Procedure for measuring thermal energy transported by fluid flow

A method is provided for measuring thermal power transferred to or from a flowing fluid, independent of the density and specific heat of the fluid. The method may also measure the thermal power transferred to or from the fluid independent of the flow rate of the fluid itself too. This is accomplished by providing a bypass in the fluid stream, determining thermal power transferred to or from the fluid flowing in this bypass line, and then relating the thermal power transferred in the bypass line to the thermal power of the principal fluid stream itself, by calculating temperatures at various points along the principal fluid stream and along the bypass line.

BACKGROUND OF THE INVENTION 
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 uniformly 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 
.DELTA.T=T.sub.1 -T.sub.2 and separate measurement of a volumetric flow V, 
the rate of change of thermal energy content then being: 
EQU Q=.rho..multidot.CV(T.sub.1 -T.sub.2) 
where .rho.=density of the fluid and C=specific heat. The measurement of 
density of the fluid and specific heat 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 with additive substances, it is well conceivable that .rho.C 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 .DELTA.T; 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. 
SUMMARY OF THE INVENTION 
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, with 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 introduced or therefrom withdrawn thermal 
energy at a given rate; and that in said incoming line, in said return 
line and in said by-pass line respectively are measured the temperatures 
of the fluid flowing through said incoming line, the fluid flowing through 
said return line, and the fluid by-pass flow flowing through said by-pass 
line respectively, 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. 
A number of significant advantages are gained by using the procedure of the 
invention. 
The procedure of the invention contains in actual fact no measurement 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). In the procedure of the 
invention, one measures the rate at which the fluid's heat content changes 
in the object, merely by measurement 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 percent 
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.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In the embodiment depicted in FIG. 1 of the drawings, the thermal energy 
flow Q.sub.1 is withdrawn from the flowing fluid V.sub.1, 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.sub.2 for the fluid through a 
by-pass line 14. The fluid return line 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.sub.1,T.sub.2,T.sub.3,T.sub.4 and 
T.sub.5 are measured at the points a,b,c,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.sub.2 at which 
thermal energy is carried into or withdrawn from the by-passing fluid 
V.sub.2. The procedure of the invention is based on the following 
equations: 
EQU Q.sub.1 =.rho..multidot.C.multidot.V.sub.1 .multidot.(T.sub.1 -T.sub.2) (1) 
EQU Q.sub.2 =.rho..multidot.C.multidot.V.sub.2 .multidot.(T.sub.4 -T.sub.5) (2) 
EQU .rho..multidot.C.multidot.(V.sub.1 +V.sub.2).multidot.T.sub.3 
=.rho..multidot.C.multidot.V.sub.2 .multidot.T.sub.5 
+.rho..multidot.C.multidot.V.sub.1 .multidot.T.sub.2 (3) 
If these equations are solved for Q.sub.1 as a function of Q.sub.2 and of 
the temperatures T.sub.1,T.sub.2,T.sub.3,T.sub.4 and T.sub.5, one finds: 
##EQU1## 
As can be seen from equation (4), the fluid density .rho. 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 .rho.C is nearly completely 
eliminated. Potential changes of the ratio of distribution, k=V.sub.2 
/V.sub.1, 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.sub.1. (It should be 
noted that the bypass 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.sub.2. When the by-pass flow is made small, the 
differential temperature T.sub.3 -T.sub.2 will be small. Since Q.sub.2 
represents that energy which the measuring process consumes, it should 
also be minimized, whereby then the differential temperature T.sub.4 
-T.sub.5 will also be small. All considered, the differential temperatures 
T.sub.3 -T.sub.2 and T.sub.4 -T.sub.5 are small, whereby they are the main 
sources of error in the measuring procedure. The error incurred in the 
measuring of Q.sub.2 is essentially dependent on the method by which 
Q.sub.2 is introduced and transported to be incorporated in the by-pass 
flow. Since in actual fact T.sub.1 and T.sub.4 are identical, four 
temperature measuring pick-ups are required in the procedure. 
When the flow distribution ratio k(=V.sub.2 /V.sub.1) is low and Q.sub.2 is 
so regulated that T.sub.4 -T.sub.5 .apprxeq.T.sub.3 -T.sub.2, then is it 
possible to say that the power (energy rate) needed for measurement is 
Q.sub.2 .apprxeq.k.sup.2. Q.sub.1, and T.sub.3 -T.sub.2 .apprxeq.T.sub.4 
-T.sub.5 .apprxeq.k. (T.sub.1 -T.sub.2), and the total volumetric flow 
rate (i.e., the pumping requirement) has increased by the factor 1+k. 
In frequent instances, e.g. in area heating energy transmission, the 
differential temperature T.sub.1 -T.sub.2 is about 50.degree. C. If it is 
possible to measure the differential temperatures T.sub.4 -T.sub.5 and 
T.sub.3 -T.sub.2 accurately enough even when they are about 1.degree. 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.degree. C. higher than in the case that 
Q.sub.1 were measured by conventional procedures. Q.sub.2 would only 
amount to 0.04% of Q.sub.1, whereby for instance if Q.sub.1 were 1 MW, 
then Q.sub.2 would only be 400 W. This energy, too, will be returned to 
the power plant and partly utilized. The temperature dependence of .rho.C 
introduces, with the parameters of this example, a correction coefficient 
amounting to a few tenths of one percent 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.sub.3, since if the mixing of the flows 
V.sub.1 and V.sub.2 is not quite thorough, even remarkable temperature 
gradients may be encountered. 
The measuring of the power rate Q.sub.2 depends on the way in which energy 
is introduced in the by-pass flow or if in fact Q.sub.2 is negative, 
withdrawn from it. 
For instance if Q.sub.2 is introduced into the by-pass flow by means of a 
heater resistance therein installed, it will suffice for the measurement 
of Q.sub.2 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: 
##EQU2## 
In principle, at least, energy may be carried into the by-pass flow or 
taken therefrom, by a heat conductor as well (see FIG. 2). It is possible 
in that case to measure the power rate Q.sub.2 in the form of the 
differential temperature .DELTA.T.sub.R.sbsb.th building up across a given 
thermal resistance R.sub.th. In that case, the entire measuring of thermal 
energy rate Q.sub.1 would reduce to measurement of temperatures 
exclusively, and then: 
##EQU3## 
If the flow distribution ratio can be assumed to be known, measuring the 
temperature T.sub.3 becomes superfluous. Then: 
##EQU4## 
The effects which the zero point creep of the temperature pick-ups has on 
the critical differential temperatures T.sub.3 -T.sub.2 and T.sub.4 
-T.sub.5 may be eliminated, as required. When Q.sub.2 =0, T.sub.4 -T.sub.5 
has to be zero. Similarly, for V.sub.2 =0, T.sub.3 -T.sub.2 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.