lost in each step doesn't disappear, but it is transformed into heat within the working fluid.

In the course of continuous operation the heat builds up more quickly than it is able to escape unless the system has been specifically designed to control it. The effects multiply quickly:

• Viscosity breakdown occurs when the temperatures rise; the viscosity decreases, which causes thinning of the film that lubricates moving parts and causes more internal leakage.
• Acceleration of Oxidation: The degradation rates are roughly doubled for each 18°F (10°C) increase over the temperature at which the fluid is operating at its optimal as a general rule that is widely employed for maintenance of hydraulic fluids.
• Seal and hose degradation: Elastomeric seals as well as hose compounds are able to harden, crack, or lose elasticity if exposed to high temperatures for long periods, which can cause leaks and premature failure.
• Dimensional instability: The thermal expansion of components within tight tolerances, such as servo valves and piston pumps, can impact clearances and repeatability of performance.

Where does the heat source come from within the hydraulic circuit? 

Effective thermal management begins with understanding the source of heat generation. Controlling heat from its source will always be more effective than trying to eliminate it later.

Pressure drop across restrictions

Every time a fluid goes through a limitation such as an escape valve, a flow control valve, or a small orifice such as a throttling orifice, pressure energy transforms into heat. This is among the biggest and easiest to control sources of heat load within an entire system. This is the reason why load sensing and proportional valve technology, which reduces the risk of pressure drop that is unnecessary, is a common feature of the design of systems that are focused on efficiency.

Motor and pump inefficiency

The motor or pump cannot operate in perfect volumetric and mechanical efficiency. The difference in the power input and the output power is absorbed as heat. Pumps that are worn out produce significantly more heat than fresh ones because leakage inside (slippage) grows as clearances wear. This means that an engine that is nearing the end of its life usually appears first as a thermal issue before it becomes an issue with performance.

The cycle of the relief valve

Systems in which relief valves dump the flow back to the tank at full pressure instead of the pump releasing or destroying the tank -- use a lot of energy in the form of heat. This is particularly true in fixed-displacement pumps operating with the same load but not always needing continuous flow.

Line loss and friction

Tubes and hoses that are not sized correctly as well as a high number of fittings and plumbing runs that are long contribute to the generation of frictional heat. It is an irritating mistake when systems have been expanded or modified with time, without revisiting the line sizing.

Strategies for managing core heat

heat exchangers

The primary instrument for taking heat out of hydraulic fluid. Selecting the appropriate type is dependent on the ambient conditions, the frequency of operation, and the availability of services.

The air-cooled (air blast) coolers make use of the power of a fan to push air from the outside across the finned core, which is then carrying hot fluid. They're self-contained and do not require a water source and are the most common option for mobile equipment and facilities that do not have the benefit of a chilled or process water source. The primary drawback of these units is that their cooling capacity is dependent on the temperature of the air in which they are located. Performance is reduced in hot climates and enclosed equipment rooms.

The water-cooled (shell-and-tube) coolers circulate the hydraulic fluid through tubes, which are then surrounded by cooling water. They provide greater consistency in cooling capacity, regardless of the temperature of the air. These are typically found in industrial facilities that are stationary and have access to an air cooling tower or plant water loop and typically have tighter temperatures than air-cooled units of similar capacity.

The correct size of a cooler requires the calculation of actual heat loads (typically calculated in BTU/hr, or kW) according to system inefficiency and not just pump horsepower. Oversizing will waste capital, while undersizing can leave the system exposed to thermal radiation in peak times of operation.

Reservoir design

The reservoir can do more than just store fluid; it's also an active heat exchanger in itself. A properly sized reservoir provides the following:

• Time for air trapped to escape and heat to dissipate from the tank's walls
• Surface area for convective as well as radiant heat loss
• The thermal mass buffers rapid temperature swings in temporary high-load conditions

A standard industry guideline sizes the reservoir at around three times the flow rate of the pump per minute for systems with continuous duty, but this will vary with the conditions in the surroundings, the duration of duty, and whether additional cooling is available. In addition, the confusion within the tank affects the flow, as it splits both suction and return lines, allowing more time for the fluid to cool and settle before being redirected back into the pump.

The selection of fluids and the viscosity grade

Selecting a hydraulic fluid that has an index of viscosity suitable to the operating temperature range of the system ensures a steady fluid lubricating layer across the thermal and load swings. High-VI liquids can withstand thinning at high temperatures better than conventional mineral oils and are particularly important in continuous-duty environments in which the fluid is not given an opportunity to cool in between. In some high-heat-load applications, synthetic or high-temperature-rated fluids are specified specifically for their improved thermal stability and oxidation resistance.

Optimization of efficiency in the system

Since each piece of the hydraulic efficiency turns into heat, reducing the load on heat typically results in improving the overall efficiency of the system:

• Specifying variable-displacement pumps that reduce flow (and therefore heat generation) when full flow isn't needed
• Utilizing flow-sharing or load-sensing structures to minimize losses due to pressure drop
• Correctly sizing fittings and hoses to minimize friction losses
• It is important to address worn components quickly because internal leakage can be a self-reinforcing thermal issue; heat thins fluid. The thinner fluid causes leakage, and leakage creates more heat.

Preventive and routine maintenance

Regular or continuous temperature monitoring at critical points within the system—such as the pump reservoir, drain of the case, and cooler outlet—permits operators to spot temperature trends before they lead to problems. An increase in operating temperatures over weeks or months is usually the first indication of wear on the pump or a clogged cooler or the onset of a restriction prior to the onset of other signs.

Bring it all together

Controlling heat during continuous hydraulic processes does not require a singular solution; it's a combination of proper component sizing, an efficient design of the circuit, the right fluid selection, and regular monitoring. Systems that consider the management of thermal energy as a primary design element, not something to be addressed as an afterthought with an overly large cooler, are more likely to experience longer service life for the fluid as well as fewer seal and hose failures, as well as more consistent performance over longer duty cycles.

Which is the best working temperature for the hydraulic fluid?

The majority of industrial hydraulic systems work best when temperatures of the fluid are kept between 120°F and 140°F (49°C and 60°C). The continuous operation at temperatures above 160°F (71°C) dramatically accelerates the rate of sealing degradation and oxidation.

What happens when the system heats up?

Overheating reduces the viscosity of the fluid and increases leakage into the fluid. It also speeds up the process of oxidation and the formation of varnish and deteriorates seals and hoses—which can result in lower efficiency, increased wear on components, and eventually failure if not addressed.

Do I require an air-cooled or water-cooled hydraulic cooler?

Coolers with air cooling are more simple and are suitable for portable facilities or equipment that lack access to water, whereas coolers that are water-cooled provide an uninterrupted, consistent, and ambient-independent cooling. They are ideal for stationary systems that have high heat loads and access to a water loop in the plant.

How does reservoir size affect heat management?

A bigger well-balanced reservoir offers greater surface area to disperse heat and allows for a longer period of time for the fluid to cool prior to recirculation, which makes sizing the reservoir an important passive aspect in overall temperature control.

Does changing the hydraulic fluids reduce the operating temperature?

Making the switch to a fluid having an increased viscosity index or a better thermal stability may ensure proper lubrication and cut down on heat generated by leakage within the circuit, but it can't make up for a sluggish cooler or an inadequate circuit design.