A THERMAL FLOW RATE SENSOR

Field of the Invention

This invention relates generally to fluid flow rate sensors, and more particularly to a thermal sensor device to detect high flow rates by restricting the heat transfer surface area.

Background

Thermal flow rate sensors have been available for several decades and the principles employed are well known in the field of flow rate detection and measurement. Examples of such devices are US patents 3,366,942, 4,449,403, and 5,600,528, among many others.

Some prior art thermal flow rate sensors have been susceptible to water hammer damage, some are relatively complicated, and some lack agency certifications.

A limitation of many thermal sensors is that they tend to saturate at a velocity which is below that which may be possible within a particular conduit. That is, the sensor may have a velocity upper limit below the maximum velocity that may be encountered.

Summary of Embodiments of the Invention

Embodiments of the invention provide improvements in high fluid flow sensing installations. One advantage is that such improvements can be applied to already proven sensor structures. Another is that such improvements are very low cost while at the same time allowing up to twenty times increased flow rate.

As shown and described, the various embodiments herein encase the thermal wells, preferably with a low heat transfer material shield to reduce the heat transfer area by approximately 20: 1. The embodiments described herein are not limited to a particular material for the shields.

Brief Description of the Drawing

The objects, advantages, and features of the invention will become more apparent from the following detailed description, when read in conjunction with the accompanying drawing, in which:

Fig. 1 is an example of a prior art thermal fluid flow rate sensor;

Fig. 2 is a partial cross section of the device of Fig. 1 in situ;

Fig. 3 is a cross sectional view of a first embodiment according to the invention; Fig. 4 is an end view of the Fig. 3 embodiment;

Fig. 5 is a cross sectional view of a second embodiment according to the invention; Fig. 6 is an end view of the Fig. 5 embodiment;

Fig. 7 is a cross sectional view of a third embodiment according to the invention; Fig. 8 is an end view of the Fig. 7 embodiment; and

Fig. 9 is a cross sectional view of a fourth embodiment according to the invention.

Detailed Description of the Invnetion

With reference now the drawing and more particularly to Fig. 1 , a typical prior art thermal fluid flow sensor 11 is shown. The sensor device has thermal wells 12 and 13 extending from insertion element 14. Threads 15 engage a mating threaded opening in a conduit, and housing 16 encases the operational electronics of the sensor in a typical installation. Within the usual wells are temperature sensitive elements, such as resistance temperature detectors (RTDs), which provide a AR as the amount of heat in the active or heated RTD changes with respect to the reference RTD.

Fig. 2 shows the head of the sensor structure of Fig. 1 mounted in conduit 21, and the flow direction is indicated by arrow 22. The heat transfer area of thermal wells 12 and 13 constitutes most of their lengths.

For the prior art devices of Figs. 1 and 2, they can function for gases or liquids. However, since the thermal wells depend on a temperature, and consequently a resistance difference (ΔΤ or AR), they are much more responsive in gases. Liquids, such as water, carry away much more heat by simple immersion, leaving relatively little ΔΤ available to indicate fluid mass flow.

A purpose of embodiments of the invention, as shown herein, is to concentrate the heat dissipation area of the thermal wells, thereby maximizing the heat source to enhance the heat transfer effect. A result is that the same thermal wells, modified as shown herein, can have as much as a twenty- fold increase in mass flow range as well as improved resolution.

With reference now to Fig. 3, insertion element 31, having threads 32 to secure the device in the wall of a conduit, as shown in Fig. 2, may be configured with conventional thermal wells 33, 34 shield apparatus comprising 35 is added in this embodiment, which may be made of any material. Preferably, it is thermally insulative such as a low heat transfer plastic, which reduces the heat transfer area as compared to the full area of the thermal wells as in the standard instruments of the type shown in Figs. 1 and 2. The heat transfer area is merely the exposed tips of thermal wells 33 and 34.

Each thermal well within the shell may be surrounded with a relatively stagnant clearance volume 36 which evinces little heat transfer. The fluid flow direction is shown by arrow 37, which could as well be in the opposite direction.

The heat transfer area (A) which is exposed to fluid flow is about 5% of the exposed thermal well area of Figs. 1 and 2. The result is a less efficient way to carry heat away from the heated sensor, and thereby extending the range in a 20:1 ratio, for example. Where unshielded thermal wells might have a gas sensing flow range of only 0.0 to about 1.0 feet per second before there is no ΔΤ and resulting zero AR, the embodiment has a mass flow detecting range of zero to about 20 feet per second.

It should be understood that these numbers and ratios are set out here only for exemplary purposes and they could higher or lower. Further, the sensors according to the invention embodiments herein can provide velocity or mass flow outputs. Velocity is more applicable to liquids, while either velocity or mass flow readings can be applicable to gases.

The equation for heat transfer is Q = ^h c ^Α (Ά - T2) where Q is the fixed wattage of heater power, ^h c is the heat transfer coefficient for forced convection and is a function of the mass flow rate, A is the fixed area of the thermal wells, and (Ά - ^T 2) varies with the flow rate. ^T l is the heated RTD temperature and ^T 2 is the reduced temperature resulting from fluid flow.

At any (Ti -T2) , or ΔΤ, as measured during flow rate pre-shipment calibration, the smaller the area A the higher the flow rate can be. By reducing the heat transfer area by approximately 20: 1 and thereby increasing the ^h c , the flow rate range increases from approximately 0.5 ft/sec to a much more commercially useable 10 ft/sec in water.

An alternative embodiment is shown in Figs. 5 and 6. The shield apparatus here is comprised of 41 which is formed with an open center. It may be made up of two or more elements. The function is the same as that described with respect to Figs. 3 and 4.

Two separate sleeves 44 and 45 comprising the shield apparatus are shown in Figs. 7 and 8, which surround thermal wells 33 and 34. The function is the same as previously described.

Another alternative embodiment of the shield apparatus is shown in Fig. 9. Rather than the thermal well tips being exposed to the flowing fluid, opening 51 and 52 are formed in sleeves 53 and 54. The thermal wells here are exposed to the flowing fluid in a different way, but are still exposed in a way that dampens the heat transfer effect.

The direction of flow in the Fig. 9 embodiment may be from the right or from the left or it may be into or out of the paper, perpendicular to the direction shown here. The fluid flow may be at any angle with respect to the orientation of openings 51 and 52. Alternatively, only one such side opening may be used, and it may have any orientation to the fluid flow.

While the above description is directed to an exposure of 5% of the heat transfer area of the thermal wells, there is room for substantial variation in that exposure amount to the flowing fluid. For example, 25% exposure might offer a 4: 1 increase in detection range as compared with the prior art sensor of Figs. 1 and 2. While 15% shielding is not as end point, it is a practical value for less than the high flow rates to which the embodiments of Figs. 3-9 exemplify. Thus, shielding embodiments of the invention that can have practical value can be as low as 50% to as high as 95%, and anywhere in between. It is preferred that the sensors be 85% to 95%) shielded from the flowing fluid.

While the preferred embodiment with an active and a reference sensor is described, a single sensor functioning on a time-shared basis could also be employed. This structure would have one of sensors 33, 34 in any of the embodiments of the shield (35, 41 , 44, 45, 53, 54) and would alternatingly be unheated (reference) and heated (active). These time-share steps can be perfomied relatively close together in time so the two readings would be reasonable accurate and relevant.

As another alternative, the two sensors could be structured so that only the active one is shielded and the reference sensor would be open to fluid flow. Again, the structure of Figs. 3-9 are preferred.