mec203 heat transfer lab report

8/9/2019 MEC203 Heat Transfer Lab Report

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Heat exchanger Lab Report

Authors: Sijin He, Xihao Huang,

George Wright and Joseph Wookey

1. Introduction1.1 Heat exchanger A heat exchanger is a thermodynamic device

which concerns the rate and extent of heat

exchange between two surfaces, or fluids. he

function of a heat exchanger is to either remove heat from a hot fluid or to add heat to

the cold fluid. here are normally three

categories of flow arrangement correspondingto the direction of fluid motion inside the heat

exchanger! parallel flow, counter flow and

cross flow. In parallel flow, both the hot and

cold fluids enter the heat exchanger at thesame end and move in the same direction. In

counter flow, the hot and cold fluids enter the

heat exchanger at opposite ends and flow inopposite directions. his experiment

concerned the counter flow.

Reduce the theory part" Add the graph #Flowthrough a plate heat exchanger$ in

lab sheet"

1.% he &ffectiveness'() *ethod

he +'() method is used to evaluate the

performance of heat exchangers. his methodis based on a dimensionless parameter called

the heat transfer effectiveness +, defined as

+ Q - Qmax 1/

(), the number of transfer units is

expressed asthose r in nest page/

() U A s

( m C p)min

; %/

0hen m V · ρ 2/

Another dimensionless 3uantity called the

capacity ratio c is defined as

c= ( m ·C p)min

( m ·C p)max

; 2/

he &ffectiveness, +, of a heat exchanger is a

function of the number of transfer units ()

and the capacity ratio c, that is, + 4（ (),

c ） . 5ur ob6ect is to study how the

effectiveness varies with capacity ratio for aconstant (). his will be done on a plate

heat exchanger which has a flow arrangement

of counter flow.

%. 7rocedure 8 9ata Analysis

%.1 7rocedure

&nsure the thermostat is set to :;℃ . <et both

the hot and cold water flow rates to 1 l min '1.

a=e all six pieces of data once the system has

stabili>ed. Repeat increasing the hot flow ratein ;.: l min'1 increments. he maximum flow

rate the instrument can achieve is ? l min'1.

%.% 9ataT1/℃ ) T2/℃ T3/℃ T4/℃ Vh/L min^-

1

Vc/L min^-

1)

[email protected] 2:.: 1:. %;.B 1 1

?C.1 ?%. 1:. %1.C 1.: 1

[email protected] ?2.C 1:.B %%.@ % 1

?@.: ??.C 1:. %2.B %.: 1

[email protected] ?:. 1:.B %?.B 2 1

Table 1: Measured !no"n para#eters

)se c p ?.1@ DE Dg'1D '1 and the density of

water to be CC@ =g m'2. )se As ;.;:m%,

4;.C:

In addition to the 3 page report, ona third page I would like you to putthe name of each member of thegroup and briey say what they did. I

will use this information to scaleback the marks of anyone who hasnot engaged with the labspresentation and report./

(otice that the specific heat of a fluid in

general changes with temperature. Fut in aspecified temperature range, it can be

treated as a constant at some average valuewith little loss in accuracy.

Table $: %al&ulated 'alues (ro# data gi'en in Table 1)

table % maybe useless, all the results areshown in appendix/

%.2 Analysis

Applying effectiveness'() method, + isdefined as

+ Q - Qmax ; 1/

(), the number of transfer units isexpressed as

Qhot (KJ)

Qcol(KJ)

+ T m(℃ )

)0-mG

℃ ）

() mc

mh

0.89 ;.2: ;.? %2.:? ;.2% ;.%2 1

0.68 ;.?? ;.: %B.1 ;.2? ;.%? ;.B

0.61 ;.?C ;.@1 %.@2 ;.2C ;.%@ ;.:

0.63 ;.: ;.C %.CC ;.?? ;.2% ;.?0.65 ;.2 1.; %.@? ;.?C ;.2: ;.22



() U As

( mC p)min

; %/

0hen m V · ρ 2/

Another dimensionless 3uantity called the

capacity ratio c, defined as

c= cmin

cmax

= ( m·C p)min

( m·C p)max

; ?/

In this experiment, as cmin e3uals to

ccold , cmax e3uals to chot , and C p

is assumed to be the same for hot and hold

water, e3uation ?/ can be shown

c=ccold

chot

=( m ·C p)cold( m ·C p)hot

= mc

mh

; :/

o compute effectiveness, rate of heat

transfer is needed. Qhot , Qcold and

Qmax can be calculated with following

e3uations!

Qhot mh·C p h ,∈¿−T h,out

T ¿/

/

Qcold mc ·C p · c ,∈¿

T c, out −T ¿/ B/

Qmax mc ·C p

c ,∈¿h ,∈¿−T ¿

T ¿

/ @/

where

mh V h · ρh C/

mc V c · ρc 1;/

In order to find () value, firstly compute

the logarithmic mean temperature difference

T m

h ,∈¿−T c,out T ¿¿

c ,∈¿T h,out −T ¿

¿h ,∈¿−T c,out

c ,∈¿T h,out −T ¿

T ¿ /¿¿

ln ¿¿¿

11/

he overall heat transfer coefficient ) for hot

and cold water respectively are!

U h Qhot

F A s ΔT m 1%/

U c Qcold

F A s ΔT m 12/

4inally, () value for both hot and cold

water can be found with e3uation %/.

NTU hot

U h As

mc C p 1?/

NTU cold U c As

mc C p 1?/

Jompare + between theoretical value and

value in practice!

Applying () e3uation for <hell and ube!

one pass %, ? passes with a selected ()() ;.2 in this case/ K,+%

{1+c+√ 1+c

2 1+exp [− NTU √ 1+c2]

1−exp [− NTU √ 1+c2

]

}

−1

1:/

&rror and 7lot analysisError anal!i!"

9ifferences in Qhot and Qcold

According to analysis and calculation, there is

a larger than expected discrepancy between

Qhot Qcold . heoretically,

´Qhot / ´Qcold effectiveness/ should be

constantly e3ual to 1 this indicates that thereis no extra resistance during the transmission.

)nder this assumption, all the energy transfers

from hot water to cold. In practice, whilstmaintaining the expected trend of a lower

capacity ratio mass flow rate cold water-

mass flow rate hot water/ giving a higher

effectiveness as more hot water is passing thecold each second these values, ;.%B2 at a

capacity ratio of ;.22 to ;.1: at a capacity

ratio of 1, are lower than expected. hisdiscrepancy needs to be analysed and potential

losses explained.

he generation of heat losses!

Mengel K%;;, states that to calculate

heat generation in practice, the thic=nessof the tube wall can no longer be

considered as small, the slightly low

thermal conductivity of the tube material

has to be ta=en into consideration, andthermal resistance of the tube should not

be neglected. hese factors were not

considered when calculating the values

described above which contributed to thediscrepancy between the calculated data



and the ideal circumstances. K2K?

Heat losses in heat exchangers areaffected by 2 ma6or factors!

1/ 4ouling inside tubes

%/ (atural convection

2/ Radiation around tubes

Resistance e3uation can be applied!

1

U A s

= 1

U i Ai

= 1

U o Ao

= R= 1

h i Ai

1/

Heat differences caused by fouling!

<cales on the plate heat exchanger inner

tube surfaces can be seen in the labe3uipment. 4ouling adds additional

resistance to the heat transfer process,

indicating that theoretical ) is muchsmaller than ) in practice. It shows that

if hard water is present, solid deposits in

a fluid must participate K:.

Heat differences caused by natural convection

and radiation!

Air cooling is another significant factor inlosses, Mengel %;;, p@12/, stated that #Heat

lost to the surroundings ＝ heat given by hot

fluid water/ N heat ta=en by cold fluid air/.$

K In an ideal heat exchanger the pipescarrying the hot water would be perfectly

insulated from this process however they were

not in this case leading to significant heatlosses to the surrounding air and a less ideal

transfer to the cold li3uid.

Oraph Analysis

Jonsidering the operating temperature as wellas the length of service does not change in this

case, the hot water flow rate is the only factor to fouling. 4ouling decreases due to increasing

velocity K%K:. 0hen accumulating the hot

water flow rate, it wea=ens the overall heat

transfer coefficient. &vidence of this can befound in AwadPs article %;11/, low fluid

velocities less than ;.Cm-s/ allow suspended

solids to settle on the heat exchange surface.

In 7lot 1/, the theoretical Red line/ and

measured values Flue line/ begin relatively

close to one another, since fouling has less of an effect when capacity ratio is reduced.

However, as the capacity ratio increases the

theoretical and lab data spread further apart

showing that fouling is having more of aneffect at a higher capacity ratio.

9iscussion

4or a given (), the effectiveness becomes amaximum for capacity ratio c; and a

minimum for c1 5R for a constant (), the

effectiveness increases as the capacity ratiodecreases.

0hen comparing 7lot. 1 to the effectiveness

graphs in the textboo=, we can see that theinverse relationship between capacity ratio

and effectiveness holds for all types of heat

exchangers.

0.2 0.3 0. 0.! 0." 0.# 0.$ 0.% & &.&0

0.0!

0.&

0.&!

0.2

0.2!

0.3

0.3!

Effectiveness (ε) VS Capacity Ratio (c)

Capacity Ratio (c)

*lot +1 +-ed line sho"s the e.pe&ted

theoreti&al results, blue line sho"s

#easured results (ro# the lab

Jonclusion

he effectiveness increases as the

capacity ratio c decreases which means if we want to achieve a high performance of

heat exchanger, we should increase the

mass flow ratio in this experiment weincrease the mass flow rate of hot water/.

In addition the reduction of losses through

the mechanisms discussed above should

be =ept to an absolute minimum. his can be achieved by using controlled

conditions in con6unction with a well'

insulated exchanger this is to minimi>e

heat loss to the surroundings.4urthermore, using purified water and

mec203 heat transfer lab report

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