Heat Exchanger Sizing Fundamentals

Hydraulic oil coolers are sized based on the heat load that must be rejected and the temperature difference available between the oil and the cooling medium (air or water). The Log Mean Temperature Difference (LMTD) method provides accurate sizing for heat exchangers.

LMTD Calculation

The LMTD accounts for the changing temperature difference along the heat exchanger length:

LMTD = (dT1 - dT2) / ln(dT1 / dT2)

Where dT1 is the temperature difference at the inlet end and dT2 is the difference at the outlet end. For counter-flow exchangers (most efficient):

dT1 = T_oil_in - T_coolant_out

dT2 = T_oil_out - T_coolant_in

Required Heat Transfer Area

Once LMTD is known, the required heat transfer area can be calculated:

Q = U x A x LMTD

Therefore: A = Q / (U x LMTD)

Where U is the overall heat transfer coefficient, which depends on the cooler type and fluid properties.

Air-Cooled vs Water-Cooled Comparison

Air-Cooled Heat Exchangers:

• Lower U-values (20-60 BTU/hr-ft2-F typical)
• Larger physical size required
• Limited by ambient air temperature
• No water consumption or treatment needed
• Simpler installation, lower operating cost
• Fan noise considerations

Water-Cooled Heat Exchangers:

• Higher U-values (100-200 BTU/hr-ft2-F typical)
• More compact design
• Can achieve lower oil temperatures
• Requires water supply and treatment
• Shell-and-tube or plate type construction
• Higher efficiency but more complex

Typical U-Values for Hydraulic Oil Coolers

Design Considerations

Safety Factor: Size coolers 20-50% larger than calculated to account for fouling, ambient temperature variations, and peak loads.

Approach Temperature: The minimum temperature difference between oil outlet and coolant inlet. Air-cooled: 15-25 deg F minimum. Water-cooled: 10-15 deg F minimum.

Oil Viscosity: Higher viscosity oils have lower heat transfer coefficients. Adjust U-values down 20-30% for ISO 68 and higher viscosity grades.