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CET 1 > Heat & Mass transfer operations > Flashcards

Heat & Mass transfer operations Flashcards

1
Q

How do you get to Q = UA tlm

A

Take an element in the concentric heat exchanger. Then, rate of heat transfer from inner fluid to fluid in outer shell = UA (t-T). Talking about q not Q.

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2
Q

What happens when mcp = MCp

A

Operating line gives t-T= t1-T1

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2
Q

What systems does Q = UA (T)lm apply to

A

Both co- & counter- current flows. Identical result when Tw is kept constant ( fluid condensing).

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3
Q

Pressure drop across a tube formula

A

4cfL/D1/2pu2

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4
Q

General assumptions to measure pressure drop

A

Constant Tw, U and cross surface area

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5
Q

Operating line for co-current case

A

Perform overall heat balance over the exchanger. i e equate the qs.

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6
Q

How do the operating temperature lines change (co-current) with A&u changing?

A

Slopes = 1/W (-ve for A) and ta=tb when Q tends to infinity. Limitation is that there is no temperature cross-over.

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7
Q

Operating line for the counter current case

A

temperature change in B is now negative. Slopes maintain the same magnitude (but negative for both A and B). Possible to have temperature crossover.

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8
Q

difference between Q and q

A

Q is the total heat transferred or total duty, whereas q is the rate of heat transfer. Q = UA(T)lm or MCp(t1-t2) and q = U(t1-t2)

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9
Q

difference between design and rating calculations

A

Design calculations give the magnitude of UA required to achieve a given heat transfer duty (Q). A rating calculation is used to determine how a fixed configuration performs given a set of inputs.

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10
Q

Steps to solve single pass concentric cylinder questions

A
  1. Energy balance Q and log mean Q 2. Multiply through by Wb to get xi. psi is the exponent term.
  2. Two unknowns from the operating line expression, eliminate one using psi and xi.
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11
Q

Why is anything a rating case and not a design case?

A

When the inlet temperatures are known, UA and xi are specified and we need to know how the exchanger performs.

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12
Q

How do you calculate the efficiency of a concentric tube heat exchanger?

A

After the unknowns are represented in terms of psi and xi, the same can be done with Q. Qmax can be found when UA tends to infinity and psi tends to 0. efficiency = Q/Qmax

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13
Q

What do psi, xi and epsilon represent?

A

psi is the exponent term which is mainly affected by UA. xi is the ratio of the heat capacity flow rate W for A and B. epsilon stands for efficiency.

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14
Q

How much extra energy is required for increase in heat transfer (through increasing flow rate)?

A

This energy can be calculated using the pressure drop on the water side. Generally, an incremental rate of return for energy savings.

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15
Q

When is blassius valid?

A

Re < 20000

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16
Q

When is Gnielinski valid?

A

Re < 2300, 0.5<Pr <2000

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17
Q

Which temperature is used to calculate Re and Nu for Gnielinski?

A

Properties are calculated at the film temperature Tf = 1/2(Twall+Tbulk)

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18
Q

In the design equation Q = UA (T)lm, what area do you use?

A

We use the curved surface area A = pi dL

19
Q

Overall heat transfer coefficient formula

A

1/U = 1/ha +fouling resistance + 1/hb
As fouling increases, U decreases.

20
Q

What is the pressure drop proportional (length and diameter) to in a simple heat exchanger?

A

pressure drop prop to L2/d5

21
Q

How do pressure drop and heat transfer coefficients vary with d?

A

Heat transfer coefficient increases with decreasing d and so does the pressure drop. Hence, important to find optimal point.

22
Q

What happens as n (number of tubes) changes?

A

The area changes in the design equation. it is now A = n pi dL. Extra degree of freedom in the design. u = m/(cross section areadensityn)

23
Q

What constitutes a feasible heat exchanger design?

A

Re about 9000 and 3<L<10m

24
Q

Why are multi pass heat exchangers generally better than single pass heat exchangers?

A

Large duties require large L and n, however n is limited by its effect on U and pressure drop. So, we can incorporate counter current and co-current flow in the same shell (like a U tube). One tube pass means going from left to right once, two means two times. Number of tubes (n) is different from number of tube passes.

25
Q

What is the prime objective in heat exchanger design

A

Optimising A (as low as possible)

26
Q

In heat transfer coefficients when do we use h and when do we use k

A

h for convection and k for conduction, however both can be related using Nu.

27
Q

What is F?

A

F is the correction factor for A. It accounts for the mixed co and counter current flow. To find F, R and S both functions of temperature differences have to be calculated.

28
Q

If there is milk and water, which one goes in the tube?

A

Milk goes in the tubes as it is most likely to cause fouling and hence require cleaning.

29
Q

What does temperature cross-over mean?

A

Approach temperature is defined by the difference between the outlet temperatures. When this is negative, a temperature cross-over occurs. This is the point where the stream would change from being heated to cooled or vice versa.

30
Q

How do you deal with a temperature cross over

A

Use two shell passes (using a horizontal baffle), which offsets the decrease in shell temperature by increasing the avg shell temp in each section. Generally two separate shells are used.

31
Q

How does fouling cause over-designing?

A

The extra area due to the fouling resistance is extremely sensitive (changes by a lot for a small change in fouling), leading to over designing (costly). The uncertainty in h values is normally already nullified by the fouling resistance.

32
Q

How can over-designing cause problems?

A

Over designing would mean that when the unit is clean, it would over perform, generally not yielding the desired result.

33
Q

When does kinetics dominate mass transfer?

A

Design is based on rate of mass transfer in continuous contact systems. If resistances to mass transfer are significant, then kinetics determine the amt transferred.

34
Q

How do Wetted wall columns work?

A

Liquid down and gas up. Any soluble component in gas will dissolve in the liquid. Surface area of gas-liquid interchange clearly defined.

35
Q

Why are packed columns better than wetted wall columns?

A

Much larger interfacial area for mass transfer.

36
Q

Random vs Structured packing

A

Random (rings saddles) vs grid packing. Grid offers larger transfer area per unit volume.

37
Q

Plate vs packed columns

A

Plate - can handle wider ranges especially at lower flow rates where as for packed liquid hold up is smaller. Number of plates can be established with more certainty whereas packed columns are particularly useful for small diameters. Plat columns have easier cooling and heating with coils and easy to clean. Packed columns have lower pressure drops.

38
Q

EQBM Relation (Gas vs lie)

A

Concentration of gas and liquid are related by Hc.
Cgas = Hc Cliq

39
Q

Difference between Hc and K /Hm

A

Hc is for concentration of solute whereas Hm is for molar fraction of solute in gas and liquid.

40
Q

When bulk gas and bulk liquid are in contact, where is eqbm assumed?

A

Blue concentrations won’t be eqbm so have to consider rate of mass transfer between them. Eqbm is assumed at the interface itself.

41
Q

Mass transfer calculations at the interface, how to proceed.

A

Perform a gas side and liquid side mass transfer (using k dC) and consider hypothetical eqbm stage.

42
Q

Difference between k and K

A

K is the overall mass transfer coefficient (between the bulk concentrations, for example Kg would be the gas side coefficient N = Kg(Cg-Cg) where Cg is gas equivalent concentration in eqbm with the liquid bulk).
k is just the coefficient of mass transfer between the bulk and interface.

43
Q

How does Kg and Kl change with varying solubility of the solute?

A

When gas is not very soluble Kl is almost the same as kl (Resistance to mass transfer is on the liquid side, or Cg = Cgi).
When gas is very soluble, then Kg is almost equal to kg (gas-side controlled)
Intermediate solubility - in the middle (neutral)

44
Q
A

CET 1 (2 decks)

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