Reformulación del Plan Jefas y Jefes de Hogar Desocupados

1 : Passage fin efficiency for single banking (-)

2 : Passage fin efficiency for double banking (-)

h

: Passage fin height (m)

t

: Passage fin thickness (m)

n

: Passage fin density (m-1)

o : Effective heat transfer coefficient of a stream (W/m2K)

Unperforated fraction [1 - (percentage perforation)/100] (-)

m : Thermal conductivity of fin material (aluminium) (W/m K)

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Thermal and Hydraulic Design

STANDARDS OF THE BRAZED ALUMINIUM PLATE-FIN HEAT EXCHANGER MANUFACTURER'S ASSOCIATION 55

7.4.6 Rough Estimation of the Core Volume and Weight

To obtain a quick indication of the heat exchanger volume required for a certain duty, the following simple relation may be used:

C MTD Q

V / (14)

where

V

: Required volume of heat exchanger or heat exchangers (without headers) (m3)

Q

: Overall heat duty (W)

MTD : Mean temperature difference between streams (K)

C

: Coefficient; 100,000 for hydrocarbon application (W/m3K) 50,000 for air separation application

The values of 100,000 and 50,000 represent the product, UAd, assuming an overall

heat transfer coefficient of 200 W/m2K and 100 W/m3K respectively, and a mean geometric heat transfer surface density of 500 m2/m3.

The weight of a complete heat exchanger may be assumed to be 1000 kg per unit core volume (m3). This value varies in practice between 650 and 1500 kg/m3.

7.5 HYDRAULIC RELATIONS

The Purchaser usually specifies the allowable pressure loss for each stream, within the Manufacturer's scope of supply.

In the hydraulic design of the heat exchanger, the fin type and passages are chosen to meet this pressure loss requirement. In order to ensure uniform flow distribution of a stream among its passages, the components of the pressure drop are evaluated. Uniform distribution of a stream over the width of a layer is provided by good design of the distributors.

7.5.1 Components of Pressure Loss

The individual pressure losses within a heat exchanger typically consist of (See Figure 7-6):

1. Expansion loss into the inlet header 2. Contraction loss at the entry to the core 3. Loss across the inlet distributor

4. Loss across the heat transfer length 5. Loss across the outlet distributor 6. Expansion loss into the outlet header 7. Contraction loss into the outlet nozzle 8. Gravitational loss (or gain).

Additional pressure losses in piping and/or manifolding outside the Manufacturer's scope of supply are to be accounted for by the Purchaser. The Purchaser usually specifies the allowable nozzle to nozzle pressure drop.

General methods for predicting these pressure losses are given in Reference (4). Manufacturers make use of their experience to select the most appropriate method of estimating the losses given in Items 1 to 7 above.

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56 STANDARDS OF THE BRAZED ALUMINIUM PLATE-FIN HEAT EXCHANGER MANUFACTURER'S ASSOCIATION

Figure 7-6: Pressure Loss Components 7.5.2 Single-Phase Pressure Loss

The frictional pressure loss across a plate-fin passage and at any associated entry, exit and turning losses, can be expressed by:

2 2 d f 4 2 2 h m m p G K G l P (15) where

f

: Fanning friction factor (-)

lp : Passage length (m)

d

:

Hydraulic diameter of passage (m)

G

m : Mass velocity (mass flux) of stream (kg/m2s)

: Density of a stream (kg/m3)

K

: Expansion, contraction or turning loss coefficient (-)

P

: Overall pressure drop (Pa)

Note: By convention, the upstream mass flux is used for estimating expansion losses and the downstream for contractions.

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Thermal and Hydraulic Design

STANDARDS OF THE BRAZED ALUMINIUM PLATE-FIN HEAT EXCHANGER MANUFACTURER'S ASSOCIATION 57

7.5.3 Two-Phase Pressure Loss

In brazed aluminium plate-fin heat exchangers with two-phase streams where fluid quality and physical properties are changing, it is necessary to divide the heat exchanger into suitable increments of length in order to assess the overall pressure gradient simultaneously with the thermal design calculations.

The pressure gradient in a two-phase flow can be divided into three components: • The frictional component,

• The static head component, • The accelerational component.

Each Manufacturer uses suitable design correlations for estimating these components from experience. General estimating methods are given in Reference (4).

7.6 GENERAL CONSIDERATIONS IN THE THERMAL AND HYDRAULIC DESIGN 7.6.1 Choice of Fin Geometry

Each fin must conduct the required amount of heat and also withstand the design pressure at the design temperature as a structural component.

Fin geometry is therefore selected to meet both requirements. Details of the required structural performance are given in Chapter 5. Details of the fin's required thermal performance are given earlier in Section 7.4.4.

The choice of fin will also influence the most economical design of an exchanger for a specific application. Table 7-1 provides general information on common applications for each type of fin (see Figure 1-6, Chapter 1).

Table 7-1: Common Applications for Each Type of Fin

Corrugation Description Application Features Relative pressure drop

Relative heat transfer Plain Straight For general use lowest lowest Perforated Straight with small

holes

Most frequently used for any purpose. Sometimes used for the "hardway" finning

low low

Serrated Straight, offset half a pitch - usually about every 3-4 mm

Frequently used, especially for low pressure gas streams in air separation plants highest highest Herringbone or long-lanced serrated

Smooth but in waves of about 10 mm pitch (can be perforated), or serrated with long serration pitch

Often used for gas streams with low allowable pressure drop

high high

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58 STANDARDS OF THE BRAZED ALUMINIUM PLATE-FIN HEAT EXCHANGER MANUFACTURER'S ASSOCIATION

7.6.2 Layer Stacking Arrangement

With multi-stream heat exchangers, the choice of the stacking sequence or layer- pattern must take into account the local heat balance among streams and any local non-linearity of the Enthalpy-Temperature Curves of each stream. A thermally well- balanced stacking arrangement would result in a nearly uniform metal temperature at any cross section of the heat exchanger, thus allowing the detailed design to proceed with the assumption of a common wall temperature.

The deviations from a uniform metal temperature can be evaluated by using a more detailed (layer-by-layer) analysis, taking into account heat being transferred by metal conduction between non-adjacent layers.

7.6.3 Two-Phase Distribution

When a stream at the exchanger inlet is made up of a mixture of vapour and liquid, it can be necessary to separate the two phases first and then to distribute them in separate distribution systems that mix the two phases in a controlled way within the exchanger. Re-mixing of the two phases can be done in the header or in the passages depending on the distribution system being used. Details of typical systems are shown in Section 1.2.8.

7.6.4 Thermosyphon

A thermosyphon design is available based on design information from the Purchaser.

7.6.5 Manifold Assemblies

For multiple core assemblies, so-called U-piping configuration is preferred to Z-piping for improved flow distribution. Figure 7-7 illustrates this point.

U-piping Z-piping

Figure 7-7: Manifold Assemblies

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In document Informe sobre los Programas de Empleo Provinciales 2006 (página 66-69)