FR2663408A1 - Method of regulating a heat generator with air and fume washers - Google Patents

Abstract

The method is characterised in that: - the temperature of the washing water is determined at the outlet of the air washer (or fume washer) TEAF (TEFA), - the pinching T1 (T2) of the oxidant air entering (leaving) the air wahser (3) is determined in relation to the temperature of the water leaving (re-entering) this washer (3), - the pinching T4 (T3) of the fumes leaving (re-entering) the fume washer (5) and of the water entering (leaving) the fume washer is determined, - the flow of water QE in circulation is regulated to make the difference between the pinchings between ¦T1¦ - ¦T4¦ (or ¦T2¦ - ¦T3¦) tend towards zero.

Description

"Process for regulating a heat generator with air and smoke washers"
The present invention relates to a method of regulating a heat generator comprising a hearth supplied with fuel and with combustion air and releasing fumes, a heating circuit whose energy demand is variable, an air washer receiving the combustion air upstream of the fireplace, a smoke washer receiving the smoke downstream of the fireplace, a washing liquid circuit connecting the washers to take the washing liquid (water) at the outlet of one of the washers and introduce it at the entrance to the other, this liquid circulating in the washers against the current with the gas (air, fumes), the excess washing liquid being extracted from the circuit to maintain a constant quantity of liquid there.

 Such a process is already known. This process makes it possible to recover energy from the fumes to inject this energy into the combustion air and increase the efficiency of the installation.

 However the yield depends, very importantly, on the regulation. This regulation must ensure an optimum between the circulation of the washing liquid or heat transfer liquid and the energy demand, variable demand, imposed on the boiler and determining the flow of combustion air and consequently the flow of smoke.

 The present invention proposes to create a regulation method for optimizing combustion.

To this end, the invention relates to a process characterized in that
- the temperature of the washing water at the outlet of the air washer (or of the smoke washer) is determined,
the pinch of the combustion air entering (leaving) in the air washer is determined relative to the temperature of the water leaving (returning) from this washer,
- the pinch of the fumes leaving (entering) in the smoke washer and of the water entering (leaving) in the smoke washer is determined,
- the flow rate of the circulating water is adjusted to make the difference between pinches tend to zero.

 As this regulation optimizes the flow of heat transfer liquid or washing liquid, in circulation as a function of the demand for power imposed on the boiler, that is to say the demand for combustion air and the emission of fumes, this achieves optimum exhaustion of the fumes and therefore an optimum efficiency of the thermal installation.

 Indeed, this regulation takes into account all the variations of energy contained in the fumes of the boiler, whether these variations come from the variations of flow of the fumes (variation of power) or the variations of enthalpy of the fumes (variation of the temperature boiler irrigation).

The present invention will be described in more detail with the aid of the accompanying drawings in which
FIG. 1 is an overall diagram of a heat generator to which the control method of the invention is applied,
- Figure 2 is a detail of the air saturation curve in water and enthalpy curves, to explain the implementation of the regulation method according to the invention.

- Figure 3 is a diagram of a resistance bridge, formed of thermistors R1-R2-R3-R4 respectively measuring the temperatures TAe (R1), TEAF (R2), TEAF (R3), TAs (R4); in fact the resistances
R2 and R3 measure the same reference temperature which in this case is TEAF.

 According to Figure 1, the invention relates to a method of regulating a heat generator. This heat generator consists of a hearth 1 connected to a heating circuit 20 via an exchanger placed in the hearth. This fireplace is supplied with fuel and combustion air; it provides smoke.

 Upstream of the combustion air inlet 2 is an air washer 3 and downstream of the hearth 1, on the smoke outlet 4, there is a smoke washer 5.

 Before supplying the boiler 1, the combustion air passes through the washer 3, the air inlet of which bears the reference 5. Similarly, the fumes leaving the hearth 1 arrive via the inlet 4 in the fume washer 6 and leave it through the smoke outlet 7.

 The installation also includes a washing liquid circuit respectively connecting the outlet of each washer to the inlet of the other washer. More specifically, this circuit consists of a first branch connecting the outlet 8 of the air washer 3 to the inlet 9 of the smoke washer. This branch may include a circulation pump 10. Similarly, the outlet 11 of the smoke washer 6 is connected to the inlet 12 of the air washer by the second branch of the washing liquid circuit. This second branch may also include a circulation pump 13.

Finally, the washing circuit includes an overflow 14 for discharging the excess liquid formed by the condensates of the combustion products.

 The regulation according to the invention relates to the flow rate of washing liquid since the other parameters of the installation are given or imposed by combustion.

 The heat demand of the heating circuit imposes the operation of the boiler, i.e. the demand for combustion air and the emission of fumes. On the other hand, the flow rate of washing liquid is adjusted from the outside to ensure optimum operation of the installation, that is to say combustion with the best thermal efficiency.

To describe the process according to the invention, the definition of various parameters of the installation will be given below.
All the air and smoke temperatures used are humid isenthalpic temperatures.

QA = dry air flow through the air washer (air
oxidizer)
QF = mass flow rate of dry smoke leaving the
fireplace and passing through the smoke washer.

QEFA = (equal to QEAF) water flow from the smoke washer
to the air washer
QEAF = water flow from the air washer to the
fumes.

TEFA = water temperature (washing liquid) at
the entrance to the air washer.

TEAF = temperature of the water leaving the washer
of air.

TAe = air temperature at the inlet of the washer
of air.

HAe = enthalpy of air at the inlet of the air washer.

TAs = air temperature at the outlet of the washer
of air.

HAs = enthalpy of air at the outlet of the air washer.

T1 = pinch between the air at the inlet of the washer
of air and the temperature of the water leaving
this same washer, TEAF - TAe.

H1 = enthalpy pinch corresponding to the deviation
of temperature T1.

T2 = pinching between the air leaving the air washer
and the water entering the air washer.

H2 = enthalpy pinch corresponding to the deviation
T2 temperature.

TEAF = water temperature at the inlet of the washer
fumes (this temperature is the same as the
washer outlet water temperature
air).

TEFA = temperature of the water leaving the washer
fumes (this temperature is the same as the
water temperature at the inlet of the washer
air).

TFe = smoke temperature at the inlet of the washer
fumes.

HFe = enthalpy of fumes at the inlet of the scrubber
fumes.

TFs = temperature of the fumes leaving the washer
fumes.

HFs = smoke enthalpy at the outlet of the scrubber
fumes.

T3 = pinch between the fumes at the inlet of the washer
fumes and water coming out of the fu washer
mées.

H3 = enthalpy pinch corresponding to the deviation
temperature T3.

T4 = pinch between the fumes coming out of the washer
fumes and air entering the washer
fumes.

H4 = enthalpy pinch corresponding to the deviation
temperature T4.

Aim
We want HAs to be as high as possible and HFs to be as small as possible.

Principle of regulation
The regulation must adjust the flow rates of the thermal capacities in the scrubbers to have the hottest water at the inlet of the air washer (heat recovered from the flue gases) and the coldest at the inlet of the washer's water. fumes (heat exchanged with combustion air).

 In other words to arrive at an optimal regulation resulting in the best combustion efficiency, it is necessary to balance the flow rates of the capacities of the gas fluids (air or fumes) and of the washing water fluids in each of the washers, that is to say that is, to achieve the best compromise between good water temperatures and good pinching.

For example, in the air washer 3 the heat exchanges are as follows
For water
- The lowering of the water temperature between the inlet 12 and the outlet 8 corresponds linearly to the energy exchanged, that is to say the energy supplied to the air for a flow of given water.

For the air
- The rise in air temperature between the combustion air inlet 5 and the humid air outlet 2 supplying the hearth 1 is a non-linear function of the energy received (i.e. l energy supplied by water). This non-linear function depends on the saturation curve shown in Figure 2.

The regulation process will be illustrated below using a few examples of malfunction in terms of energy balance before defining the process in its general characteristics.
1st Case: wash water flow is too low compared to the air flow.

 This situation has different consequences for air and smoke washers; regulation will be done from the air washer.

 - In the air washer the pinch T1-H1 will be weak. The water leaving the air washer 3 is very cold, which is in itself a good result but the pinch T2- (H2) is strong so the combustion air having passed through the air washer 3 has not received enough energy.

 In the smoke washer, the water introduced at the TEAF temperature is cold, which is a good characteristic in itself, but the water flow being low, pinching T4 (H4) is strong: the fumes come out of the smoke washer without be sufficiently exhausted for this temperature.

 On the other hand, the pinch T3 (H3) is low and the water temperature is high, which is apparently good.

 The water leaving the smoke washer and which arrives in the air washer 3 is at a high temperature (but the flow is low); this water therefore provides little energy to the air and the pinch T2 (H2) is strong.

 Regulation: to reduce in this case the strong pinches T2 (H2) and T4 (H4) it is necessary to regulate the circulation of washing water in the direction of a stronger flow.

2nd case: Water flow too high compared to the air flow in the air washer 3
Pinch T1 (H1) will be strong because by hypothesis the water flow is too strong for the air flow; the air is very quickly charged in temperature and enthalpy and the water, at high flow, will undergo only little variations in temperature.

 The pinch T2 (H2) is weak, which apparently is good, provided that TEFA is a sufficiently high temperature.

 - The temperature of the water leaving the TEAF washer is high because the water is insufficiently cooled by the air due to the excessive flow of water.

In the smoke washer, the situation is as follows
- the water received is at TEAF temperature it is hot, which is a bad temperature.

 - the pinch T4 (H4) is weak, which is apparently good.

 - the smoke leaves the smoke washer 6 at too high a temperature. They will have been exhausted by the exchange in this washer but at too high a water temperature, so that they will be badly exhausted.

 - the pinch T3 (H3) is strong and the TEFA water temperature at the outlet of the smoke washer will be close to TEAF, far from TFe.

 For regulation, the TEFA-TEAF temperature level must be increased, therefore the water flow must be reduced.

The purpose of the process according to the invention is to optimize the levels of the enthalpies of the air and the fumes. The energy balance is as follows
QA (HAs - HAe) = QEFA (TEFA - TEAF) .C = E
C = specific heat of water.

 As the fumes come from the combustion air, QA is roughly equivalent to QF; QEFA will be kept equal to QEAF.

E represents the energy exchanged.

 As already indicated, the process must make E maximum.

 According to the method, in the air washer 3, the aim is to make HAs as close as possible to the enthalpy of saturated air at the temperature of TEFA while having the temperature TEAF of the water leaving the washer d air, such that the enthalpy of air saturated at this temperature TEAF is as close as possible to HAe.

 On the side of the smoke washer 6, one consequence is that HFs becomes very close to the enthalpy of air saturated at TEAF temperature and the enthalpy of air saturated to TEFA is very close to HFe.

 The regulation made from the air washer could also be done from the smoke washer.

 Thus, in parallel in the flue gas washer, the enthalpy HFs must be made as close as possible to the enthalpy of air saturated at the TEAF temperature and the TEFA temperature of the water leaving the flue gas washer must be obtained. 6 is such that the enthalpy of air saturated at this temperature TEFA is as close as possible to the enthalpy of the HFe fumes.

 The regulation according to the invention is done by relying on the saturation curve (Figure 2).

The saturation curve and the enthalpy curves show, as is known, that for saturated air, the enthalpy does not vary linearly with respect to the rise in temperature or to the differences in wet temperatures. But it is easier to measure temperatures than enthalpies. Consequently, according to the invention, the regulation of the enthalpies is simulated by temperature variations, close so that the AT ratio is practically constant. In other words, starting from an operating point M of the curve, corresponding to the temperature TEAF, we place ourselves on either side of this point on the saturation curve. At the point level
M the curve can be compared to its tangent. We then have almost constant.

 The reference temperature for regulation can be either TEAF or TEFA.

According to the invention, if TEAF is chosen as a benchmark, regulation is carried out which compares T1 and
T4 so that | T1 | - | T4 | tends to 0 and so | H1 | | H1 | ~ | H4 | | H4 | tends to 0 because T1 T4. For this, pumps 10 and 13 were supplied so that the QEAF or
QEFA (which are identical since it is a circulation of washing liquid without increasing the volume of liquid) tends towards reducing the differences in the pinches T1 and T4 until the cancellation.

 It will result from it in the same way, that the pinches T2 and T3 will be close or equal in absolute value and, consequently, the enthalpies H2 and H3 will be equal. This constitutes the best optimization of the system allowing to achieve a maximum elevation of the enthalpy of the water and a maximum lowering of the enthalpy of the fumes.

 Although described above in the case of pinches T1 and T4, the control could also be done on the pinches T2 and T3 by choosing the TEFA reference.

According to a variant of the invention which is only a less good solution than the solution described above, the temperature differences are equalized by a regulation which compares the lowering of the water temperature with the rise in the air temperature in the air washer 1T = TEFA - TEAF
A2T = TAs - TAe
We are trying to make the difference A1T - T null.

To do this, you set the QE rate. In the smoke washer, as we have A3T = TEAF - TEFA and A4T = TFe
TFs, the difference A3T - A4T will tend roughly to 0. As another variant, one could start from the temperature differences A3T - # 4T and result in a difference A1T -A2T which roughly tends to 0. It is also possible to achieve a balance such that A1T = A2T on the air washer and for the other washer we will have: A3T = - At t6tU washer operates at temperatures higher than those of the air washer (due to pinching); the slope of the saturation curve will therefore be steeper and A3T will be different from A4T for the same energy exchanged.

 Finally, according to another variant, the nips are taken in temperature in the air washer T2 - Tî and it is adjusted to make this difference tend towards 0 by acting on the flow rate QE. We will therefore obtain T4 - T3 tending towards 0.

The reverse is also possible from the smoke washer. It should be noted that if we seek a balance between T1 and T2 or T3 and T4, this method would be very crude because the optimum level of H2 H1 energies is not achieved because T2 T1
The bridge assembly thus determines the pinches T1 and T4 between the resistors R1 -R2 and R3-R4.

The above regulation uses the TEAF temperature as a reference; it is also possible to use the TEFA temperature and the pinches T2,
T3.

Claims (2)

 1) Method for regulating a heat generator comprising  - a hearth supplied with fuel and with combustion air which gives off smoke,  - a heating circuit whose energy demand is variable,  - an air washer receiving the combustion air upstream from the fireplace,  - a smoke washer receiving the smoke downstream from the fireplace,  - a washing liquid circuit connecting the washers to take the washing liquid (water) at the outlet of one of the washers and introduce it at the inlet of the other, this liquid circulating in the backwashers with the gas (air, fumes), the excess washing liquid being extracted from the circuit to maintain a constant quantity of liquid therein, process characterized in that  - the temperature of the washing water at the outlet of the TEAF air washer (or smoke washer) is determined,  the pinch T1 (T2) of the combustion air entering (leaving) in the air washer (3) is determined relative to the temperature of the water leaving (returning) from this washer (3),  the pinch T4 (T3) of the fumes leaving (entering) is determined in the smoke washer (5) and of the water entering (leaving) in the smoke washer,  - the flow of water QE in circulation is adjusted to make the difference between pinches between | T1 | | T4 | (or | T2 | - | T3 |).  2) Method according to claim 1, charac terized in that the temperature of the fluids for regulation is measured using a resistance bridge (thermistor) R1, R21 R31 R4.