EP0547363A1 - Metal heat-exchanger tube for cooling viscous fluids - Google Patents

Abstract

The invention relates to a metal heat-exchanger tube (1) having integral fins (2 or 3), running round on the outside and inside in the shape of a helix, for cooling viscous fluids (4), in particular oil in servo circuits of vehicles, the fluid (4) to be cooled flowing through the tube (1) and the outside thereof being cooled by air (5). A discontinuous rise in heat removal is achieved according to the invention by the following features: a.) the ratio of the external heat transfer area Aa of the tube (1)/internal heat transfer area Ai of the tube (1) is: <IMAGE> b.) the relationship: <IMAGE> holds for the clear internal diameter di of the tube (1), with internal diameter di (mm), volumetric <IMAGE> of the fluid (4) (m<3>/h), kinematic viscosity nu (mm<2>/s). <IMAGE>

Description

The invention relates to a metallic heat exchanger tube according to the preamble of claim 1.
In vehicles with power steering systems, the amount of oil required for the steering function is delivered via an oil pump, which maintains an almost constant delivery rate in the oil circuit regardless of the engine speed.

Depending on the engine load and steering actuation, the oil is heated. The highest oil heating occurs in trailer operation when driving uphill. Heat exchanger tubes are used to cool the heated oil, and air is applied to them from the outside.

In a known embodiment, it is a heat exchanger tube made of metal, which has integral ribs encircling the inside and outside on the inside and outside. With increasing oil temperature and constant air velocity, this heat exchanger tube has a heat dissipation proportional to the temperature difference of the oil / air temperature. With extreme steering stress, the oil temperature will continue to rise despite the heat exchanger tube. A critical point is reached when the oil temperature is equal to or higher than the resistance temperature of the sealing materials used. The result is oil loss due to leaks and thus reduced functionality or failure of the power steering, not to mention the environmental impact of escaping oil. An extension of the heat exchanger tube is due to the cramped Space in the engine compartment of the vehicle is usually excluded and increases the cost of the heat exchanger.

The invention is therefore based on the object of designing a heat exchanger tube of the type mentioned in such a way that the heat dissipation increases disproportionately as the oil temperature rises, in order to avoid high oil temperatures and at the same time to enable high heat output in a compact size.

• a.) das Verhältnis der äußeren Wärmeübertragsfläche A_a des Rohres / inneren Wärmeübertragungsfläche A_i des Rohres beträgt:
  
  ${\text{A}}_{\text{a}}{\text{/A}}_{\text{i}}\text{> 4,3;}$
  
  
  
  
• b.) für den lichten Innendurchmesser des Rohres gilt die Beziehung:
  
  mit Innendurchmesser d_i (mm), Volumenstrom V̇ des Fluids (m³/h), kinematische Viskosität ν (mm²/s). (Die Größen A_a und A_i sind jeweils auf 1m Rohrlänge bezogen).

This object is achieved according to the invention by a heat exchanger tube with the two characterized features of claim 1:

• a.) the ratio of the outer heat transfer area A _{a of} the tube / inner heat transfer area A _{i of} the tube is:
  
  ${\text{A}}_{\text{a}}{\text{/ A}}_{\text{i}}\text{>4.3;}$
  
  
  
  
• b.) the relationship applies to the inside diameter of the pipe:
  
  with inside diameter d _i (mm), volume flow V̇ of the fluid (m³ / h), kinematic viscosity ν (mm² / s). (The sizes A _a and A _i are each related to 1m pipe length).

It has surprisingly been found that a sudden increase in heat dissipation and thus the activation of the inner fin surface occurs when the inner tube diameter d _{i is} adapted to the fluid temperature to be limited and to the existing fluid volume flow. The ratio A _a / A _i according to the invention ensures the necessary heat dissipation to the outside. In addition, this disproportionate increase in performance means that the heat exchanger can be built in a compact and thus space-saving manner.

According to a preferred embodiment of the invention, the ratio A _a / A _i = 5-8.

The relationship applies to a usual fluid volume flow V̇ of about 0.4 m³ / h

$$\text{di ≦}\frac{\text{60}}{\text{ν}}\text{.}$$

dabei beträgt der Drallwinkel der inneren Rippen vorzugsweise

$\text{α = 15 - 30°,}$

insbesondere

$\text{α = 18 - 25°.}$

With regard to the formation of the inner fin surface, it is recommended if the ratio of the inner (finned) heat transfer surface A _{i of} the tube / inner smooth heat transfer surface A _i , _{smooth of} the tube is:

${\text{A}}_{\text{i}}{\text{/ A}}_{\text{i}}{\text{,}}_{\text{smooth}}\text{= 1.4-2.5;}$

the twist angle of the inner ribs is preferably

$\text{α = 15 - 30 °,}$

in particular

$\text{α = 18 - 25 °.}$

(A _i and A _i , _smooth are again related to 1 m pipe length).

In order to achieve optimal heat transfer coefficients, the square root of the ratio of the inner heat transfer area A _i / inner (free) cross-sectional area A _{i is} preferably _transverse

$\text{√}\overline{{\text{A}}_{\text{i}}{\text{/ A}}_{\text{i}}\text{,}}\overline{{}_{\text{across}}}\text{= 24-34, especially 26-32.}$

Fig.1 a

ein Wärmeaustauscherrohr im Teillängsschnitt,

Fig.1 b

einen Querschnitt eines Wärmeaustauscherrohres und

Fig.2

den sprunghaften Leistungsanstieg eines erfindungsgemäßen Ölkühlers im Vergleich zu einem Ölkühler nach dem Stand der Technik.

The invention is explained in more detail using the following exemplary embodiment. It shows

Fig.1 a

a heat exchanger tube in partial longitudinal section,

Fig.1 b

a cross section of a heat exchanger tube and

Fig. 2

the sudden increase in performance of an oil cooler according to the invention compared to an oil cooler according to the prior art.

A_a:

äußere Wärmeübertragungsfläche des Rohres 1,

A_i :

innere (berippte) Wärmeübertragungsfläche des Rohres 1,

A_i, _quer :

innere (freie) Querschnittsfläche des Rohres 1 und

d_i :

lichter Rohrinnendurchmesser des Rohres 1.

The metallic heat exchanger tube 1 according to Fig. 1a / 1b is provided on the outside and inside with integral ribs 2 and 3, which run helically. The swirl angle of the inner ribs 3, which is related to the longitudinal axis of the tube, is denoted by α. The following are also entered:

A _a :

outer heat transfer surface of the tube 1,

A _i :

inner (finned) heat transfer surface of the tube 1,

A _i , _across :

inner (free) cross-sectional area of the tube 1 and

d _i :

inner tube diameter of tube 1.

The viscous fluid 4 to be cooled flows inside the tube, air 5 outside.

The advantages of the invention are explained with reference to FIG. 2:
Measurements were carried out on a heat exchanger tube 1 according to the invention (oil cooler A) and comparative measurements were carried out on a heat exchanger tube according to the prior art (oil cooler B).

The following table shows the geometrical data of the pipes examined (oil coolers A and B):

The volume flow V̇ of the fluid (oil) in the pipes was approximately 0.4 m³ / h. The external air speed was constant at 5 m / s (which corresponds to a driving speed of 18 km / h).
2 clearly shows the sudden increase in performance in the oil cooler A according to the invention.

Claims (8)

Metallic heat exchanger tube (1) with integral ribs (2 or 3) running around the outside and inside for cooling cooling viscous fluids (4), especially oil in servo circuits of vehicles, whereby the fluid (4) to be cooled passes through the Pipe (1) flows and the outside of it is cooled by air (5),
characterized by the following features: a.) The ratio of the outer heat transfer area A _{a of} the tube (1) / inner heat transfer area A _{i of} the tube (1) is:

${\text{A}}_{\text{a}}{\text{/ A}}_{\text{i}}\text{>4.3;}$

b.) for the inside diameter d _{i of} the tube (1) the relationship applies:

with inside diameter d _i (mm), volume flow V̇ of the fluid (4) (m³ / h), kinematic viscosity ν (mm² / s). Heat exchanger tube according to claim 1, characterized in
that the ratio A _a / A _i = 5-8. Heat exchanger tube according to claim 1 or 2, characterized in
that the following applies to the inside diameter d _{i of} the tube (1):

$${\text{d}}_{\text{i}}\text{≦}\frac{\text{60}}{\text{ν}}\text{.}$$

Heat exchanger tube according to one or more of claims 1 to 3, characterized in that the ratio of the inner (finned) heat transfer surface A _{i of} the tube (1) / inner smooth heat transfer surface A _{i, smooth of} the tube (1) is:

${\text{A}}_{\text{i}}{\text{/ A}}_{\text{i, smooth}}\text{= 1.4 - 2.5.}$

Heat exchanger tube according to claim 4, characterized in
that the twist angle of the inner ribs (3)

$\text{α = 15 - 30 °.}$

Heat exchanger tube according to claim 5, characterized in
that the twist angle α = 18 - 25 °. Heat exchanger tube according to one or more of claims 1 to 6, characterized in that
that the square root of the ratio of the inner heat transfer area A _i / inner (free) cross-sectional area A _i , _transverse is:

$\text{√}\overline{{\text{A}}_{\text{i}}{\text{/ A}}_{\text{i}}\text{,}}\overline{{}_{\text{across}}}\text{= 24-34.}$

Heat exchanger tube according to claim 7, characterized in
that

$\text{√}\overline{{\text{A}}_{\text{i}}{\text{/ A}}_{\text{i}}\text{,}}\overline{{}_{\text{across}}}\text{= 26 -32.}$

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