Heat exchanger is main equipment in thermal system. As we know, energy is related by heat for any condition. For example power plant with gas turbine. Getting bigger power proportional with higher temperature working in system. Heat affect temperature, when something of spot has different temperature, so that heat flow from higher temperature thought lower temperature. When we produce heat such as from combustion, the problem we must convert that vapor of fluid from liquid condition. Heat is function of conductivity thermal, different temperature, cross area, thickness, coefficient convective. and if we say convective, other function include on proses such as velocity, fluid, hydraulic diameter, heater, and temperature of fluid.
Heat exchanger can be achieved with two method, the first just different of temperature and the second include changes of phase. In general heat exchanger just with through heat by temperature without change of phase,if any different temperature, it exactly change heat to equilibrium of thermal. And process which include change of phase is called by evaporation and condensation. Evaporation is process to accept heat that makes change of phase form liquid through vapor and effect form this process is cooling for other fluid because throw heat to evaporate of other fluid. In contrast, condensation is opposite of evaporation that work for throw heat and makes fluid from vapor become liquid.
The benefit if we play phase for heat exchanger because we don’t increase of temperature to get bigger energy through on system. Because that relate with enthalphy of fluid which function of phase and temperature. And process can be better if convection process is high, maybe with enhance pipe with similiar roughness, so that makes fluid be turbulent early enter the pipe.
Figure above depict temperature of fluid in and out of heat exchanger. This figure form Fundamentals of Heat and Mass Transfer by Incropere et al. Picture (a) is heat exchanger with condensing vapor to liquid, other fluid increase temperature because accept heat. Picture (b) is heat exchanger with evaporating fluid from liquid through vapor. and picture (c) is normal heat transfer which change temperature to flow of heat.
Case Study about Heat Exchanger
How many temperature inlet and outlet for fluid that don’t evaporation
Temperature Evaporator 5oC or278 K
- Heat of Heat Exchanger 50 kW
- UA equipment of exchanger 12 kW/K
- w.cp from fluid will be cooled 8 kW/K
Q = w.cp.dT
dT = Q / (w.cp)
To-Ti = -50/8 = -6.25 Equation I
negative value is direction of heat that is absorbed by evaporator and it is mean other fluid is cooled (Ti > To)
To-Ti = (Te-Ti)(1-e^(-UA/(w.cp)))
To-Ti = (278-Ti)(1-e^(-12/(8)))
To-0.22Ti=216.84 Equation II
Elimination of Equation I and II
To-Ti = -6.25
Ti = 13.01 C
To = 6.76 C
So that, in cooling process uses heat exchanger with evaporation process as cooling media get temperature inlet around 13.01 C and outlet 6.76 C, and constrain temperature evaporator is 5 C
t1,o = to = ?
q = 500 W/m2
h1 = 45 W/m2.K, h2 = 12 W/m2.K
A = 1m . x = x (m)
t1,i = ti = ambient temperature = 15oC
w = 0,02 kg/s, cp = 1000 J/(kg.K)
to = ti + (tc – ti)(1 – e-UA/wcp)
Δtc = q/U = 500/9,47 = 52,8oC
tc = 52,8 +15 = 67,8oC
So that, to = 15 + (67,8 – 15)(1 – e-9,47.x/0,02.1000)
to = 15 + 52,8 (1 – e-x/2,11)