Heat losses through lagging

The rate of heat loss per unit area is given by  

For the case of heat loss to the atmosphere from a lagged stemn pipe, the thennal resistance is due to that of the condensate film and dirt on the inside of the pipe, that of the pipe wall, that of the lagging, and that of the air film outside the lagging. Thus for unit length of a lagged pipe  

Thus the temperature on the outside of the lagging is (444 — 134) = 310 K, which approximates to the assumed value. 

Thus (hr -(-hc)= 11.1 W/m K, which is close to the assumed value. In practice it is rare for forced convection currents to be absent, and the heat loss is probably higher than this value. 

If the pipe were unlagged, (hr + (i,) for AT = 150 K would be about 20 W/m K and the heat loss would then be  

---

In an operating column the effective reflux ratio will be increased by vapour condensed within the column due to heat leakage through the walls. With a well-lagged column the heat loss will be small and no allowance is normally made for this increased flow in design calculations. If a column is poorly insulated, changes in the internal reflux due to sudden changes in the external conditions, such as a sudden rain storm, can have a noticeable effect on the column operation and control. 

It has been demonstrated that polymers can exhibit fluid as weU as solid behavior. The viscous fluid behavior of polymers becomes evident under dynamic loading where the viscous damping property reduces the amplitude of free vibrations. It is also observed that the strain will lag the stress and that there can be an energy loss through heat dissipation during periodic loading. 

Some early calorimeters use thermal methods based on principles of heat and mass balance (12) and temperature rise of a constant flow of air through the combustion chamber (13). These calorimeters suffer from many drawbacks associated with their design. Heat and mass balance requires numerous measurements to account for all heat and mass flows. In most cases, thermal lag and losses in the equipment occur, which are not easily calculated. 

---