Chapter 11 Flashcards
Types of heat exchanger
counter flow and parallel flow
what is assumed for ideal heat exchangers
complete heat transfer from one to another
Draw the graph of temperature change for hot and cold fluid for parallel and counter flow heat exchangers
see book
what does a compact heat exchanger have
large heat transfer surface area per unit volume
area density beta > 700
difference between cross flow and counter flow
cross flow fluids flowing perpendicular to each other
what does unmixed mean for heat exchangers
has cross flow steam plates, reduce separation, small turbulence, lower pressure gradient but less mixing and cooling
What is the most common type of heat exchanger
shell and tube
what does a shell and tube heat exchanger contain
large number of tubes, (several hundred) packed in a shell, axis parallel to shell
regenerative heat exchanger
involves the alternate passage of hot and cold fluid steams through the same flow area
how are shell and tube heat exchangers further classified
by number of shell and tube passes
Dynamic type regenerator
rotating drum and continuous flow of the hot and cold fluid through different portion of the drum so that any portion of the drum passes periodically through the hot stream, storing heat and then through the cold steam rejecting this stored heat
Condenser is
one of the fluids is cooled and condeses as it flows through the heat exchanger
boiler is
one of the fluid absorbs heat and vaporizes
plate heat exchangers consist of
series of plates with corrugated flat flow passages the hot and cold fluids flow in alternate passages and thus each fluid stream is surrounded by two hot fluid streams
very effective heat transfer
What type of analysis can be used on heat exchangers
assume 1d steady radial flow, no heat generation no radiation and use network analysis
in double pipe heat exchangers what is the total thermal resistance
Rtot = Ri + Rwall + Ro
convection inner flow, conduction through wall, convection second surface
Rwall in double pipe heat exchangers
lnDo/Di / 2pikL
what is often assumed when using network analysis on heat exchangers
thickness of the wall -> therefore resistance of wall -> 0
therefore total resistance = Ro + Ri
What is U
the overall heat transfer coefficient
In double pipe heat exchangers what does Q. =
DELTAT/R = UA*DELTAT
relationship between U and Rtot
1/UAs = R
relationhsip between U and heat transfer coefficient
when Rwall approx = 0 and Ai approx = Ao approx = As
1/U approx = 1/hi + 1/ho
What does 1/U approx = 1/hi + 1/ho mean
U is dominated by the smaller convection coefficient
what happens when a fluid and gas heat exchanger is used
U tends towards the h of the gas, such cases fins used on gas side to enhance product UA, and inc heat transfer
When tube is finned what happens to As
As = A total = Afin + A unfinned
for short fins of high thermal conductivity
As = Aunfinned + fineff Afin
why does performance of heat exchangers decreases with time
accumulation of deposits on heat transfer surfaces
additional resistance to heat transfer
how is accumulations treated
with a fouling factor R
With fouling factor 1/UAs =
R = 1/hiAi + Rfi/Ai + lnDo/Di / 2pi()kL + Rfo/Ao + 1/hoAo
fouling factor increases with
operating temperature, length of service and decreases with velocity of the fluids
If fluids stream have same capacity rates
experience the same temperature change in well insulated heat exchanger
Symbol of heat capacity rate
Ch or Cc for hot or cold
C =
m. *c mass flow rate * heat capacity of fluid
Heat transfer in heat exchanger
Q. = Cc (Tcout - Tcin) Q. = Ch (Thin - Thout)
Ideal scenario what does Q. =
UAs * LMTD
How is U found
1/U = 1/hi + 1/ho, found from Nu of internal and external flow from Re and Pr number
Non ideal scenario what does Q. =
FUAs * LMTD what F is a geom effect
In a condenser or boiler tube what does Q.
m. * hfg where m. is the rae of evaporation or condensation of the fluid, hfg is the enthalpy of vaporization of the fluid at the specified temperature or pressure
Graph of condenser or Boiler heat transfer
see book
Derive the equation for LMTD
see book
what is LMTD
exact representation of the average temperature difference between the hot and cold fluids
DELTAT1 for parallel and counter flow
parallel Thin - Tcin
Counter Thin - Tcout
DELTAT2 for parallel and counter flow
parallel Thout - Tcout
Counter Thout - Tcin
Difference between LMTD and arithmetic mean temp difference
AMTD overestimate rate of heat transfer
when DETLAT1 and DELTAT2 differ by no more than 40% less than 1% error in using AMTD
What will never happen in counter flow heat exchangers
outlet temperature of cold fluid can never exceed inlet temperature of the hot fluid
for specified inlet and outlet temps
LMTD for counter flow is always greater than parallel therefore greater heat transfer with sme area
when heat capacity is equal in counter flow
LMTD = DETLAT1 = DETLAT2
F= 1 for
a condenser or boiler
How is F found
using the charts
LMTD method
select heat exchanger that will meet prescribed heat transfer req
- Select type of heat exchanger
- determine any unknown inlet/outlet temps and heat transfer rate using energy balance
- calculate LMTD and F is necessary
- Calculate the heat transfer surface area As
Whats NTU method used fo
determining heat transfer rate and outlet temps
heat transfer effectiveness
Q. /Q.max = actual heat transfer rate / Maximum possible het transfer rate
DELTATmax for NTU =
Thin - Tcin
Q.max =
Cmin (Thin - Tcin) where Cmin is smaller of Ch and Cc
Why is Cmin used
smaller heat capacity = larger temp jump see book
If Ch = Cc
then DELTATh = DELTATc
if Cc = Cmin
eff = Q/Qmax = Cc (Tcout - Tcin)/Cc(Thin - Tcin) =
Tcout - Tcin/Thin - Tcin
if Ch = Cmin
eff = Q/Qmax = Ch (Thin - Thout)/Ch(Thin - Tcout) =
Thin - Thout)/(Thin - Tcout
NTU (Number of transfer units)
UAs/Cmin
what is NTU
a measure of the surface area As thus the larger the NTU the larger the heat exchanger
capacity ratio
c =Cmin/Cmax
the effectiveness of a heat exchanger is a function of
NTU and capacity ratio
how does effectiveness vary with NTU
rapid increase at low values, at high values more gradual, large size cannot be justified large increase in NTU small increase in effectiveness
effectiveness max when c =
0, boiler or condenser
minimum for c = 1
highest effectiveness
Counter flow heat exchanger, then cross flow unmixed
