Hydraulic reservoirs do a whole lot more than simply hold fluid. They can also help or hinder efforts to keep oil clean and cool, depending on the design of the reservoir and other relevant hydraulic components. While the ideal hydraulic reservoir would be nice and large, in the constraints of a mobile system this is seldom practical due to the increased weight and bulk it would add to the machine. Therefore, in practice mobile equipment operators must balance the desire for optimum reservoir performance with the need to keep immediate operational costs down (specifically fuel consumption).
Nonetheless, it is important to understand the many functions of a hydraulic reservoir in a mobile system, as this can help you evaluate whether or not your reservoir is providing the performance you desire.
The time spent in the reservoir is an important opportunity for a fluid to cool after passing through other parts of the system. Reservoirs facilitate heat loss by radiating heat out through the reservoir walls, so the greater the surface area of these walls and the longer the fluid is allowed to remain in the tank, the greater the cooling power will be. Ideally, the reservoir would be able to accommodate three times the pump flow. So for a pump doing 25 gpm, the ideal tank size would be 75. However, in practice the reservoir in such a mobile system would be closer to 30 or 40 gal, and a cooler might be added to the system to accommodate for the reduced size.
A larger reservoir is also better for removing air from hydraulic fluid, because the longer the fluid remains in the tank the more time there is for bubbles to rise out of the fluid. When a smaller reservoir is necessary, a baffle plate can be installed. This will help trap bubbles on the underside of the plate and prevent them from recirculating and causing cavitation damage to cylinders and other hydraulic parts.
The longer fluid remains in the reservoir, the more opportunity there is for particulates to settle out of the fluid, which helps to reduce wear and tear on the system that would otherwise be caused by these contaminants. Again, the solution is to compensate for a smaller reservoir with a baffle plate. This will force fluid through longer paths to the pump inlet to help capture particles and prevent them from recirculating.