Fin tubes----Finned heat exchanger tubes' heat transfer capacity and resistance capacity

Fin tubes----Finned heat exchanger tubes' heat transfer capacity and resistance capacity

The last two lecture make peace, has repeatedly referred to the outer tube finned tube heat transfer coefficient (h) the concept and mentioned due to air-side or gas side heat transfer coefficient is very low, we need a wing piece tube truth. This section will explain the heat transfer coefficient is calculated. Further, when the fluid flows through the fins bundles, to overcome certain flow resistance, which would cause pressure drop △ P, the greater the pressure drop, indicating that the greater the power consumed. Therefore, the calculation of pressure drop is one that should be of concern, this section also describes the pressure drop is calculated.

1. Fluid flow around the fin tube bundle heat transfer coefficient

First, the definition of the heat transfer coefficient recap: heat transfer coefficient is when fluid flows through the solid wall, the unit time per unit area and the temperature difference between the heat. It should be noted here, said unit temperature difference is between the solid wall and the fluid temperature. In this session, the heat transfer coefficient is represented by h, the unit is: W / (m2. ℃).

It should be noted that the finned tubes are arranged in rows and staggered along the points, as shown below. As rows and staggered along the flow of fluid are different, and thus the heat transfer coefficient calculation formula is different.

Staggered along the row of current flow

All finned tube bundle heat transfer coefficient calculation are obtained from the test out, tests to consider many factors, which are the result of known experimental correlations. Different researchers conducted tests may yield different experimental relational form, but its results should be similar. Our task is to choose a trustworthy correlations calculated. Here, it is recommended Briggs and Young's experimental correlations. They had more than ten kinds of annular finned tube bundle studied experimentally, all experiments by fork tubes are arranged in a row, tube pitch equilateral triangular arrangement. The standard error of about 5%. The experimental results are described below:

For high finned tubes:

When (df / db) = 1.7 -2.4, db = 12 -41 mm, the

h = 0.1378 (λ / db) (db Gmax / μ) 0.718 (Pr) 0.333 (Y / H) 0.296

For low-finned tubes:

When (df / db) = 1.2 -1.6, db = 13.5 -16 mm when

h = 0.1507 (λ / db) (db Gmax / μ) 0.667 (Pr) 0.333 (Y / H) 0.164 (Y / t) 0.075

Where, df, db: fin and base outer tube diameter, m; Y, H, t: fin clearance height and thickness, m;

λ, μ and Pr are the thermal conductivity of the fluid, the viscosity coefficient and the Prandtl number. Temperature of the fluid by the fluid properties of the table obtained Cha; formula Gmax is the narrowest cross-section at the fluid mass flow rate, the unit is Kg / (m2. s).

The so-called narrowest section refers caught between two adjacent finned tubes in cross-section. From the above equation, affecting heat transfer coefficient h biggest factor is the flow rate: 0.718 or with Gmax of 0.667 power is proportional. How to apply this correlation is calculated, followed by an example to illustrate this.

2. Fluid flow around the flow resistance of finned tubes

Robinson and Briggs for more than ten kinds of staggered circular finned tubes under isothermal conditions for a flow resistance tests. Experiments are:

     Re = (db Gmax / μ) = 2000 - 50000

     Pt / db = 1.8 -4.6, where, Pt = S1, the transverse tube spacing;

     df / db = 1.7 -2.4

     db = 12 -41 mm

Resistance that the pressure drop expression:

     △ P = f × (N G2max) / 2 ρ unit is Pa

The above formula, N is the number of vertical tube rows, f is the friction coefficient, is a dimensionless number. For the equilateral triangular arrangement of the tube bundle, by the following experimental correlation formula:

     f = 37.86 (db Gmax / μ) - 0.314 (Pt / db) - 0.927

Seen from the above two equations, the impact pressure drop finned tube bundles main factors: first, the flow rate is proportional to the power 1.7 Gmax; second tube spacing, is almost inversely proportional to a Pt. Therefore, in order to reduce drag, you can use the larger tube spacing and lower fluid flow rate.

3 Calculation example

A finned tube bundle are arranged staggered equilateral triangle, which the frontal area of ​​2m × 2m, air flow flowing through the tube bundle is 32000 kg / h, the finned tube bundle geometry is:

CPG (φ38 × 3.5 / 70/6/1) ------ (meaning expressed on this look Lecture)

Tube spacing: 92, longitudinal (flow direction) of tube rows 10 rows. Air inlet temperature is 20 ℃, while the outlet temperature of 100 ℃.

Calculate the air flowing through the tubes when the convective heat transfer coefficient and pressure drop.

3-1 under investigation to take the average temperature of the fluid properties:

   Average temperature = (20 +100) / 2 = 60 ℃, the air at this temperature is the physical properties:

   Density: ρ = 1.06 kg / m3 viscosity: μ = 20.1 × 10-6 kg / (ms)

   Thermal conductivity: λ = 0.029 W / (m. ℃) Prandtl number: Pr = 0.696

3-2 calculated flow rate:

   The windward side of the air mass flow rate: Gf = 32000/3600 / (2 × 2) = 2.22 kg / m2 s

   Narrowest cross-sectional area / frontal area

     (Pt × 1000) - (2 × db / 2) × 1000 - (1000/6) × T × H × 2

   = ------------------------------------------------- -------------------------- =

                   Pt × 1000

   = (92 × 1000-38 × 1000-166.6 × 1 × 16 × 2) / (92 × 1000) = 0.529

   The mass flow rate of the narrowest cross-section:

   Gmax = Gf / 0.529 = 2.22 / 0.529 = 4.2 kg / m2.s

3-3 calculating heat transfer coefficient

   h = 0.1378 (0.029 / 0.038) (0.038 × 4.2 / (20.1 × 10-6)) 0.718 × 0.6960.333 ×

(5/16) 0.296 = 34.3 W / (m2 ℃)

 The calculation of the fin of the finned tubes ratio of 8.72, the fin efficiency of 0.78, so that the outer surface of the base tube to the heat transfer coefficient on the basis of:

  h = 34.3 × 8.72 × 0.78 = 233 W / (m2. ℃)

3-4 calculate pressure drop

 

  First calculate the friction coefficient, f = 37.86 [(0.038 × 4.2) / (20.1 × 10 -6)] -0.314 (92/38) -0.927

                     = 0.9946

  Flowing through the discharge tubes 10 of the pressure drop, △ P = f × (10 × 4.2 2) / (2 × 1.06) = 82.76 Pa

  For each discharge pipe pressure drop = 82.76 / 10 = 8.276 Pa

4 Calculation Form

The above calculation shows that the outer tube finned tube heat transfer coefficient and pressure drop calculation is quite complicated, and for non-professionals, there will be some difficulties. Therefore, the following is already calculated data in a number of groups, for use and reference. Below.

When the gas flow around the fin tube bundle heat transfer coefficient and flow resistance calculation table

Explanation of the table: a calculation of the average temperature of the fluid 100 ℃; applicable with air or gas.

            Two equilateral triangular fin staggered arrangement; N represents the flow direction of tube rows.

            3 "fin specifications" see the second lecture in the description;

            4 "head mass flow rate" refers to the fluid before entering the bundle, the unit flow area per unit time (second) of the fluid flowing through the mass (kg).

Application Method: a calculation or select "face mass flow rate" and "fin Specifications";

           2 In the table to find the corresponding values ​​similar to check access h, △ P values.

If you are still in the application when the table was a bit of trouble, may also wish to last row from the table "average value" direct selection. However, there will be about 20% of the error, while the average value is only suitable for high frequency welding finned tube bundles common specifications.

5 About the finned tube heat transfer coefficient and pressure drop discussions

(1) and the other heat exchanger, as described high heat transfer coefficient can save the heat transfer area, reduce equipment an investment; the large pressure drop, indicating that the device resistance is large, a large operating cost, increasing the second investment.

(2) the structural parameters of certain circumstances, the heat transfer coefficient and pressure drop are mainly affected by the fluid flow (here the mass flow rate) effects, both as the flow rate increases. Stresses recommended by the formula shows that the fin tube bundle heat transfer coefficient h and the mass flow rate of 0.718 power is proportional; while the pressure drop △ P and the mass flow rate of 1.684 proportional.

(3) the pressure drop when the user has a very demanding requirements, such as requiring the entire finned tube heat exchanger pressure drop shall be controlled at 100 Pa or less, then the measures to be taken are: 1) to expand the frontal area, reducing small face velocity; 2) to expand the tube spacing, further reducing the mass flow rate at the narrowest section. Please note that when you do not pay the price are: heat transfer coefficient decreases, increasing the heat transfer area.

(4) the pressure drop when the user does not explicitly require or request more relaxed when you as the designer can choose a larger mass flow rate, to make the structure more compact.

(5) In the case of more serious fouling, but also need to ensure that a certain velocity in the fluid (gas) with a certain self-blowing capability.

The end of the talk, describes a special type of fin tubes --- oval finned tube, which substrate tube is elliptical, and the square of the fins. As shown below.

This tube has the advantage of: Heat transfer coefficient and resistance. The exterior design is like the same car, the base pipe connector

Near streamlined, less resistance than the tube. The disadvantage is the high cost of manufacturing. 

Key Words: Finned Tube, Fin Tubes, Extruded Fin Tubes, Embedded Fin Tube, L Fin tube, Tubos Aletados, Finned Tube Heat Exchanger