Many of the circuit examples in this chapter use fixed-volume pumps. Fixed-volume pumps always generate heat when used with any flow-control circuit. Also, flow controls always generate some heat regardless of the type pump used. Figures 10-52 through 10-55 show meter-in flow-control circuits and explain how different type pumps affect heat generation.

Checking horsepower loss is one way to determine the amount of heat generated by a circuit. To figure horsepower in a hydraulic system use the formula:
(horsepower) = 0.000583 (pressure in psi) (flow in gpm).
Multiply the horsepower by 2545 to calculate the amount of heat produced in British Thermal Units per hour.

fig 52

Fig. 10-52. Meter-in flow-control circuit with fixed-volume pump – cylinder extending.

In the circuit in Figure 10-52, 7 gpm flows across the relief valve at 1000 psi. When this oil reaches system pressure, it flows to tank without doing useful work. If you multiply 0.000583 (7 gpm)(1000 psi)(2545 BTU/hr), the heat loss comes to 10,386 BTU/hr. Add to this the heat loss due to the 3 gpm at 900-psi pressure drop passing through the flow control (4006 BTU/hr). Thus, when the cylinder is moving under the conditions shown in Figure 10-52, more than 14,000 BTU/hr of energy turns into heat. The maximum heat that the unit could produce is 0.000583(10 gpm)(1000 psi)(2545 BTU/hr) or 14,837 BTU/hr. Therefore, approximately 97% of the system’s energy is wasted heating the fluid.

fig 53

Fig. 10-53. Meter-in flow-control circuit with pressure-compensated pump – cylinder extending.

The pressure-compensated pump in Figure 10-53 produces only the flow needed, so there is no oil going over a relief valve. The only heat loss in this circuit is the 900 psi across the flow-control valve. There are 10,000-BTU/hr fewer entering the system just by changing the type of pump.

fig 54

Fig. 10-54. Meter-in flow-control circuit with load-sensing, pressure-compensated pump – cylinder extending.

To cut even more heat from the system, the circuit in Figure 10-54 uses a load-sensing pressure-compensated pump. This pump has a sensing line that monitors the pressure required to move the cylinder, then sets the compensator 150 to 200 psi higher. With a 100-psi cylinder requirement, the pump would operate at approximately 250 psi. This low pressure drop across the meter-in flow control generates a heat loss of just 668 BTU/hr.

To sense the load at both ends of the cylinder, or if there is more than one cylinder to control, the sensing lines come back to the pump through check valves. These check valves allow the pump to see the system’s highest pressure requirement and set the pump pressure 150 to 200 psi above it.

When using a load-sensing pump, always use a meter-in flow-control circuit. A meter-out circuit shows pressure at both ends of the cylinder all the time it is moving. With pressure at the load-sensing port, the pump would go to compensator setting and stay there. (For more information on load-sensing pumps, look in Chapter15.)

fig 55

Fig. 10-55. Meter-in flow-control circuit with variable-volume pump – cylinder extending.

Heat generation for the circuit in Figure 10-55 is the lowest possible when variable speed is necessary. In this circuit, a variable-volume pump set for 3-gpm flow replaces the flow controls. Because the cylinder uses all the flow produced, system pressure only goes to the 100 psi required to move the cylinder. There is no excess horsepower, so no heat is generated. (In actual practice, pump inefficiency, pressure drop in the lines, and friction between parts generate some heat. These losses may cause the system temperature to rise 5 to 15° above ambient temperature.)

When practical, a variable-volume pump provides the best way to control actuator speed. Variable-volume pumps may require some electronic controls if there is more than one cylinder, but lower operating cost for the life of the machine quickly offsets this one-time first cost.