Fuel circuit with protected main filter

Details Fuel circuit with protected main filter Fuel circuit with protected main filter

2000-01-12

2001-06-26

Lithgow, Thomas M. (Department: 1724)

Liquid purification or separation

With heater or heat exchanger

For filter

C210S416400, C210S443000, C244S13500B

Reexamination Certificate

active

06251270

ABSTRACT:

DESCRIPTION
1. Field of the Invention
The invention involves the area of filters for fuels and particularly filters used in fuel circuits of aeroplane engines. It also involves the fuel circuits which use such filters.
2. Discussion of the Background
Filters, and particularly fuel filters, have long been known. They have an air-tight chamber, in a cylindrical shape for example. This chamber contains a filtering element. A fuel input and output are located on either side of the filtering element so that the fuel which leaves must go through the filtering element. An example of an aeroplane fuel circuit using such a filter is shown schematically in FIG.
1
.
Fuel from a fuel tank which is not shown is brought by a pipe
1
to a low-pressure pump
2
which is a booster pump for a main pump
3
located downstream in the circuit. A main filter
4
receives the fuel from the low-pressure pump
2
and also the fuel from the return circuits which will be described later. The filter
4
includes, in a known fashion as described above, a fuel input
5
, located upstream, a fuel output
6
located downstream, and an air-tight chamber
7
delimited by a case
40
. In a manner which is known but not shown in
FIG. 1
, the main filter
4
has a filtering element
8
. At the output from the main pump
3
, the fuel goes through a doser
9
. The doser
9
, which is servo controlled, releases only the quantity of fuel needed for operation of the engines as a function of the flight conditions. Branches
10
,
11
located upstream from the doser
9
and downstream from the main pump
3
, direct the fuel to the servo mechanisms
12
which use the pressure of the fuel from the main pump
3
for their operation, and also to the heat exchanger
13
through a regulating valve
15
.
For the explanations which follow, some indications regarding fuel temperatures will be needed. Temperatures in degrees Celsius corresponding to a lower limit exterior temperature for a type Al fuel are given for information purposes for different locations in the circuit shown in FIG.
1
. The fuel in the fuel tank is at a temperature close to the outside temperature at the flight altitude during a flight at 54° C. for example as shown in FIG.
1
. This low temperature could interfere with proper functioning of the servo mechanisms
12
and for this reason, means
14
for fuel heating are installed upstream from the servo mechanisms
12
. This low temperature is however advantageously used to cool various means of operation of the airplane and particularly the engines. This cooling occurs by means of the main heat exchanger
13
mentioned above. The fuel leaving the servo mechanisms
12
is taken by a pipe
16
upstream from the main heat exchanger
13
when it is combined with the fuel leaving the regulating valve
15
. When it leaves the main heat exchanger
13
, the fuel may be in the conditions shown in
FIG. 1
at a temperature of about 7°. This fuel is drawn downstream from the low pressure pump
2
, and upstream from the main filter
4
, by a pipe
17
, which forms the downstream extremity of the return circuit mentioned above. This return circuit includes the pipe
16
downstream from the servo mechanism
12
, the branch
11
leading to the regulating valve
15
and the circuit of the main heat exchanger
13
terminated by the pipe
17
. Given the flow of fuel in the various elements of the circuit which has just been described and the operating conditions for the example chosen, the fuel temperature upon entry into the main filter
4
is about −17° C. There can of course be wide fluctuations in this temperature because it is the result of a mixture of the fuel coming from the fuel tank and for which the temperature depends on the initial flight conditions, the flight time, and the outside temperatures encountered, and also the fuel from the return circuit for which the temperature depends on the flight time, the outside temperatures encountered, but also the fuel flow needed for the engines, servo mechanisms, and heat exchanges with the cooling systems. It is thus possible that the fuel entering the main filter
4
of the fuel circuit would have temperatures close to or equal to −2° C. or −9° C., which are temperatures which favor the formation of flakes of frost which rapidly soil the filtering element
8
of the filter
4
to the point of clogging it. Detection of this phenomenon leads to the opening of a by-pass
18
which avoids the clogged filter. Other by-passes
18
which avoid other elements of the fuel circuit were indicated in
FIG. 1
but will not be discussed because this is not necessary for understanding the invention. In the case of ice formation on the filter
4
, the fuel continues to circulate through the by-pass
18
, and in particular it feeds the engine injectors, but it is no longer filtered, thus running the risk of soiling or even clogging one of several injectors, which would hinder engine operation. The circuit which has just been described in conjunction with
FIG. 1
is a generic example of a recent circuit with almost as many variations as there are engines. In addition, the description given is brief and only intended to clarify the reasons for which the fuel temperature is variable as a function of the parts of the circuit in which the fuel is contained. In the past there were turbomachines which regulated fuel reheating circuits to obtain fuel at a temperature which does not lead to ice formation. These regulated circuits are costly to install and their operation is energy-consuming. The filter according to the invention aims to reduce the probability of clogging or partial obstruction of the main filter of a fuel circuit by ice formation. This decreased probability is obtained without a costly or bulky system and without making the circuit complicated.
SUMMARY OF THE INVENTION
The filter according to the invention is a filter for a fuel circuit with an air-tight chamber formed by a case containing a filtering element, a fuel output oriented so that the fuel which enters the air-tight chamber can only reach the output through the filtering element, the filter being characterised in that it has two entries, a first one and a second formed by openings in the case, these two entries leading directly to the chamber containing the filtering element.
In a circuit including a filter according to the invention, the first entry is reserved for example for the fuel which has not received any heat treatment and which arrives from the fuel tank. The second entry will be reserved for the fuel which has undergone heating and which arrives by the return circuit. The filtering element thus receives directly from one side a “hot” fuel which does not risk freezing, and from the other side a cold fuel which is usually at a temperature below −9°, i.e. a temperature below the temperature at which the water contained in the fuel can freeze in the form of flakes which rapidly obstruct the filter. The hot and cold fuels are introduced directly into the chamber containing the filter element so as to avoid a prior mixing of the hot and cold fuels. The mixtures of hot and cold fuels formed by turbulence at the time of entry into the filter are unstable such that, if freezing is beginning at some place in the filtering element, it can reasonably be hoped that these frost conditions will themselves be unstable and that the freezing will not continue. If the cold fuel is at a temperature which allows freezing, the filter will not totally freeze but will rather have a cold spot which will freeze and clog first and a hotter spot which will freeze later. The double entry, hot and cold, allows for a degree of control, a certain orientation of the mechanism of freezing of the surface of the filtering element leading to prolonging of the functioning time without opening of the by-pass. This is due to the fact that the head loss at the filter is sufficiently low because a portion of the filtering surface remains frost-free.
The filter according to the invention can advantageously have within th

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