PROCEDURE FOR MEASURING THERMAL ENERGY TRANSPORTED BY FLUID FLOW

Abstract: Procedure for measuring thermal energy transported by means of a fluid flow. With a view to avoiding measurement which is dependent on the density and specific heat of the fluid, in the procedure of the invention is provided in parallel with the consumption unit (13) where thermal power (Q<s1>s) is withdrawn from the fluid (V<u1>u + V<u2>u) flowing through the incoming line (11), a fluid by-pass flow (V<s2>s) by a bypass line (14). Into the fluid by-pass flow (V<u2>u) is introduced heat energy at a given rate (Q<u2>u) or therefrom is withdrawn heat energy at a given rate (Q<u2>u), and in incoming line (11), respectively in the return line (12) and respectively in the by-pass flow line (14) are measured the temperatures (T<u1>u; T<u2>u; T<u4>u; T<u5>u; T<u3>u) of the fluid (V<u1>u + V<u2>u) flowing through the incoming line (11), of the fluid (V<u1>u + V<u2>u) flowing through the return line (12) and of the fluid partial by-pass flow (V<u2>u) through the by-pass line (14), respectively, whereby the thermal energy (Q<u1>u) transported by the fluid flow is measurable by the aid of merely the supply rate of energy (Q<u2>u) to the fluid by-pass flow (V<u2>u), or the rate at which energy is therefrom withdrawn (Q<u2>u), and the temperature measurements (T<u1>u, T<u2>u, T<u3>u, T<u4>u, T<u5>u).

Description: Procedure for measuring thermal energy transported by fluid flow The present invention concerns a procedure for measuring thermal energy transported with the aid of fluid flow. The procedure of the invention is appropriate for measurement both of the increase and decrease of the heat contents in the heat-transporting .fluid. The commonest area of application of the procedure is a commercial area heating distribution network, specifically for each house or for each area. The fluid transporting thermal energy may be any uni¬ formly flowing fluid as regards its state of aggregation and its chemical composition, its most important characteristic being a high specific heat value. Present-day heat quantity or thermal energy measurement techniques are based on separate measurement of a temperature differential * ΔT = T.. - T and separate measurement of a volumetric flow V, the rate of change of thermal energy content then being:- Q = p . C V (T τ - T 2 ) where p = density of the fluid and C = specific heat, the measurε- ment of which both last-mentioned is completely omitted nowadays, implying that no provision is made for their variations. If steps are taken to employ as thermal energy-transporting fluid, water which has been improved x-?ith additive substances, it is well con¬ ceivable that pC may not always be as constant as is the case with pure water. Of the various quantities to measured, V is substantially less accurate than ΔT; if endeavours are made to improve the measuring « accuracy of V, this leads to very expensive meters, such as e.g. the inductive (magnetic) flow meter, which has a metering error amounting to (± 0,5% of the reading) + (± 0,5% of full scale deflection) . The limitations imposed by area heating technology on the developing of the method of measurement - to mention a few of them - are:- - the meter must not cause any significant increase in demand of pumping work; the upper limit for the pressure drop is quite generally 0,1 bar; - the power consumption of the meter should be minimized, and it must not exceed 0,1% of the thermal energy rate that is being measured; - the price at which the meter sells should be concordant with the savings regarding errors in the charging, owing to heightened accuracy of measurement. The object of the invention is to achieve an improvement of previously known methods for measuring thermal energy transported by fluid flow. The more detailed object of the invention is to teach a procedure which is independent of the fluid's density and specific heat. Still one further object of the invention is to provide a procedure wherein the flow rate need not be measured at all. The rest of the objects of the invention, and the advantages gained with its aid, will become apparent in the disclosure of the- invention. The objects of the invention are attained by a procedure which is mainly characterized in that there is provided in parallel across the consumption unit (where the thermal energy is withdrawn from the fluid passing through the incoming line) , a fluid by-pass flow through a by-pass line; that into said fluid by-pass flow is intro¬ duced or therefrom withdrawn thermal energy at a given rate; and that in said incoming line, respectively in said return line and respectively in said by-pass line are measured the temperatures of the fluid flowing through said incoming line, respectively of the fluid flowing through said return line and respectively of the fluid by-pass flow flowing through said by-pass line, whereby the thermal energy transported ' with the aid of the fluid flow is measurable by the exclusive aid of the rate at which energy is introduced into the said fluid by-pass flow and of the said temperature measurements. f _1 The other characteristic features of the invention are presented in claims 2 through 7. A number of significant advantages are gained by using the procedure of the invention. The procedure of the invention contains in actual fact no measure¬ ment of flow rate at all, in the conventional sense, instead of which one permits a minor fluid short-circuit flow past the change- of-energy object under measurement (a'heat exchanger for instance). One tries, in the procedure of the invention, to measure the rate at which the fluid's heat contents change in the object, merely by measurements of differential temperatures and by those relating to the auxiliary heating or cooling of the minor shunt flow. The procedure of the invention is independent of the per cent magnitude of the shunt flow, of the density and specific heat of the fluid, and it is thus understood that these may vary without in any way deranging the measurement. The shunt flow is essential in the invention presented here, and the independence of density and specific heat which was mentioned is only achieved by heating or cooling the same fluid which serves as the actual vehicle proper. The procedure here described becomes increasingly favourable in the technical and economic respects with increasing pipeline size and energy quantity to be measured. The procedure of the invention shall be described in detail with reference being made to the principle solution presented in the figure of the attached drawing, but to which the invention is not meant to be exclusively confined. In the embodiment depicted in the figure of the drawing, the thermal energy flow Q- is withdrawn from the flowing fluid V. , and this thermal flow rate is understood to be the quantity that is being measured. The basic insight of the present invention teaches that one provides in parallel with the unit 13 where energy is being consumed at a certain rate (e.g. a heat exchanger), a by-pass flow * V„ for the fluid through a by-pass line 14. The fluid return line J " ■ - .-i .* - '- ' ' v.-- W.i-'O . N has been denoted with the reference numeral 12. In the incoming line 11, in the by-pass line 14 and in the return line 12 the fluid temperatures T χ ,T ,T„,T, and T are measured at the points a,b,c 5 d and e indicated in the figure. The thermal energy rate that one desires to determine is found with the aid of the said temperature measurements and of the rate Q„ at which thermal energy is carried _ into or withdrawn from the by-passing fluid V„ . The procedure of the invention is based on the following equations:- Q χ = p • C • V. - (.T. - T 2 ) (1) Q 2 = p • C • v- 2 • (T 4 - T 5 ) (2) p • C • (V χ + V 2 ) • T 3 = p • C • V 2 • T 5 + p • C • V χ - T 2 (3) If these equations are solved for Q, as a function of Q„ and of the temperatures T.. ,T„,T_,T, and T-., one finds : - (T χ - τ 2 ) • (τ 5 - τ 3 ) Q = . Q (4) (τ 4 - τ 5 ) ■ (τ 3 - τ 2 ) As can be seen from equation (4) , the fluid density p and specific heat C of the fluid cancel out, whereby the method is unresponsive to the potential fluctuations of these quantities. Owing to the symmetric nature of equation (4) , the temperature dependence of pC is nearly completely eliminated. Potential changes of the ratio of distribution, k = V-./V-. , will cause no measuring error either, because this ratio is measured with the aid of differential temperatures (Equation 3) . However the flow distribution ratio has practical significance in that it determines substantially the increase of the fluid pumping power required to achieve transport of thermal energy at the rate Q- . (It should be noted that the by¬ pass flow is an extra flow made necessary by the procedure.) As readable from equation (4) , the inaccuracy of measurement of the present procedure arises from the error in measurement of four differential temperatures and from the error in measuring the rate of thermal energy Q„. When the by-pass flow is made small, the differential temperature T, - T„ will be small. Since Q„ represents that energy which the measuring process consumes, it should also be minimized, whereby then the differential temperature T, - T- will also be small. All considered, the differential temperatures T„ - T 2 and T, - T are small, whereby they are the main sources of error in the measuring procedure. The error incurred in the measuring of Q„ is essentially dependent on the method by which Q„ is introduced and transported to be incorporated ' in the by-pass flow. Since in actual fact T and T, are indentical, four temperature measuring pick-ups are required in the procedure. When the flow distribution ratio k (= V./V . ) is low and Q„ is so regulated that T - T, ?* T, - T„, then is it possible to say that "4 - J — y, the power (energy rate) needed for measurement is „ Λ k . Q 1 , and T - T < * T - T ~* k . (T - T ) , and the total volumetric flow rate (i.e., the pumping requirement) has increased by the factor 1 + k. In frequent instance, e.g. in area heating energy transmission, the differential temperature T, - T„ is about 50 C. If it is possible to measure the differential temperatures T, - T-. and T~ - T„ accurately enough even when they are about 1 C, the by-pass flow ratio might even be as low as 1/50. Hereby, thus, the pumping requirement would only increase by 2% and the temperature of the return water would be about 1 C higher than in the case that Q- were measured by conventional procedures. Q„ would only amount to 0,04% of 0., whereby for instance if Q were 1 MW, then Q„ would only be 400 W. This energy, too, will be returned to the power plant and partly utilized. The temperature dependence of pC introduces, with the parameters of this example, a correction coefficient amounting to a few tenths of one per cent at the most and which is dependent on the temperature values used, with a gentle slope only. The differential temperatures may be measured by any method in common use. However, attention should be paid to making the temperature pick-up mounting such that the said temperature represents, as well as possible, the average fluid temperature over the whole pipe cross section at the point of measurement in question. This is particularly important in the measurement of the temperature T,, since if the mixing of the flows V. and V„ is not quite thorough, even remarkable temperature gradients may be encountered. The measuring of the power rate Q~ depends on the way in which energy is introduced in the by-pass flow or if in fact Q~ is nega- tive, withdrawn from it. For instance if Q„ is introduced into the by-pass flow by means of a heater resistance therein installed, it will suffice for the measurement of Q„ if the electric power uptake p of the heater resistance is measured, which can be done with adequate accuracy by well-established procedures. Then, thus:- (τ 4 - τ 5 ) (τ 3 - T 2-) In principle, at least, energy may be carried into the by-pass flow or taken therefrom, by a heat conductor as well. It is possible in that case to measure the power rate § in the form of the differential temperature ΔT building up across a given thermal R th resistance -λ . . In that case, the entire measuring of thermal energy rate Q.. would reduce to measurement of temperatures exclusively, aanndd ttthen:- ) R th (τ ι - v ■ • <v - T 3 J *1 (6) If the flow distribution ratio can be assumed to be known, measuring the temperature T_ becomes superfluous. Then:- - (T - τ ) Q χ - £ • - . . Q (7) < T 4 " V The effects which the zero point creep of the temperature pick-ups has on the critical differential temperatures T, - T_ and T, - T,. # may be eliminated, as required. When Q„ = 0, T, - T,- has to be zero. Similarly, for V„ = 0, T„ - T„ has to be zero. This implies a remarkable simplification of the requirements to be imposed on the differential temperature meters in question. In the foregoing merely the principle solution of the invention has been presented, and it is obvious to a person skilled in the art that details of the invention -may vary in numerous different ways within the scope of the inventive idea expressed in the attached claims. :V -' - A l r

Claims: Claims 1. Procedure for measuring thermal energy transported by means of a fluid flow, characterized in that there is provided in parallel with the consumption unit (13) where thermal power (Q.) is with¬ drawn from the fluid (V.. + V„) flowing through the incoming line (11), a fluid by-pass flow (V ) by a by-pass line (14); that into said fluid by-pass flow (V ) is introduced heat energy at a given rate (Q„) or therefrom is withdrawn heat energy at a given rate (Q„); and that in said incoming line (11), respectively in said return line (12) and respectively in said by-pass flow line (14) are measured the temperatures (T ;T 2 ;T, T 5 ;T ) of the fluid (?- + V„) flowing through said incoming line (11) ; of the fluid (V + V ) flowing through said return line (12) ; and of the fluid partial by-pass flow (V„) through said by-pass line (14) , respectively, whereby the thermal energy (Q,) transported by the fluid flow is measurable by the aid of merely the supply rate of energy (Q„) to said fluid by-pass flow (V-), or the rate at which energy is therefrom withdrawn (Q ) , and the said temperature measurements (1 ,T~,T ,T, ,T ) . 2. Procedure according to claim 1, characterized in that into said fluid by-pass flow (V„) is introduced power (Q„) with the aid of a heater resistance disposed in the said by-pass line (14) . 3. Procedure according to claim 2, characterized in that the electric power uptake (p) of said heater resistance is measured in order to find the power rate (Q„) supplied to said fluid by-pass flow (V 2 ). 4. Procedure according to claim 1, characterized in that into the said fluid by-pass flow (V„) is introduced power (Q ) or therefrom is withdrawn power (Q„) with the aid of a heat conductor disposed in said by-pass line (14) . 5. Procedure according to claim 4, characterized in that therein is measured the differential temperature (ΔT ) building up across the R th \-7 \ >ir * /- - thermal resistance (R , ) constituted by a part with given length of said thermal conductor. 6. Procedure according to claim 1, characterized in that therein is determined the distribution ratio (k) of the fluid (V. ) flowing through said incoming line (11) and the fluid by-pass flow (V„) going through said by-pass line (14). 7. Procedure according to claim 1, characterized in that in said return line (12) is measured the temperature of the summed flow (V. + V„) of the fluid (V-) flowing through said return line (12) and the by-pass flow (V„ ) of said fluid flowing through said by-pass line (14) to said return line (12) .

Inventors: HIRSIMAEKI O, KARRAS M
Applicants: HIRSIMAEKI O, KARRAS M, VALMET OY
Application year: 1981
Main classification: G01K17
IPC codes: G01K17