WO2009026991A1

WO2009026991A1 - Pump, in particular for cryogenic media

Info

Publication number: WO2009026991A1
Application number: PCT/EP2008/005776
Authority: WO
Inventors: Wilfried-Henning Reese; Martin BRÜCKLMEIER
Original assignee: Linde Aktiengesellschaft
Priority date: 2007-08-24
Filing date: 2008-07-15
Publication date: 2009-03-05
Also published as: DE102007040089A1

Abstract

The invention relates to a displacement machine, in particular a pump with a piston that is arranged inside a cylinder chamber. According to the invention, the cylinder wall is at least partially formed as a thin metal wall (4), suitable for absorbing the sealing forces for the piston rings (3), the metal wall (4) is encased in an insulating layer (5) and/or a carbon fibre and/or Kevlar® composite material, said insulating layer (5) having a high compressive strength, a high modulus of elasticity, a low thermal conductivity and a low thermal capacitance, and the insulating layer (5) is located in the cylinder wall (1) that surrounds said layer.

Description

description

Pump, in particular for cryogenic media

The invention relates to a displacement machine, in particular a pump, comprising a piston arranged within a cylinder space.

Generic displacement machines or pumps are well known from the prior art. When such positive displacement pumps are used to pump cryogenic media, such as liquid hydrogen (LH ₂ ), problems arise that are not an issue when pumping non-cryogenic media.

At present, especially with regard to the so-called. High-pressure hydrogen production, which is gaining in importance due to the increasingly used liquid hydrogen storage and the application for filling vehicle high-pressure accumulators, paying attention to the pump constructions used for it.

Known pump designs that allow the realization of a high-pressure hydrogen production, produce currently in addition to the (desired) high-pressure hydrogen gas - this is gaseous hydrogen, which is present under a pressure of 250 to 900 bar, to understand - significant amounts of

Low-pressure hydrogen gas - this is to be understood as gaseous hydrogen which is present under a pressure of from 1.0 to 8.0 bar. The proportion of this low-pressure hydrogen gas can be up to 30%.

The formation of this (undesired) low-pressure hydrogen gas occurs because the known pump designs permanently cool the cylinder space of the displacement piston with the medium to be pumped, in order to achieve recondensation of the gas quantity remaining as residual gas in the dead space of the pump. or due to the friction of the piston. To promote this low-pressure hydrogen gas additional compressors are required to store the low-pressure gas at about 450 bar and feed it to the vehicle tank during a filling. The current state of the present filling technology provides, due to the aforementioned disadvantages, not to use LH ₂ pumps. The stored in the storage tank at the liquid hydrogen filling station liquid hydrogen is removed at a storage pressure of about 2.5 to 5 bar, warmed in heat exchangers to ambient temperature and then fed to a pre-compressor. This generates an intermediate pressure of about 25 bar and supplies the gas to a reciprocating compressor, which compresses the hydrogen to 250 to 300 bar. The thus compressed hydrogen is temporarily stored in a memory bundle. For refueling, the vehicle tank is initially filled by overflowing from the storage banks to about 450 bar. In order to be able to realize a refueling up to 700 bar accumulator pressure, the temporarily stored hydrogen is removed from the accumulator bundle by means of a so-called booster compressor with high delivery rate, compressed to approx. 850 bar and fed to the vehicle.

The still cold low-pressure hydrogen gas can be recycled to the LH ₂ storage tank from which the hydrogen to be delivered has been taken. However, this procedure results in the case of larger amounts of cold gas to an undesirable increase in pressure and temperature of the liquid in the storage container. Thus, the tank pressure must be lowered after reaching the maximum allowable working pressure either by blowing off gas or by compressing the gas to be blown off and caching the compressed gas.

In general, each pump introduces heat energy into the medium to be pumped during the compression and delivery stroke. The heating of the medium depends on the compression ratio of the pump. Cryopumps for LH ₂ would thermodynamically isentropic compression - this means that there is no heat exchange with the environment - with the frictionless piston during the compression and displacement from 0.25 to 45 MPa heat the fluid by about 15 K, while the Medium at a compression of 0.25 to 90 MPa by about 24 K would heat. Such heating of the medium would not be detrimental to the pumping system if the heated medium can not heat exchange with these components due to adiabatic cylinder wall and piston system design. The heated medium would then be directed to the high pressure side without reaction to the pump system. There it is Heating of the medium is not harmful, since the high-pressure medium for further use is then further heated in a heat exchanger. Such "adiabatic pumping systems" can not be realized so far, because due to the high pressures of 45 to 90 MPa metallic components, eg. For the cylinder wall, are required. However, these result in a significant heat exchange in the pump system.

The cylinder walls of the pumps of the prior art are designed with a considerable wall thickness due to the high pressures occurring in the cylinder chamber. The wall thickness depends on the cylinder diameter and material and is usually at least 6 mm up to 35 mm and more when using high-strength stainless steels. Due to its good conductivity and large mass during the compression and displacement stroke, the metal cylinder wall absorbs considerable amounts of heat from the heated pumped liquid and returns it to the (deeply cold) liquid newly supplied to the suction stroke during stroke reversal. As a result, cold gas is produced on the bulb wall, which due to its low density prevents optimal filling of the cylinder with (cryogenic) liquid. This effect can lead to a significant deterioration of the degree of filling of the cylinder. In the next delivery stroke, this leads to a further, even higher heating of the fluid and a temperature cycle which can lead to zero pump delivery.

To prevent this undesirable effect, it has already been proposed to realize a cooling of the cylinder wall. Such a pump construction is known from the unpublished German patent application 102007035616.

This solution allows a thermodynamically quasi-isothermal compression, since not only the cylinder wall, but also a large part of the medium to be pumped is cooled. The cooling of the cylinder wall can be carried out in the compression and promotion of LH ₂ only by a medium with a lower temperature; Usually, therefore, the cooling is carried out with the pumping medium supplied. Theoretically, it would also be possible to cool with externally cooled helium gas; However, such a procedure would be realized only in exceptional cases because of the associated costs and effort. This cooling, when done with the supplied fluid, leads to evaporation of a proportion of the supplied fluid. The case undesirably produced cold gas whose Share may amount to up to 20% of the delivery, is lost for the promotion and must be otherwise compressed or used with the disadvantages described above.

Another technical problem of LH ₂ displacement machines or pumps is that they must first be cold-started after a standstill period before restarting. This is usually done by conveying medium, which is removed from the storage container and recycled in the circuit as cold gas to the storage container, the LH ₂ -durchmungsmaschänen or pump flows through. However, unwanted heat is also introduced into the storage container in this procedure. Since when using the LH ₂ - displacement machines or pump as a filling pump in a hydrogen filling station for vehicles several longer downtime per day are expected (must), can be introduced into the storage tank by the above-described cold driving procedure significant amounts of heat.

Object of the present invention is to provide a generic displacement machine, in particular a pump, which avoids the aforementioned disadvantages.

To solve this problem, a displacement machine, in particular a pump, comprising a disposed within a cylinder chamber piston proposed, which is characterized in that

- The cylinder wall at least partially as a thin metallic wall, which is adapted to receive the sealing forces to the piston rings, formed and

the metallic wall is covered by an insulating layer and / or a carbon fiber and / or a Kevlar composite material,

wherein the insulating layer has high compression strength, high elastic modulus, low thermal conductivity and low heat capacity, and the insulating layer is disposed in the surrounding cylinder wall.

The displacement machine or pump according to the invention enables the realization of a thermodynamically quasi adiabatically executed cylinder wall. This can be achieved by two mutually alternative design options, namely on the one hand by a combination of metallic wall and insulating layer and on the other by a combination of metallic wall and a carbon fiber and / or a Kevlar composite material, but also the realization of the two aforementioned combinations in a displacement machine or pump is possible.

By means of the displacement machine or pump according to the invention - if materials are selected for the insulating layer or the carbon fiber and / or Kevlar composite material, which have the property of neither transporting nor storing heat - now be achieved that

Heat transfer to the supplied in the intake stroke and to be conveyed medium almost impossible. As a result, the unwanted evaporation of the supplied medium. be avoided on the piston wall.

Under the above-mentioned term "thin metallic wall" are wall thicknesses of about 1 to 3 mm to understand. These wall thicknesses can not absorb the high forces and stresses that occur in the cylinder wall at an internal pressure of 45 to 90 MPa. However, they are capable of absorbing the sealing forces to the piston seals and of providing a suitable low friction friction surface for the piston seals.

The thin metallic wall is covered by an insulating layer and / or a carbon fiber and / or Kevlar composite material. These must have a sufficiently high strength and a sufficiently high modulus of elasticity. However, their heat conduction and heat capacity should be as low as possible.

The insulating layer, unlike the carbon fiber and / or Kevlar composite material, can not absorb the high forces and stresses, but can transmit them. Therefore, the insulating layer is incorporated within the surrounding metal walls of the cylinder wall, wherein the insulating layer is preferably installed gas-tight. Preferably, this space is evacuated. The insulating layer is thus surrounded by a much firmer outer metal wall, which is then able to absorb the forces transmitted by the insulating layer.

Further advantageous embodiments of the displacement machine according to the invention, which are the subject of the dependent claims, are characterized in that

the insulating layer consists at least partly of a ceramic material,

the strength of the carbon fiber or Kevlar composite material is at least 3000 N / mm ² , preferably at least 4500 N / mm ² , and

- The compressive strength of the insulating layer is at least 2000 N / mm ² , preferably at least 5000 N / mm ² and / or the modulus of elasticity of the insulating layer is at least 170,000 N / mm ² and / or the thermal conductivity of the insulating layer at most 8 Wm ^-1 K. ^'1 , preferably at most 1, 5 Wm ¹ K ^"1 and / or the specific heat capacity of the insulating layer is at most 0.7 KJKg ¹ K ^{" 1} , preferably at most 0.2 KJKg ¹ K ^"1 .

In this case, the latter statements relate to the compressive strength, the modulus of elasticity, the thermal conductivity and the specific heat capacity to a temperature of 300 K or relate to ambient conditions. Under cryogenic conditions, the values may differ significantly from the stated values.

The displacement machine or pump according to the invention and others

Embodiments of the same, which constitute subjects of the dependent claims, will be explained in more detail below with reference to the embodiment shown in the figure. The figure shows a schematic side sectional view through a hydrogen high-pressure piston pump for delivery pressures up to 1000 bar, which has a comparatively good degree of delivery of up to 90% or more.

By a cylinder wall 1, a cylinder space 6 is defined. Within this cylinder space 6, a piston 2 is arranged. The seal between the piston and cylinder wall 1 by means of one or more piston seals. 3

According to the invention, the cylinder wall is at least partially formed as a thin metallic wall 4. This must be designed such that it is able to absorb the sealing forces to the piston rings 3.

The metallic wall 4 in turn is covered by an insulating layer 5. This insulating layer 5 has a high pressure resistance, a high modulus of elasticity, a low thermal conductivity and a low heat capacity. The insulating layer 5 is arranged gas-tight in the surrounding cylinder wall 1.

The insulating layer 5 is preferably at least partially made of a ceramic material, such as silicon nitride (SI ₃ N ₄ ) or zirconium oxide (ZrO ₂ ), or similar materials.

Silicon nitride has a combination of outstanding material properties which has not been achieved by other ceramics so far, such as high toughness, high strength, even at high temperatures, excellent

Thermal shock resistance, excellent wear resistance, low thermal expansion, average thermal conductivity and good chemical resistance includes. The advantages of zirconia are: high fracture toughness, thermal expansion similar to that of cast iron, highest flexural and tensile strength, high wear and corrosion resistance, low thermal conductivity, oxygen ion conductivity and very good tribological properties.

The displacement machine or pump according to the invention, with a manageable constructional and financial outlay, provides the possibility of a generic displacement machine or pump for a wide variety of applications Use cases to avoid the disadvantages associated with the compression of certain media as much as possible. The invention also allows a faster cold-driving procedure, so that the restart of a pump after a (longer) downtime can be done within a shorter period.

Claims

claims

1. displacement machine, in particular pump, having a arranged within a cylinder chamber piston, characterized in that

- The cylinder wall at least partially as a thin metallic wall (4), which is adapted to receive the sealing forces to the piston rings (3), formed and

the metallic wall (4) is encased by an insulating layer (5) and / or a carbon fiber and / or a Kevlar composite material,

- wherein the insulating layer (5) has a high pressure resistance, a high elastic modulus, a low thermal conductivity and a low heat capacity, and

- The insulating layer (5) in the surrounding cylinder wall (1) is arranged.

2. displacement machine according to claim 1, characterized in that the insulating layer (5) is arranged gas-tight in the surrounding cylinder wall (1).

3. displacement machine according to claim 1 or 2, characterized in that the insulating layer (5) consists at least partially of a ceramic material.

4. displacement machine according to one of the preceding claims 1 to 3, characterized in that

- The compressive strength of the insulating layer (5) is at least 2000 N / mm ² , preferably at least 5000 N / mm ² and / or

- The modulus of elasticity of the insulating layer (5) is at least 170,000 N / mm ² and / or

- The thermal conductivity of the insulating layer (5) is at most 8 Wm ^'1 K ^{' 1} , preferably at most 1, 5 Wm ^-1 K ^"1 and / or - The specific heat capacity of the insulating layer (5) is at most 0.7 KJKg ^'1 K ^"1 , preferably at most 0.2 KJKg ¹ K ^{" 1} .

5. Displacement machine according to one of the preceding claims 1 to 4, characterized in that the strength of the carbon fiber or Kevlar composite material is at least 3000 N / mm ² , preferably at least 4500 N / mm ² .