Latent heat loss

Boiler Thermal Efficiency Traditionally, boiler thermal efficiency is calculated pour/pm, where in is the LHV (lower heating value) of the fuel. A rule of thumb for economizers is that boiler efficiency increases by 1 percent for every 22°C (40°F) drop in temperature of the dry flue gas. These two statements do not reveal the considerable quantity of additional heat, available to be recovered through condensation of the water vapor in the flue gas, which is lost to atmosphere with hot flue gas. Based on fuel HHV (higher heating value), the total latent heat loss can be substantial an additional 9.6 percent (natural gas), 8.0 percent (propane), 6.5 percent (heating ou). 

C. Leaf temperature will increase because of the decrease in latent heat loss accompanying transpiration. 

In a manifold-type dryer, in which the ampoules are exposed to moist room air, an aqueous vapor freezes at the part of the vessel surface where ice crystals remain. This deposited frost layer significantly retards the sublimation process, with the inhibition of heat flow from atmosphere to the drying products, resulting in a longer exposure to those portions of the product where ice crystals have disappeared and the specimen temperature has decreased, with a latent heat loss caused by sublimation of frosted moisture. 

Sensible heat loss from the skin depends on the temperatures of the skin, die environment, and the surrounding surfaces as well as the air motion. The latent heat loss, on the other hand, depends on the skin wettedness and the relative humidity of the environment as well. Clothing serves as insulation and reduces both the sensible and latent forms of heat lo.ss. The heat transfer from the lungs through respiration obviously depends on the frequency pf breatlrlng and the volume of the lungs as well as the environmental faplors that affect heat tran.sfcr from tlie skin. 

Evaporative or latent heat loss from the skin is proportional to the difference between the water vapor pressure at the skin and the ambient air, and the skin wettedness, which is a measure of the amount of moisture on the skin. It is duo to Iho combined effects of the evaporation of sweat and the diffusion of water through the skin, and can be expressed a.s... 

Assumptions 1 Steady conditions exist. 2 The latent heat loss from the person remains the same. 3 The heat transfer coefficients remain the same. 

C What is latent heat How is the latent heat loss from the human body affected by (a) skin wetledness and (b) relative humidity of the environment How is the rale of evaporation from the body related to the rate of latent heat loss ... 

A clothed or unclothed person feels comfonable when the skin temperature is about 33°C. Consider an average man wearing summer clothes whose thermal resistance is 1.1 clo. The man feels very comfortable while standing in a room maintained at 20°C. If this man were to stand in that room unclothed, determine the temperature at wliich the room must be maintained for him to feel thermally comfortable. Assume the latent heat loss from the person to remain the same. 

The average annual ratio of sensible to latent heat loss at the surface is called the Bowen ratio. With —6 units of sensible heat loss and —23 units of latent heat loss, the Bowen ratio is —0.27. 

The major sources of error in using the unsteady-state charts are the inadequate data on the density, heat capacity, and thermal conductivity of the foods and the prediction of the convective coefficient. Food materials are irregular anisotropic substances, and the physical properties are often difficult to evaluate. Also, if evaporation of water occurs on chilling, latent heat losses can affect the accuracy of the results. 

The cross-sectional area of the wick is deterrnined by the required Hquid flow rate and the specific properties of capillary pressure and viscous drag. The mass flow rate is equal to the desired heat-transfer rate divided by the latent heat of vaporization of the fluid. Thus the transfer of 2260 W requires a Hquid (H2O) flow of 1 cm /s at 100°C. Because of porous character, wicks are relatively poor thermal conductors. Radial heat flow through the wick is often the dominant source of temperature loss in a heat pipe therefore, the wick thickness tends to be constrained and rarely exceeds 3 mm. 

It is advantageous to use a low-retentivity carbon to enable the adsorbate to be stripped out easily. When empirical data are not available, the following heat requirements have to be taken into consideration (1) heat to the adsorbent and vessel, (2) heat of adsorption and specific heat of adsorbate leaving the adsorbent, (3) latent and specific heat of water vapor accompanying the adsorbate, (4) heat in condensed, indirect steam, (5) radiation and convection heat losses. 

Heat gains and losses can be only sensible or sensible and latent. Sensible heat gains result from conduction, convection, and/or radiation. Latent heat gains occur when moisture is added to the space fe.g., from evaporation . 

Heat recovery section of an AHU The part of an AHU in which a sensible or latent heat gain or loss takes place by means of a heat-transfer medium. 

Ventilation heat loss or gain The quantity of sensible and latent heat lost or gained from an enclosure due to natural or mechanical ventilation. 

If a phase change occurs in the process stream for which heat duties are being calculated, it is best to perform a flash calculation and determine the heat loss or gain by the change in enthalpy. For a quick hand approximation it is possible to calculate sensible heat for both the gas and liquid phases of each component. The sum of all the latent and sen -i-ble heats is the approximate total heat duty. 

For example, if total heat duty (sensible heat, latent heat duty, heat losses to the atmosphere) was 1 MMBtu/hr and water was being heated, a heat flux of 10,000 Btu/hr-ft would be used and 100 ft of fire tube area would be required. 

A little of this is lost by radiation if the surrounding surfaces are cold and some as sensible heat, by convection from the skin. The remainder is taken up as latent heat of moisture from the respiratory tissues and perspiration from the skin (see Table 23.2). Radiant loss will be very small if the subject is clothed, and is ignored in this table. 

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