3. Oil as a heat transfer fluid

Heat in a system may be caused by two things: friction and external temperature. A large volume of high-pressure oil spilling through a relief valve or passing through long lengths of piping is bound to create excessive heat. Very high external temperatures will transfer considerable heat into a system. For example, a hydraulic system that feeds and controls the opening and closing of furnace doors is subjected to intense heat. Heat transferred to the piston rod of the furnace’s pusher cylinder will, in turn, be transferred to the hydraulic oil. 

Heat causes oil to become less viscous and also causes it to deteriorate. Heat will cause packings to leak, valves to malfunction and pumping equipment to lose its efficiency.

Packings will become brittle, especially if they are made of synthetic material or leather.

The close-fitting precision parts of valves often seize when excessive temperatures are present. 

Oil Cooling will improve 

• Performance - The correct oil temperature will give the right oil viscosity, resulting in good lubrication and optimal system performance. 

• Lifetime - The correct oil temperature will extend the lifetime of the oil, which would otherwise degrade. 

• Maintenance -The correct oil temperature will decrease the wear on other components, minimizing the need for maintenance.

Crude oil or petroleum is a mixture of organic chemicals, derived mainly from the remains of microscopic plants and animals that lived in our seas many millions of years ago.

Resting on the seabed, the remains of these creatures formed sedimentary layers with mud and silt. These were buried beneath other sediments until they were compressed into rock, where their organic material changed into alkanes (C5H12 - C30H62) and aromatic hydrocarbons, all compounds of hydrogen and carbon, and became gas and oil. The carbon atoms link together in chains of different lengths and structures. An example of a hydrocarbon is shown in Figure 3.1.

Figure 3.1: Decane (C10H22), anl alkane

Oil as a heat transfer fluid must not only transmit power; it must also act as a lubricant and sealant.  It must therefore have certain basic properties, the most significant of which is viscosity.

Viscosity is a material’s resistance to a change in form, and it arises from the forces between molecules. This property can be thought of as an internal friction. To understand viscosity, it is important to understand laminar flow (see Basic Heat Transfer/Flow Regimes). If a fluid is flowing over a surface, the molecules next to the surface (those clinging to the walls) have zero speed. The speed of the molecules increases as their distance from the wall increases. This difference in speed represents friction in the fluid or gas as molecules are pushed past each other. The “stickiness” between the molecules – the viscosity – will be proportional to the friction. Thus, viscosity determines the amount of friction, which in turn determines the amount of energy absorbed by the flow. Figure 3.3 shows the relative viscosities of two liquids.

Figure 3.3: The viscosity of an oil controls the thickness of the oil film under hydrodynamic lubrication conditions. Oils become thinner when heated, and viscosity must always be related to temperature. 

Dynamic and Kinematic Viscosity

There are two related measures of fluid viscosity: dynamic (absolute) and kinematic viscosity.

Dynamic Viscosity is the tangential force per unit area required to move one horizontal plane with respect to the other at unit velocity when maintained a unit distance apart by the fluid. For a Newtonian fluid, the shearing stress between the layers of a non-turbulent fluid moving in straight parallel lines can be defined as shown in Figure 3.4.

Fluids for which the shearing stress is linearly related to the rate of shearing strain are designated Newtonian. Newtonian materials are referred to as true liquids, because their viscosity or consistency is not affected by shear such as agitation or pumping at a constant temperature. Fortunately, most common fluids, both liquids and gases, are Newtonian. Water and oils are examples of Newtonian liquids.

 Figure 3.4: Dynamic viscosity

The equation in Figure 3.4 is known as Newton’s Law of Friction.

In the SI system the dynamic viscosity units are N·s/m², Pa·s or kg/m·s, where: 

1 Pa·s = 1 N·s/m^² = 1 kg/m·s

The dynamic viscosity is often expressed in the metric CGS (centimeter-gram-second) system as g/cm·s, dyne·s/cm^² or poise (P), where: 

1 P = 1 dyne·s/cm^² = 1 g/cm·s = 1/10 Pa·s

The poise is too large for practical use, and is usually divided by 100 into the smaller centipoise (cP), where:

1 P = 100 cP

Water at 20.2 °C (68.4 °F) has an absolute viscosity of 1.0 cP.

Hydraulic fluids have viscosities ranging from 32 to 100 cP at 40 °C (100 °F).

 Kinematic Viscosity is the ratio of absolute or dynamic viscosity to the density.

 ν = μ/ρ,

where: 

ν = kinematic viscosity [kg/m·s]

μ = dynamic viscosity [m^²/s]

ρ = density [kg/m^³]

For the SI system the theoretical unit is m²/s, or the commonly used Stoke (St), where: 

1.0 St = 10^-4 m²/s.

The Stoke is also too large for practical use, and is usually divided by 100 into the smaller centistoke (cSt), where:

1 St = 100 cSt and 1 cSt = 10^-6 m²/s

Because the density of water at 68.4 °F (20.2 °C) is almost 1000 kg/m^³ it follows that the kinematic viscosity of water at 68.4 °F is for all practical purposes 1.0 cSt. Viscosity is highly temperature-dependent, and for either dynamic or kinematic viscosity to be meaningful a reference temperature must be quoted. The rate of change of viscosity with temperature is indicated on an arbitrary scale called the viscosity index. The higher the numerical value of this index, the less the viscosity changes for a given change in temperature, and vice versa. For liquids, the kinematic viscosity decreases with higher temperatures. For gases, the kinematic viscosity increases with higher temperature. Figure 3.5 shows how the kinematic viscosities of some oils change with temperature.

 Figure 3.5: Viscosity Index

Measuring Instrument

The viscosity of an oil is measured in an instrument called a viscosimeter. There are several types, but for commercial purposes the viscosimeter is essentially a container in which the oil is held and allowed to run out at the bottom through a hole or tube of established size.

The reading is taken for a standard quantity of oil at a standard temperature. The time of flow is measured in seconds, and the viscosity reading is expressed as Saybolt Universal Seconds (SUS or SSU). 1 SSU = 4.55 cSt. The majority of systems operate most efficiently with fluids falling within the ranges 135-165 SSU, 185-230 SSU and 275-315 SSU.

In North America, it has been the practice for many years to define the viscosity of industrial lubricating oils in Saybolt Universal Seconds (SUS) at a reference temperature of 100 °F (40 °C). However, there is now worldwide acceptance of the International Organization for Standardization’s (ISO) proposal to establish kinematic viscosity measurements.

Viscosities designated by various societies or organizations are compared in figure 3.x. This is strictly a comparison of viscosities, and in no circumstance should it be construed as a qualitative comparison. To summarize the abbreviations in Figure 3.x:

The ISO VG system is a grading system that is generally used to describe industrial oils, i.e. oils used in stationary plant (pumps, turbines, gearboxes, compressors, etc.). ISO VG is the kinematic viscosity in centistokes at 40 °C. The beauty of this system is that the name of the oil denotes its viscosity. For example, Caltex Meropa 460 is an industrial gear circulating oil with a viscosity of 460 cSt.

 Figure 3.6: ISO System

Generally, the lower viscosity oils are hydraulic fluids and the higher viscosity oils are gear fluids. There is no exact cut-off point where gear oils become hydraulic oils, but ISO 150 is a good approximation. 

The actual viscosity of an ISO oil should not be expected to be exactly the same as its nominal viscosity. According to the ISO, 10% leeway is allowed either way, so any industrial oil with a viscosity between 90 and 110 cSt would be rated as ISO 100.

There are some intermediate grades in common usage that are not ISO-approved. These oils have viscosities of 37, 56 and 77 cSt. Although this numbering system may appear illogical, the viscosity of each subsequent grade is approximately 50% higher than that of the previous grade. This gives a range of products wide enough to meet industry’s needs without flooding the market with a different grade for each centistoke increase in viscosity.

SAE is the Society of Automotive Engineers’ viscosity measurement for automotive engine and gear oils (e.g. SAE 30, SAE 90, etc.). To avoid confusion it is divided into two subclasses, one for gear oils and one for engine oils. A number higher than 60 means the oil is formulated for a gear-type component, while a lower number corresponds to oil for use in the engine.

Unlike the ISO system, the SAE system does not give the viscosity of the oil in centistokes at 40 °C, although the higher the number, the higher the viscosity. The SAE grades are more carefully quantified than the ISO oils; both dynamic and kinematic viscosities are used, as well as both 40 °C and 100 °C temperatures. Grades with the letter ‘W’ are used at lower ambient temperatures, and are classified according to a maximum low-temperature dynamic viscosity and a maximum borderline pumping temperature as well as a minimum kinematic viscosity at 100 °C. The dynamic viscosity measurement correlates with engine speeds during low-temperature cranking, while the borderline pumping temperature measures the oil’s ability to flow to the engine oil pump and provide adequate oil pressure during start up. Grades without the ‘W’ are used in more severe operating conditions, i.e. higher temperatures and pressure differences, and are based solely on their kinematic viscosities at 100 °C. SAE gear and engine numbers cover the same range of viscosities. For example, SAE 30 engine oil has approximately the same viscosity as an SAE 85W gear oil. This is because the formulation of engine oils is very different to that of gear oils in the automotive industry. Engine oil is much more stressed than gear oil because it must cope with combustion by-products and blow-by gases that severely degrade the oil. As a result, engine oils contain a much wider variety of additives than do gear oils. Although not ideal, engine oil will function in a gearbox while a gear oil will destroy an engine.

Freedom from corrosion is another important factor to be considered. The neutralization number of new oil (indicating its degree of acidity or alkalinity) may be satisfactory, but after use the oil may tend to develop corrosive tendencies as it begins to deteriorate. The base stock should have been processed with the specific aim of inhibiting the formation of harmful acids that would attack bearings or finished metal surfaces in the system.

Another form of corrosion is rusting, which must be combated in a different manner. Many systems are idle for lengthy periods after a run at elevated temperatures. This permits moisture to condense in the system, resulting in rust. Furthermore, a machine tool using a water-soluble coolant may not be entirely sealed, allowing water to leak into the system. Rust-preventive additives, usually synthetic chemicals, are often used in oils. These corrosion inhibitors are are present in the system in relatively small quantities, and reduce metal loss due to corrosive attack. Inhibitors can form a protective barrier against corrosive agents on the metal surface.

One of the most popular tests for determining the rust-preventive properties of an oil is the ASTM (American Society of Testing and Materials) test. This involves stirring a mixture of oil and water in which a cylindrical steel specimen is immersed. After 48 hours of contact with the mixture, the specimen is examined for signs of rust.

The standard material for SWEP brazed plate heat exchangers is stainless steel AISI 316, vacuum-brazed with pure copper-based filler. Stainless steels are iron-based alloys containing at least 10.5% chromium. Stainless steels can provide an extraordinary range of corrosion resistance, depending on their chromium content and the presence or absence of some ten to fifteen other elements.

Oils in general do not usually present problems with the stainless steel and copper used in brazed plate heat exchangers. However, there are some exceptions: 

1. Sulfur Content

Sulfur is an element occurring naturally in crude oil. It becomes concentrated in the residual component from the crude oil distillation process. The amount of sulfur in fuel oil depends mainly on the source of the crude oil and, to a lesser extent, on the refining process. Sulfur is one of only a few substances detrimental to copper. In general, brazed plate heat exchangers are not recommended for oil with a sulfur content above 1% (temperature <100 ºC). At temperatures >100 ºC the sulfur content should be <1%. An additive in the oil, e.g. a corrosion inhibitor, may permit a higher sulfur content in the oil.

Instead of copper, SWEP brazed plate heat exchangers can be brazed with AISI 316 (All-Stainless) or nickel alloys that are resistant to high contents of both sulfur and ammonia. The permitted sulfur content of oil (and coal) is falling steadily due to environmental regulations and taxes. For example, the maximum sulfur content of Swedish light diesel oil is 0.1%, and less than 0.01% in city diesel.

Furthermore, the presence of water and chlorides (e.g. salt water) in the oil could make the corrosion worse. A corrosion index (e.g. test with copper plate) is therefore desirable.

In the test parameters of the ASTM norm, a copper coupon is immersed in a measured amount of engine oil. The oil is aerated at an elevated temperature of 135 °C. At the end of the test, e.g. 48 hours, the coupon and the stressed oil are examined to detect corrosion. The pass limit is ≤100 ppm (parts per million) of dissolved copper in the oil. Where the oil’s tendency to corrode exceeds that value and All-Stainless is not an alternative, SWEP recommends using Minex with titanium channel plates.

2. Special Thermal Oils

Thermal oils, which provide accurate heat control for applications requiring temperatures between 125 and 400 ºC, do not usually corrode the copper in a brazed plate heat exchanger. However, some thermal oils contain additives that could be destroyed by the copper. These thermal oils should therefore not be used with components containing copper. 

3. Vegetable Oils

Vegetable oils could be used with copper without risk of it being corroded. However, copper may oxidize vegetable oils, which would impair their quality. Copper is therefore little used in the manufacture of vegetable oils.