Latent heat of condensation

Pure vapor or substantially pure vapor can be considered condensed isothermally, and during the condensate range the latent heat of condensation is uniform. 

Consider a process which has two process hot streams (Hj and H2), two process cold streams (Cj and C2), a heating utility (HUi which is a saturated vapor that loses its latent heat of condensation) and a cooling utility (CUi). The problem data are given in Table 9.8. The cost of the heating utility is 4/10 kJ added while the cost of the coolant is 7/10 kJ. The minimum allowable temperature difference is lO C. Employ graphical, algebraic and... 

Ehie to the dilute nature of the stream, we may assume that the latent heat of condensation for MEK is much smaller than the sensible heat removed from the gas. Therefore, we can apply the procedure presented in Section 10.5. Once a value is selected for ATj ", Eq. (10.19) can be used to determine T and Eqs. (10.3b) and (10.7) can be employed to calculate the value of Since the bounds on p (and consequently on AT ) are tight, we will iterate over two values of 0.96 and 1.00 (Ar2 " = 90.0 and 94.5 K). The other iterative variable is AT, . Both variables are used to trade off fixed versus operating costs. As an illustration, consider the following iteration AT," = 5 K and... 

In condensation one fluid remains at constant temperature throughout the length of the exchanger while the fluid B that is absorbing the latent heat of condensation is rising in temperature to an outlet of tg. Note that as fluid A condenses, it does not flow the length of the travel path. Fluid A drops to the bottom of the exchanger and flows out the outlet at temperature Tg, which is the same as Tj, the tempera-... 

Buoyancy caused by the release of latent heat of condensation of water. As will be seen in Section 7.7, water releases a substantial amount of energy when it condenses. 

Qt = total heat transferred latent heat of condensation + sensible heat for cooling the vapour (gas) and condensate. 

Non-condensables >70 per cent assume the heat transfer is by forced convection only. Use the correlations for forced convection to calculate the heat-transfer coefficient, but include the latent heat of condensation in the total heat load transferred. 

The cooling cycle starts when all parts of the refrigerator are at about 1.3K. At this temperature, the 3He is completely adsorbed by the pump. The pump temperature is now raised to about 25 K by means of an heater. At 25 K, the 3He is desorbed, and its pressure increases over the saturation pressure at 1.3 K. Consequently, 3He condenses in the part of the tube T internal to the copper support C and drops down into the evaporator E. In this phase, the latent heat of condensation and the enthalpy variation are delivered to the 4He bath. The cooling phase starts when all the 3He is condensed in E and the power on the pump heater is switched off. The pump starts cooling towards the bath temperature, reducing the pressure on liquid 3He in E. The adsorption heat of the 3He vapour is delivered to the 4He bath by L. 

Dry air rising in the atmosphere has to expand as the pressure in the atmosphere decreases. This pV work decreases the temperature in a regular way, known as the adiabatic lapse rate, Td, which for the Earth is of order 9.8 Kkm-1. As the temperature decreases, condensable vapours begin to form and the work required for the expansion is used up in the latent heat of condensation of the vapour. In this case, the lapse rate for a condensable vapour, the saturated adiabatic lapse rate, is different. At a specific altitude the environmental lapse rate for a given parcel of air with a given humidity reaches a temperature that is the same as the saturated adiabatic lapse rate, when water condenses and clouds form Clouds in turn affect the albedo and the effective temperature of the planet. Convection of hot, wet (containing condensable vapour) air produces weather and precipitation. This initiates the water cycle in the atmosphere. Similar calculations may be performed for all gases, and cloud layers may be predicted in all atmospheres. 

The BET theory assumes that the reasoning used for one or two layers of molecules may be extended to n layers. It argues that energies of activation after the first layer are all equal to the latent heat of condensation, so that ... 

The condensing temperature of the steam is 300°F. The process into which the heat is transferred is at a constant temperature of 20O°F. The overall heat transfer coefficient is 300 Btu/h °F ft. The reboiler has S09 tubes that arc 10 feet long and 1 inch inside diameter. The steam and condensate are inside the tubes. The density of the condensate is 62,4 Ib ft and the latent heat of condensation of the steam is 900 Btu/lb . Neglect any sensible heat transfer. 

The terms AHj, L, AH yUnd i used in Fig. 7.1 are all enthalphy changes defined as follows AHi is the heat of immersion of the solid into the liquid, L is the latent heat of condensation, AH yis the heat of adsorption when the solid is equilibrated with saturated vapor, and i is the heat liberated when solid in equilibrium with saturated vapor is immersed into liquid. Using Hess s law of heat summation... 

The next part of the problem is to determine the vapor flow to the bottom tray. If we assume that the vapor leaving the reboiler is essentially steam, then the latent heat of condensation of this vapor is 1000 Btu/lb. Hence the flow of vapor (all steam) to the bottom tray is... 

When a vapor condenses to a liquid, we say that the latent heat of condensation of the vapor is liberated. In a steam reboiler, this liberated heat is used to reboil the distillation tower. When a vapor, or more commonly a liquid, cools, we say that its sensible heat is reduced. For a small or slight temperature change, the change in latent heat might be large, while the change in sensible heat will be very small. 

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. 

As the heavier components in the vapor condense into a liquid, they give off heat. This heat is called the latent heat of condensation. This latent heat is picked up by the liquid flowing across the tray. This liquid flow is called the internal reflux. This latent heat promotes extra vaporization of the internal reflux. Naturally, the lighter, lower-boiling-point components preferentially vaporize from the internal reflux. These lighter components have a relatively low molecular weight. 

Effect of subcooling. When steam condenses at atmospheric pressure, it gives off 1000 Btu per pound of condensing steam. This is called the latent heat of condensation of steam. 

The latent heat of condensation of this vapor is absorbed by the liquid entering the reflux drum. The liquid that enters the reflux drum, comes from the condenser. The hot vapor mixes with the condenser outlet liquid, and is condensed by this cooler liquid. 

Let s assume that the liquid draining from the condenser is not quite cold enough to absorb the entire latent heat of condensation of the vapors flowing through the hot-vapor bypass line. The vapors will then be only partially condensed. Vapor will start to accumulate in the reflux drum. This accumulation of vapor will increase the reflux drum pressure by a small amount. The higher drum pressure will back up the liquid level in the condenser by a few inches. The higher height of liq-... 

Steam has a high latent heat of condensation. This permits flowing steam to transmit about eight times more energy than that transmitted by a pound of butane. 

When any vapor expands, due to a pressure reduction (other than H2 and C02), it cools off. This is called a Joule-Thompson expansion. The reduction in temperature of the steam is called a reduction in sensible-heat content. The sensible heat of the steam is converted to latent heat of condensation. Does this mean that the latent heat of condensation of 10-psig steam is much higher than that of 450 psig steam Let s see ... 

Latent heat of condensation of saturated 10 psig steam = 980 Btu/lb. 

If a multiple-effect evaporator system produces 10 pounds of fresh water per pound of saturated steam at 35 p.s.i.a. (t = 259° F.) and t0 = 70° F., the work equivalent per 1000 gallons of fresh water is 60 kw.-hr. and the energy efficiency using the differential process with 50% recovery as the standard, is 6.9%. This calculation assumes that the available heat is simply the latent heat of condensation at the constant temperature of 259° F. 

Figure 3. Except for the latent heat of condensation released at the transparent surface, they all are forms of energy loss. Of primary significance in design and in evaluation of performance is the energy balance drawn around the distiller basin. This input is seen to be the incident solar energy minus reflection from the cover and the very small absorption in the cover. The feed water might also be considered a sensible heat supply, but it would usually be cooler than the product streams, and hence at a convenient base temperature, having zero energy input.

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