Energy-wasting pressure drops are an unavoidable feature of hydraulic systems.
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The point is that energy wasting-pressure drops are a fact of life in hydraulic systems. They can (and should) be minimized, but they can’t be completely eliminated. So let’s stop ignoring the elephant in the room. Because if left unchecked, energy contamination is just as problematic as particle contamination, and arguably more so.
Adequate lubrication of hydraulic components and efficient power transmission both depend on appropriate oil viscosity. If hydraulic fluid temperature is allowed to exceed that required to maintain viscosity at around 20 centiStokes (cSt), the likelihood of boundary lubrication—resulting in friction and wear—increases dramatically.
The temperature at which this point is reached depends on the fluid’s viscosity grade and its viscosity index (VI). The VI is a measure of an oil’s resistance to change in viscosity with a change in temperature. An oil with a high VI is often called a multi-grade oil. Multi-grade oils are often specified for equipment that must operate in cold. The high VI helps prevent the oil’s viscosity from increasing (thickening) at low temperatures. However, a high VI also helps prevent its viscosity from decreasing (thinning) at high temperatures.
In other words, the critical temperature as far as viscosity is concerned can be relatively low or high, depending on the oil being used. Furthermore, the higher the maximum operating temperature, the wider the temperature operating window becomes. And the wider the temperature operating window, the more difficult it is to maintain oil viscosity within allowable limits.
For example, consider a hydraulic system with a cold start temperature of 5°C (41°F) and a maximum operating temperature of 110°C (230°F). To maintain viscosity between 800 cSt at cold start and 25 cSt at maximum operating temperature requires an ISO VG 150 oil with a viscosity index of 229. This is not a type of hydraulic oil you can just call in and pick up from your local oil supplier. Hydraulic oils in common use have ISO viscosity grades of 22, 32, 37, 46, 68, and 100. A typical monograde hydraulic oil has a VI of around 100, and a multigrade around 150. So even if it was readily available, a VG 150 oil with a VI of 229 would be a premium-priced product, to say the least.
Regardless of viscosity limits, which are extremely important for proper lubrication and efficient operation, when it comes to oil, seal, and hose life, the top-end danger temperature is less of a moveable feast. According to Arrhenius's Law, for every 10°C (18°F) increase in temperature, the rate of reaction doubles. The chemical reactions we are concerned with concerning hydraulic oil life are oxidation (from the presence of air) and hydrolysis (from the presence of water). So the hotter the oil, the faster the rate of these reactions, and exponentially so.
By way of illustration, if you pour some cooking oil into a glass, it’ll take days (even weeks) before it darkens in color—a sign of oxidation. But if you pour the same amount of cooking oil into a frying pan, which gives the oil a large contact area with air, and then heat it up, and the oil will go black in a much shorter space of time. If oxidative failure of hydraulic oil is allowed to occur, the byproducts of oil oxidation—varnish and sludge—cause reliability problems, such as clogged filters and valve spool stiction.
Effect of operating temperature on hydraulic oil life. Measurements taken in return-side of reservoir. (Source: Facts Worth Knowing About Hydraulics, Danfoss Fluid Power, p.17.)
The dramatic effect operating temperature has on hydraulic oil life is illustrated in the nomogram shown. If a hydraulic system operates at 85ºC (185ºF), oil life is 12% of what it would be if the system operated at 60ºC (140ºF). If the system operates at 102ºC (216ºF), oil life is just 3% of what it would be if the system operated at 60ºC (140ºF).
Heat can damage oil in two ways. The first was just discussed: oxidation. Oxidation is a chemical reaction that is accelerated by high bulk oil temperature and the presence of air. This is “normal” oxidative degradation of the oil.
The second involves intense local heating of the oil. Common causes of localized intense heat are:
What’s important is that the chemical process of these two forms of oxidative degradation are different, so they show up differently on oil analysis reports. Degradation involving intense local heating produces carbonaceous nitro-nitrates in the oil. Its normally shows up as nitration on a Fourier transform infrared (FTIR) spectroscopy oil analysis report. On the other hand, oil that degrades through “normal” oxidation produces metal carboxylates and carboxylic acids. The amount of these acids—and, therefore, the remaining oil life—is what the Total Acid Number (TAN) test seeks to quantify.
Hydraulic oil is not the only thing affected by energy contamination. The elastomers used to make hydraulic seals and hoses are improving all the time. But oil temperatures above 82°C (180°F) accelerate the degradation of most of these polymers. In fact, according to seal manufacturer Parker Pradifa, operating temperatures 10°C (18°F) above recommended limits can reduce seal life by 80% or more.
Similarly, according to hose manufacturer Gates, exposing a hydraulic hose to an operating temperature 10°C (18°F) above its recommended maximum cuts its expected service life by 50%. This means a single, over-temperature event of significant magnitude can damage all the hoses and seals, “crack” the oil, and result in scuffing and wearing of lubricated surfaces.
If all this isn’t bad enough, the constant cycle of heating and cooling—a process known as aging—is more severe when temperature extremes are greater. Aging causes the polymers used in seals and hoses to lose their elastic properties. The ultimate result is leaking hoses and seals.
So what is the dangerous operating temperature number for hydraulic systems? For reasons already explained, to avoid compromising oil, hose, and seal life, I always work on a maximum of 85°C (185°F). However, to avoid compromising viscosity, lubrication, and system efficiency, a much lower temperature may have to be respected: from 85°C (185°F) down to around 50°C (122°F)—or perhaps lower, depending on the grade and type of oil being used, and by extension, the climatic conditions the machine operates in. Staying below these maximum temperature limits year-round will, in most applications, require that the machine’s hydraulic system uses an oil cooler—and in many cases, a big one.
Walter Isaacson’s biography of Steve Jobs tells a story about Jobs demanding that one of the early Macintosh computers could not contain a cooling fan. It was Jobs’ contention that a cooling fan detracts from the user experience—and I’m inclined to agree. When the cooling fan on my Alienware X51-R2 revs up, I do find it mildly annoying. Anyway, despite the initial resistance from his engineers at the time, Jobs prevailed, and the model shipped did not contain a cooling fan on its enclosure.
I mention this story because many hydraulic machine builders and end users share the late Jobs’ dislike for heat rejection devices—oil coolers, in our case. But it can’t be said that an oil cooler on a hydraulic machine detracts from the user experience. Instead, an oil cooler—or an oil cooler of sufficient size—is resisted due to one or more of the following:
In fact, during a time when I was designing and building hydraulic power units for end users, I can recall dealing with customers whose desire was no oil cooler and the smallest tank possible!
The idea that increasing tank oil volume can eliminate the need for an oil cooler is flawed—on all but the smallest of systems, in terms of input power. The formula for calculating heat convection from the tank, in SI units, is:
P = ΔT × A × h ÷ 1,000
Where:
P is heat rejected, kW
ΔT is temperature difference between the oil and air, °C
A is surface area of the tank, excluding base, m2
H is the convective heat transfer coefficient for air, W/(m2°C)
Use 12 for a normally ventilated space, 24 for forced ventilation, or 6 for poor air circulation.
Let’s consider a tank with an oil volume of 200 liters and area (excluding its base) of 1.7 m2, an ambient air temperature of 35° C, and an operating oil temperature of 85° C. In a “normally ventilated” space, the theoretical heat rejection of the tank is:
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(85-35) × 1.7 × 12 ÷ 1,000 = 1 kW
For the purposes of illustration, let’s say this calculation is too conservative, so we’ll double the above number. In other words, we expect the 200-liter tank will dissipate 2 kW of heat. Working backwards from this number, if we want the hydraulic system to have an installed cooling capacity of 25% of input power (and the tank is the only installed cooling), then the maximum, continuous input power allowable is just 8 kW! Obviously, the idea of a big (or bigger) tank in lieu of an oil cooler is not realistic in most applications.
If you accept the fact that energy-wasting pressure drops are unavoidable in any hydraulic machine, then so, too, is an appropriately sized oil cooler in most applications. Because when it comes to hydraulic machine reliability, uncontrolled energy contamination can be just as bad, or worse, than uncontrolled particle contamination.
Brendan Casey has more than 26 years of experience in the maintenance, repair, and overhaul of mobile and industrial hydraulic equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his website.
Heat kills hydraulic systems. That’s why hydraulic oil coolers are widely used in a diverse range of agricultural, mobile, manufacturing, and industrial settings. They remove excess heat generated by energy losses in a system, as well as that from external sources such as engines, furnaces, and even the surrounding environment.
In fact, coolers are often essential for designing temperature-optimized hydraulic systems that keep oil temperatures within a limited range. Such circuits are basic prerequisites for cost-efficient operation, as they provide a number of performance, economic, and environmental benefits. These include:
Considering the benefits coolers offer, it’s apparent that accurately sizing them is a paramount concern for design engineers. Undersizing obviously allows higher-than-recommended oil temperatures. But oversizing hurts system efficiency as well, by reducing temperatures below the recommended range and increasing costs with a larger-than-necessary purchase. Here are some tips on selecting the right type and size cooler for a specific application.
Several different types of coolers are suitable for hydraulic systems, with air and water cooled being the most common.
Air-cooled systems dissipate heat with flowing air. This type of cooler operates by forcing cold air over hot fluid inside a heat exchanger’s coil or core. Low running cost and simple maintenance are attractive benefits of air-cooled systems. They also eliminate water-contamination problems and minimize corrosion. Additionally, heat generated from air-cooled systems can be used for other purposes.
On the downside, the upfront expense of an air-cooled system exceeds that of a water-cooled unit. Air-based versions are also larger and generate more noise, so they can degrade the work environment. What’s more, these coolers require ventilation and clean air, and variations in ambient temperature can greatly affect cooling capacity — making steady-state operation more difficult to maintain.
Water-cooled systems use cold water to remove heat, with hot and cold fluids separated by a barrier. Compared to air-cooled systems of equivalent capacity, water-cooled systems have lower up-front costs. They’re also quiet, compact, and do not alter the surrounding ambient temperature. Variations in air temperature have little or no effect on cooling capacity, which allows for greater consistency. Meanwhile, the system’s heated water can be used for other on-site purposes.
Although water-cooled coolers cost less, they generally require clean water. Continuously running water can be expensive, leading to higher operating costs. Other concerns include risk of corrosion and erosion, and increased maintenance. Water-cooled units can also freeze in cold weather, letting water contaminate the oil.
Another consideration is whether the cooler will be plumbed in the return line or in an off-line recirculation loop. It can make a big difference. Sizing coolers installed in return lines is rather straightforward. Coolers can either be sized using maximum system temperatures or maximum reservoir temperatures. Oil temperature entering the cooler is the system temperature, and oil temperature exiting the cooler is the reservoir temperature.Sizing off-line coolers can be much more complicated, depending on pump inlet location in relation to the return-line outlet, and whether or not there are any baffles between the two. In a worst-case scenario, the cooler is sized such that inlet temperature to the cooler matches the required reservoir temperature. This means oil exiting the cooler is colder than the desired reservoir temperature. But this cold oil mixes with hot return-line oil that has not been cooled at all. Mixing the two sources of oil, along with the locations of the oil lines relative to each other, complicates modeling of actual reservoir conditions.
In addition to the above considerations, other application factors are also important. When opting for air-cooled hardware, be aware of altitude, maximum ambient air temperature, and the type of motor driving the fan. Ac, dc, and hydraulic motors are all suitable.
On the surface, the type of motor may seem of little consequence to an oil cooler’s capacity, as long as a motor provides the necessary power to drive the fan. But motor type can affect the size of a cooler. Typically 12 and 24-Vdc fan motors do not have the power density of ac motors. That means dc fan drives cannot push or pull as much air across a core as can ac drives. Because less air crosses the core, cooling capacity is lower as well. As a result, a dc fan drive may require a larger cooling package than would an ac drive.
With water-cooled coolers, first determine the type of water the unit will use — fresh, salt, or water/glycol mixture. If opting for water/glycol, it is important to clarify the type of glycol as well as the water-to-glycol ratio. Also determine the maximum water flow rate and maximum allowable waterside pressure drop. These figures are not always obtainable, but they help improve cooler selection.
Given the necessary data, engineers can then turn to sizing. Coolers can be sized using software programs or by interpolating graphs, both of which are available from cooler manufacturers. Software offers the simplest and quickest solution. Users need only enter the recorded data into the proper fields, letting the software calculate the optimal size.
For example, Parker has an air-cooled cooler sizing program that engineers can download at http://tinyurl.com/mf5nv73. Inputs for this program include the type of oil and flow rate, inlet and ambient temperatures, heat load, and altitude. Outputs include inlet and outlet oil and air temperatures, specific heat dissipation, oil pressure drop, and noise level.
The program can select a cooler based on input data, or the user can pick a certain cooler and analyze its capability. However, a software program is only as “intelligent” as the data it receives. For this reason, knowing how to interpolate graphs can help engineers better understand cooler sizing and selection.
The heat-rejection capacity of a cooler varies based on the type of fluid, fluid flow rate, difference between the two fluid temperatures, and other factors. Graphs typically express heat load versus fluid flow rate at a given set of conditions.
Graphs are based on either entering temperature difference (ETD) or approach temperature. ETD refers to the temperature of the hot fluid entering the cooler minus the temperature of cold fluid (water or ambient air, depending on the cooler type) entering the cooler. ETD curves present data on a “per degree” basis or a nominal temperature difference, typically 50 or 100°F. This means, for example, that a 50°F ETD is the result of 150°F hot-fluid temperature minus 100°F cold-fluid temperature.
Approach temperature refers to the temperature of the hot fluid at the cooler’s outlet minus the temperature of the cold fluid entering the cooler. Approach temperature curves are generally based on an approach temperature of 40°F.
To simplify, ETD looks at the temperature difference prior to the cooling process; whereas, approach temperature uses the final temperature attained through cooling.
Graphs may require users to convert units of measure to match a graph’s units for heat load and flow rate. Common units of measure for heat load include:
Often when interpolating graphs, engineers must apply correction factors for parameters like oil viscosity, altitude (for air-cooled), motor frequency (for air-cooled with ac motor) and oil-to-water ratio (for water-cooled without individual curves for various oil-to-water ratios). There may be safety factors to consider as well.
For example, graphs on page 36 show cooling capacity and pressure drop for Parker OAW water-type coolers. The cooling curves are based on specific conditions. These include ISO VG 32 oil, an oil/water ratio of 2:1, and an oil/water inlet difference of 60°F. For other conditions, engineers must use the following correction factors.
Correction factors for other oil types.
Cooling capacity: Multiply the desired cooling capacity by correction factor Kv.
Oil-pressure drop: Multiply pressure drop by correction factor Kp.
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