|

HEAT EXCHANGER CLASS NOTES FOR MECHANICAL ENGINEERING

HEAT EXCHANGER CLASS

NOTES FOR MECHANICAL

ENGINEERING

Heat exchanger transfers heat between

two fluids separated by a solid wall in

between. Cools the hot fluid. Heats the

cold fluid. There are many types of heat

exchangers. Different heat exchangers

are used in different applications.

Heat exchanger exchanges heat between

two fluids with a solid wall in between.

One fluid is heated while the other

fluid is cooled. There are two methods

of analysis. These are LMTD and NTU

methods. LMTD is used where inlet

and outlet temperatures of the two

fluids are known.

Definition of a heat exchanger

It is a device for heat transfer between two fluids.

Recuperator

 It is a device to transfer heat between two fluids which do not mix.

Compact heat exchanger

(i) A heat ex-changer which transfers maximum heat per unit volume of space.

Or

Surface area in (m2)/m3 of space is maximum for a compact heat ex-changer .

(ii)Heat Transfer area density

The surface area in m2/m3 of space is heat transfer area density.

TYPES OF HEAT EX-CHANGERS

Four basis of classification

(i) On the basis of flow

(a) Parallel flow: Initial q. is very high but then decreases, not used presently.

(b) Counter flow: q. is high throughout, area required is less, more efficient, universally used.

(c) Cross flow: where one fluid changes state namely condenser/evaporator. These are quiet common.

Fig. Types of Heat Exchagers

(ii) On the basis of function these perform

(a) Condenser

(b) Evaporator

(c) Boiler

(d) Preheater

(e) Super-heater

(iii) On the basis of construction

(a) Double pipe HEX

(b) Shell and coil HEX

(c) Shell and tube HEX: maximum in use because of easy de-scaling.

Fig. Shell & Tube Heat Exchanger (Counter Flow)

(d) Multiple shell pass/multiple tube pass HEX

It is a type of cross flow heat ex-changers to increase heat transfer.  It is very common in condensers and evaporators.

(iv) On the basis of operating principle

(a) Mixing type: Cooling towers, Jet condensers, Direct contact heat water feeders, desert coolers

(b) Non mixing type: Radiators, pre- heater economizer and pre-heater

Practical Applications of Heat Exchangers

Radiators

Inter-coolers in multi-compressors

Air preheaters

Economizers

Super-heaters

Waste water heat recovery system

Oil coolers in transformers

Condensers and evaporators in refrigerating units

HEAT EXCHANGER-OVERALL HEAT TRANSFER COEFFICIENT

DEFINITION 

Overall heat transfer coefficient is a single coefficient for convection, conduction and convection taking place simultaneously. Its symbol is ‘U’. Its units are W/m2 K.

1/UA =1/hiAi+fi + ln (r2/r1)/2πkL +1/hoAo + fo

Where fi  and  fo are fouling resistances on the inner and outer surfaces.

CALCULATION OF U

(i) On the basis of Ai

(ii) On the basis of Ao

CALCULATE resistances 1/hiAi  and 1/hoAo. Find which resistance is greater. Greater of the two becomes the basis of calculating U.

Overall heat transfer coefficient based on outer area

1/Uo =ro/ri hi + 1/ho +( ro/ri) fi +(r0 /k) ln (r2/r1) + fo

Overall heat transfer coefficient based on inner area

1/Ui =1/hi +( ri/r0) (1/ho) + fi +(ri /k) ln (ro/ri) + ( ri/r0) fo

If the tube is very thin, then with no fouling

1/U= 1/hi +1/ho

CALCULATION OF hi and ho for finding U

CALCULATION OF  hi

Find Reynolds number in the tube. Find laminar or turbulent flow. Apply corresponding Nusselt Number equation to find hi

(a) For laminar flow in a tube                             Nu = 1.32(∆T/d)0.25

(b) For turbulent flow in the tube                     Nu = 0.023 Re0.8 Pr 0.4

(c) For turbulent two phase flow in a tube

Nu = 0.023 (kL/D)Re0.8Pr0.4 F

Factor F is correction factor for two phase flow

Case 1       F =1 for 1/χ  > 0.1

Case 2      F= 2.35(1/χ + 0.213) for 1/χ  < 0.1

Where χ is Martinelli-Nelson Parameter

1/χ = (x/ (1-x))0.9 (ρL /ρV)0.5 (µgL)0.1

NOTE: Original Dittus-Boelter Equation is for liquid phase flow only. For a two phase flow, it uses a correction factor ‘F’.

CALCULATION OF  ho

I.  Sensible heat exchange on the outside of the tube or FOR A SINGLE PHASE HT

(a) Laminar flow over horizontal plate

Nu=  hoD0/kf  = 0.54 (Gr Pr)0.25

(b) Turbulent flow over horizontal plate

Nu=  hoD0/kf  = 0.14 (Gr Pr)0.33

(c)  Laminar flow over a vertical plate

Nu=  hoD0/kf = 0.59 (Gr Pr)0.25

(d) Turbulent flow over a  vertical plate

Nu=  hoD0/kf = 0.10 (Gr Pr)0.33

II.  Where phase change takes place on the outside of the tube Or For two phase HT   

hNBHTC =0.00122[(kL0.79 cpl0.45 ρL0.49)/ (σ0.19µL0.79hfg ρV0.79)] ΔTex 0.24 Δp sat 0.75

Where hNBHTC is Nucleate boiling H T coefficient Or two phase heat transfer coefficient

ΔTex = Tsur –Tsat

Δpsat =  psur –psat

(iii)Calculation of hoin case of condensation

(a) Laminar film condensation on a vertical plate

Average HT coefficient from NUSSELT EQUATION

ho = hav = 0.943 [ k3ρ2g hfg/ (μ L (Tsat—Tsur))]0.25

(b) Turbulent film condensation on a vertical plate

Film heat transfer coefficient

ho =hav = 0.0077(Re)0.4 [ k3ρ2g / (μ2]1/3

After finding hi and ho, U can be calculated

AT EXCHANGER ANALYSIS

There are two methods.

(a)    LMTD METHOD

(i)      when all the four temperatures are known.

q.=U A LMTD

(ii) When fouling is to be considered

q.=Udirty A LMTD

where f=1/U dirty  –1/U clean

Values of f and U clean are given in standard tables.

(i) ANALYSIS OF MULTIPASS HEAT EXCHANGERS

For multi-pass shell/multi-pass tube / multi-pass both in shell and tube

/ cross flow HEX

q.=(UA LMTD counter flow) F

Factor F is read from charts having F along y-axis

Parameter P along x-axis

  Curves for various values of parameter Z.

Parameter P =Temp range of tube fluid/max. temp diff in HE

Parameter Z = Temp range of shell fluid/Temp range of tube fluid

Values of F <0.75 should be read from graphs since graphs are not accurate for F<0.75.

Then, use complex mathematical equations to find ‘F’.

Low value of F means more area of heat ex-changer.

(b)NTU METHOD

Use this method when LMTD is not known. When the four temperatures are not given.

NTU = Number of transfer units (dimensionless) which represents the size of heat exchanger. NTU measures the effectiveness of the heat exchanger. It is a fictitious term.

NTU= U A/ (m.cp)min = U A/ C min

  value of (m.cp)hot =Chot

(m.cp)cold=Ccold

Where C min  is smaller of C hot  and C cold  

C = C min/C max

where C is  heat capacity ratio

EFFECTIVENESS

Effectiveness  Є = q. actual /q. max

q. max = C min(T max – T min)= C min(T hot in – T cold in)

(i) Effectiveness for parallel flow heat exchanger

Є = (1 –e—NTU(1+C))/(1+C)

(ii)                Effectiveness for counter flow heat exchanger

Є = (1 –e—NTU(1–C))/(1–C e—NTU(1—C))

where        C = C min/C max

https://mesubjects.net/wp-admin/post.php?post=14136&action=edit                    MCQ HEAT EXCHANGERS

https://mesubjects.net/wp-admin/post.php?post=2686&action=edit                      Q.ANS. HEX

https://mesubjects.net/wp-admin/post.php?post=1344&action=edit          LMTD PARALLEL & CONUTER

 

Similar Posts