In our last discussion (see January 1997, DP) we looked at the "how" of contamination control as applied to the hydraulic systems of mobile equipment. We reviewed typical forms of contamination such as particulates, water, oxidation products and many others, as well as ways these contaminants can destroy hydraulic systems. We mentioned, but did not explore to any great extent, one of the most destructive forms of hydraulic system contamination, heat.

A fundamental cause of heat generation in a hydraulic system is raising and lowering the pressure level of the fluid in the system without accomplishing work. This is not to ignore external or ambient sources of heat transferred into the hydraulic system. These typically fall into the province of the equipment manufacturer and include: mechanical losses in gearing, clutches, brakes, etc.; engine heat; high environmental temperatures and overload design conditions which are part of equipment performance envelope design.

Heat always moves from an area of higher temperature to one of lower temperature. Three modes of heat transfer include: conduction, convection, and radiation.

Conduction is the transfer of heat energy along, through or between physically continuous bodies - shafts, tubes, plates, structural members, valve bodies, pump housings, fittings, hoses, etc. The rate at which heat is conductively transferred is defined by a conductive heat transfer coefficient. Different materials have different conductivity coefficients.

Convection is the transfer of heat by means of actual motion or displacement of gases or liquids upon being heated. Common examples are the column of air you feel rising from a hot surface or the circulation of hydraulic fluid in a reservoir due to hot oil being returned near the bottom into the cool oil. Each fluid has a convection heat transfer coefficient assigned to it.

Radiation is the transfer of energy from a hot surface in wave form. As before, each body has a characteristic radiation heat transfer coefficient.

When an engineer tackles the heat problem he has to look at what is called a heat balance. Basically, that is: heat generated ([q.sub.e]) + heat absorbed by system ([q.sub.a]) + heat dissipated ([q.sub.d]) = heat transferred to ambient environment,or:

[q.sub.e] = [q.sub.a] + [q.sub.d] = heat to the environment.

We don't lose the energy, we just transform it and move it around.

So:

[q.sub.e] Btu/hr = [q.sub.a] Btu/hr + [q.sub.d] Btu/hr;

and lose the use of some of it in the process. Horsepower = 42.44 Btu/min = 2546.4 Btu/hr where Btu (British Thermal Unit) is a defined quantity of heat energy. To attack the heat balance calculations analytically is very complex.

In order to make the hydraulic system heat calculations more manageable, some fluid power companies have conducted extensive tests and reduced results to empirical curves. The curves shown in Fig. 1.1 are based on tests run on different types of pump-reservoir designs with pump delivery discharged over a relief valve to the reservoir.

While test results have been normalized to relate square feet of external surface required to transmit 1 hp equivalent heat energy from oil-to-steel-to-air, judgment should be used in applying the curves to specific hydraulic installations. The curves shown in Fig. 1.2 relate rate of heat generation by auxiliary pumps in hydraulic systems to flow and pump efficiency. Fig. 1.3 provides heat generation for power pumps at varying flow rates and efficiencies. The curves of Fig. 1.4 show power loss in hydraulic system piping for oil with a viscosity of 200 SSU.

Factors generating heat in a hydraulic system include flow restriction or throttling of the system or excessive flow velocities within the system. For example, with a 0.5 in. o.d. smooth pipe, a flow rate of 10 gpm generates heat at the rate of about 25 Btu/ft.hr. Doubling the flow to about 20 gpm increases heat generation eight times to about 200 Btu/ft.hr. Note however, that if you specified a 1 in. o.d. pipe, the flow rate of 20 gpm generates heat at the rate of only about 10 to 15 Btu/ft.hr. With a 1 in. pipe, the flow rate would have to be about 65 gpm.

Slip in pumps will cause heat generation as will internal leakage in valves and gas-filled accumulators. Pulsating accumulators may develop high temperatures on the gas side, well above the temperature of the oil.

Entrained air and the expansion and compression of entrained air can also cause localized heating. Unlike dissolved air, entrained air is held mechanically by the fluid. Tests have shown that these processes can raise temperatures as much as 25 percent.

Nonregenerative release of potential energy also causes heat generation because when a load is lifted hydraulically, potential energy is stored in the load. Release of the load usually involves nonregenerative throttling, which generates heat.

Mechanical losses in rotating or reciprocating components have been shown to cause heat generation.

Auxiliary pumps, do no useful mechanical work, the entire energy input to such a pump contributes to heat generation in the system.