Fisicoquimica

5-95E Steam is condensed by cooling water in a condenser. The rate of heat transfer in the heat exchanger and the rate of condensation of steam are to be determined.
Assumptions 1 Steady operating conditions exist. 2 The heat exchanger is well-insulated so that heat loss to the surroundings is negligible and thus heat transfer from the hot fluid is equal to the heat transfer to the cold fluid. 3Changes in the kinetic and potential energies of fluid streams are negligible. 4 Fluid properties are constant.
Properties The specific heat of water is 1.0 Btu/lbm.F (Table

A-3E). The enthalpy of vaporization of water at 85F is 1045.2
Btu/lbm (Table A-4E).
Analysis We take the tube-side of the heat exchanger where cold water is flowing as the system, which is a control volume. The energybalance for this steady-flow system can be expressed in the rate form as

Steam
85F

73F

E&in

− E&

out

 ∆E&

 0
 0
Ê0 (steady)
system

14243
Rate of net energy transfer by heat, work, and mass
E&in

1442443
Rate of change in internal, kinetic, potential, etc. energies
 E&out

60F

Water

Q&in  m& h1  m& h2

(since∆ke ≅ ∆pe ≅ 0)

85F

Q&in  m& c p (T2 − T1 )

Then the rate of heat transfer to the cold water in this heat exchanger becomes
Q&  [m& c p (Tout − Tin )]water  (138 lbm/s)(1.0 Btu/lbm.F)(73F − 60F) = 1794 Btu/s

Noting that heat gain by the water is equal to the heat loss by the condensing steam, the rate of condensation of the steam in the heatexchanger is determined from

Q&  (m& h fg

)steam

 → m&

steam 

Q&
h fg

 1794 Btu/s
1045.2 Btu/lbm

 1.72 lbm/s

El vapor se condensa por enfriamiento de agua en un condensador. La tasa de transferencia de calor en el intercambiador de calor y la velocidad de condensación de vapor de agua se han de determinar.
Hipótesis 1 Condiciones de funcionamientoestacionario existe. 2 El intercambiador de calor está bien aislado de manera que la pérdida de calor a los alrededores es insignificante y por lo tanto la transferencia de calor desde el fluido caliente es igual a la transferencia de calor hacia el fluido frío. 3 Los cambios en las energías cinética y potencial de corrientes de fluido es despreciable. 4 Las propiedades del fluido son constantes.Propiedades El calor específico del agua es 1,0 Btu / lbm.  F (Tabla
 
A-3E). La entalpía de vaporización del agua a 85  F es 1045,2
Btu / lbm (Tabla A-4E).
Análisis Tomamos el tubo del lado del intercambiador de calor donde el agua fría fluye como el sistema, que es un volumen de control. El balance de energía para este sistema de flujo estable se puede expresar en la forma como tasa
 vapor
85  F

5-96 Two streams of cold and warm air are mixed in a chamber. If the ratio of hot to cold air is 1.6, the mixture temperature and the rate of heat gain of the room are to be determined.
Assumptions 1 This is a steady-flow process since there is no change with time. 2 Kinetic and potential energy changes are negligible. 3 There are no work interactions. 4 The device is adiabaticand thus heat transfer is negligible.

Properties The gas constant of air is R = 0.287 kPa.m3/kg.K. The enthalpies of air are obtained from air table (Table A-17) as
h1 = h @278 K = 278.13 kJ/kg h2 = h @ 307 K = 307.23 kJ/kg hroom = h @ 297 K = 297.18 kJ/kg

Cold air
5C

Warm air
34C

Room 24C

Analysis (a) We take the mixing chamber as the system, which is a controlvolume since mass crosses the boundary. The mass and energy balances for this steady-flow system can be expressed in the rate form as
Mass balance:

m& − m&  ∆m&
m& − m&  ∆m&
Ê0 (steady)
in out system

Energy balance:

 0 → m& in  m& out → m&1  1.6m&1  m& 3  2.6m&1

since

m& 2  1.6m&1

E&in

−...