These calculations show that thermal resistance of stainless steel tubes in the fuel oil preheater will be less then thermal resistance of copper tubes, accumulated coked hard crust deposit of hydrocarbons. The thermal resistance of the tubes is only a part of total resistance of heat transfer, and the effect of fouling and tubing material can be accurately calculated in the process of the thermal design of a heat exchanger.
|k- Btu/(hr ft deg. F)||225.00||8.4|
|t/k – (hr sq. ft deg. F)/Btu||0.0000181||0.0004861|
|Fouling||0.005||0.005 (per TEMA)|
Where: t/k-Thermal Resistance of the tubes, t-wall thickness, k- coefficient of thermal conductivity.
Regretfully, TEMA Recommended Good Practice values of Fouling Resistances for Industrial Liquids do not take in consideration effect of tube material.
If the actual Fouling Resistance of # 6 Fuel Oil in the heater made from stainless steel tubing equals to 0.0025, then:
The coking of hydrocarbons accelerated at elevated temperature of heating tubes. The evaporation of light hydrocarbon residue fractions definitely accelerates coking.
As a good practice, The Alstrom Corporation recommends to maintain the temperature of heating media about 120 deg. F above the outgoing temperature of the #6 heating oil or less.
|#6 Fuel Oil Outgoing Temperature, deg.F||Recommended Temperature of Heating Media, deg.F||Maximal Steam Pressure, psig|
The excessive steam pressure can be used to size temperature regulator.
These recommendations are valid for asphalt and other highly viscous liquids. For instance, heating syrups may result in carmelization of the fluid.
Similar, but in less degree, phenomena occurs in shell & tube heat exchangers for water heating. Indeed, copper tubes are oxidized, resulting in fast accumulation of solid deposits.
Copper has poor mechanical properties, particularly on elevated temperatures common in heat transfer.
Tube-to-tube sheet joint of copper tubes with other materials commonly made by tube expansion. ASME Boiler & Pressure Vessel Code estimates efficiency if expanded joint 60-65%. Stainless steel tubes can be not only expanded but also seal welded to the tubesheet, resulting in 100% joint efficiency. In this case, according to the Standard for Power Plant Heat Exchangers of Heat Exchange Institute, the metal temperature of welded joint can reach the maximum value permitted by ASME Code.
After introducing 316 stainless steel tubes as a standard material of fabrication shell & tube heat exchangers and using expansion-welding technology The Alstrom Corporation was never reported about tube damage or leak in the tube-to-tubesheet joint.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)