Sensible heat increase

The next point to realise is that the available enthalpy for absorbing the various heat-flows is the sum of (a) the latent heat of vaporisation, plus (b) the available cold in the vapour (or the sensible heat increase between boiling point and ambient temperature. 

The use of hot gas clean-up methods to remove the sulfur and particulates from the gasified fuel increases turbine performance by a few percentage points over the cold clean-up systems. Hot gas clean-up permits use of the sensible heat and enables retention of the carbon dioxide and water vapor in the... 

A more obvious energy loss is the heat to the stack flue gases. The sensible heat losses can be minimized by reduced total air flow, ie, low excess air operation. Flue gas losses are also minimized by lowering the discharge temperature via increased heat recovery in economizers, air preheaters, etc. When fuels containing sulfur are burned, the final exit flue gas temperature is usually not permitted to go below about 100°C because of severe problems relating to sulfuric acid corrosion. Special economizers having Teflon-coated tubes permit lower temperatures but are not commonly used. 

In a submerged-tube FC evaporator, all heat is imparted as sensible heat, resulting in a temperature rise of the circulating hquor that reduces the overall temperature difference available for heat transfer. Temperature rise, tube proportions, tube velocity, and head requirements on the circulating pump all influence the selec tion of circulation rate. Head requirements are frequently difficult to estimate since they consist not only of the usual friction, entrance and contraction, and elevation losses when the return to the flash chamber is above the liquid level but also of increased friction losses due to flashing in the return line and vortex losses in the flash chamber. Circulation is sometimes limited by vapor in the pump suction hne. This may be drawn in as a result of inadequate vapor-liquid separation or may come from vortices near the pump suction connection to the body or may be formed in the line itself by short circuiting from heater outlet to pump inlet of liquor that has not flashed completely to equilibrium at the pressure in the vapor head. 

Under steady-state conditions the temperature of the evaporating surface increases until the rate of sensible heat transfer to the surface equals the rate of heat removed by evaporation from the surface. To calculate this temperature, it is convenient to modify Eq. (12-26) in terms of humidity rather than partial-pressure difference, as follows ... 

Sensible heating only Cionstant moisture content increase in specific volume, dry- and wet-bulb temperature, and specific enthalpy decreased % saturation Steam, hoc-water coils, or electric heating... 

The amount of heat absorbed or lo.st by a substance that causes a change in the temperature of the substance is sensible heat, ft is called sensible heat because it can be measured by the change in temperature it causes. For example, as heat is added to a piece of steel the temperature of that steel increases and can be measured. The general equation for calculating sensible heat is ... 

Economizer. The economizer is a tubular heat exchanger used to recover heat from the exhaust gases from boilers or some processes. It is used in boilers to recover much of the sensible heat for use in preheating the boiler feedwater. An increase in boiler efficiency of 4-6 per cent is typical. The design and materials of construction depend on the application. 

The specific enthalpy will increase with dry hulh (sensible heat of the air) and moisture content (sensible and latent heat of the water). The adiabatic (isoenthalpic) lines for an air-water vapour mixture are almost parallel with the wet bulb lines so, to avoid any confusion, the enthalpy scale is placed outside the body of the chart, and readings must be taken using a straight-edge. (See Figure 23.7.)... 

Making a heat balance over the column, it is seen that the heat of vaporisation of the liquid must come from the sensible heat in the gas. The temperature of the gas falls from 6 to the adiabatic saturation temperature 6S, and its humidity increases from to Jfv (the saturation value at 9S). Then working on the basis of unit mass of dry gas ... 

Thus, the isothermal is a straight line of slope [Cw(9 — On) + A] with respect to the humidity axis. At the reference temperature 90, the slope is X at higher temperatures, the slope is greater than X, and at lower temperatures it is less than X. Because the latent heat is normally large compared with the sensible heat, the slope of the isothermals remains positive down to very low temperatures. Since the humidity is plotted as the ordinate, the slope of the isothermal relative to the X-axis decreases with increase in temperature. When 9 > Bq and Jf > Jf0, the saturation humidity, the vapour phase consists of a saturated gas with liquid droplets in suspension. The relation between enthalpy and humidity at constant temperature 9 is ... 

By far the most common way of heat storage is as sensible heat. As Figure 100 shows, heat transferred to the storage medium leads to a temperature increase of the storage medium. The ratio of stored heat to temperature rise is the heat capacity of the storage medium. 

This temperature increase can be detected by a sensor and the heat stored is thus called sensible heat. Sensible heat storage in most cases uses as storage materials solids (stone, brick,...) or liquids (water,...). Gasses have... 

Figure 100. Heat storage as sensible heat leads to a temperature increase when heat is stored...

The runaway reactor is treated as entirely adiabatic. The energy terms include (1) energy accumulation resulting from the sensible heat of the reactor fluid as a result of its increased temperature due to overpressure and (2) the energy removal resulting from the vaporization of liquid in the reactor and subsequent discharge through the relief vent. 

Many of the conservation measures require detailed process analysis plus optimization. For example, the efficient firing of fuel (category 1) is extremely important in all applications. For any rate of fuel combustion, a theoretical quantity of air (for complete combustion to carbon dioxide and water vapor) exists under which the most efficient combustion occurs. Reduction of the amount of air available leads to incomplete combustion and a rapid decrease in efficiency. In addition, carbon particles may be formed that can lead to accelerated fouling of heater tube surfaces. To allow for small variations in fuel composition and flow rate and in the air flow rates that inevitably occur in industrial practice, it is usually desirable to aim for operation with a small amount of excess air, say 5 to 10 percent, above the theoretical amount for complete combustion. Too much excess air, however, leads to increased sensible heat losses through the stack gas. 

But what is vaporizing The reflux, of course. The sensible-heat content of the vapors, which is reduced when the reflux rate is increased, is converted to latent heat, as the vapors partially vaporize the incremental reflux flow. 

Heat exchange provided by sensible-heat transfer is improved when velocities are higher. Especially when the heating fluid is on the tube side of an exchanger, sensible-heat-transfer rates are always increased by high velocity. 

The 160°F BFW is efficiently mixed with the incoming steam, in what is effectively a small, vertical stripping tower, mounted above the large deaerator drum. The majority of the steam condenses by direct contact with the 160°F BFW and in so doing, the latent heat of condensation of the steam is used to increase the sensible-heat content of the 160°F BFW to 230°F. 

Heat balance—e.g., condensing steam gives up latent heat, or sensible heat, to increase the temperature in the feed preheater. 

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