GB2112517A - Heating apparatus - Google Patents

Abstract

Heating apparatus comprises a pre-heater 1 containing loose packing material 5, which is connected to a flue passageway 2, also containing loose packing material 14, and a heat exchanger 3, disposed in the flue passageway 2. In operation, cool water is pumped through pipeway 18 to inlet 16 of the flue passageway 2 and is trickled through ports 17 and packing material 14, to cause water vapour present therein to condense. The water, heated by combustion products also present therein, is then pumped through pipeway 21 to inlet 7 of pre-heater 1 and is trickled through packing material 5, thereby heating and moistening incoming air for combustion. The cooled water is then re-cycled. The heat exchanger 3 conveys heat from the combustion products to a domestic space heating system including radiators. <IMAGE>

Description

SPECIFICATION Heating apparatus This invention relates to heating apparatus and it relates especialiy, although not exclusively, to combustion-fired water heating apparatus for use in association with a domestic space and water heating system, and to a method of using heat generated in such heating apparatus.

In such a heating system, heat conveyed by the combustion products or gases, generated by a fuel burner, is commonly transferred to water utilised in radiators of the heating system, by the agency of a heat exchanger disposed within a flue passageway of the burner which carries the combustion products. It is found in practice, that the temperature of water returned from the radiators for reheating is typically in the order of 60-70"C and this therefore represents the lowest temperature to which the combustion products can be cooled in such apparatus. In particular, with fuels which contain hydrogen, some of the heat conveyed by the combustion products takes the form of latent heat of vapourisation of water produced by combustion and this heat can only be released when condensation occurs, at the dewpoint and below.For the case of natural gas burned at the air/fuel ratio commonly employed in heating apparatus the dewpoint is about 55"C. The dewpoint is thus below the lowest attainable flue gas temperature, and condensation does not occur. The latent heat of vapourisation of the water produced by combustion is about 10% of the calorific value of the fuel and is thus a significant portion of heat lost to atmosphere.

It is known that condensation can be made possible by employing a cooler return water temperature, but this cannot always conveniently and economically be arranged in practice.

It is an object of the present invention, therefore, to utilise more of the available heat from the combustion products.

According to the present invention there is provided heating apparatus comprising gas supply means to a gas burner, means for moistening in-coming air for combustion, heat exchange means for conveying heat from the gas burner to a location requiring said heat, means for exhausting combustion products from the gas burner to atmosphere, and fluid means for transferring heat from the combustion products, before being exhausted, to the in-coming air for combustion.

Further according to the present invention there is provided a space heating system including heating apparatus as described in the immediately preceding paragraph.

By the present invention heat is capable of being extracted from the combustion products in the flue passageway by cool water, the water being heated by the combustion products and then utilised to heat and moisten airforcombustion in the pre-heater.

The heated water in the pre-heater is cooled by the air for combustion and then this cooled water is circulated back to the flue passageway. Heat which would otherwise have been lost through the flue gas exit is now absorbed by cool water and conveyed to the air for combustion, thereby increasing the efficiency of heat extraction from the hot combustion gases. In addition, moistening the air for combustion raises the dew point so that condensation can now occur at or below this higher temperature. This means that the latent heat of vapourisation of the water produced by combustion may begin to be extracted by lowering the temperature of the combustion products by an amount less than would previously have been envisaged.

Preferably, the pre-heater comprises a first column having packing material arranged therein so that heated water from the first inlet is constrained to permeate down through the packing material as air for combustion rises through the material. In this way heat from the water is efficiently transferred to the combustion air. Similarly, the flue passageway may comprise a second column having packing material arranged therein so that as combustion products rise heat is transferred to cooled water permeating down through the material.

The arrangement may be such that there is a net build up of water circulating through the pre-heater and flue passageway, for example, if the temperature of the combustion products is brought below the dew point with the effect that the latent heat of vapourisation is given up in condensation. In such arrangements it is preferred that a drain is provided to allow the escape of excess water or condensate, and this drain may be provided at the bottom of the pre-heater.

Preferably, water is allowed to trickle into the preheater at the first inlet and/or into the flue gas passageway at the second inlet. Alternatively, the water may be sprayed into the pre-heater and/or flue gas passageway.

Preferably, where water is sprayed into the preheater and/or flue gas passageway packing material is not provided but air/water drop separators are provided, to filter out water drops from the air for combustion in the first case, and to stop warm water drops being expelled through the flue gas exit in the second case.

Where convenient, one or more circulating pumps may be provided to circulate water around the pre-heater and flue passageway.

An embodiment of heating apparatus in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic sectional view of the embodiment for use in a domestic space heating system; Figure 2 shows a graph of the variation in enthalpy (total heat content) related to the gross calorific value of methane with temperature of the air for combustion and with temperature of the gaseous combustion products, before (as in the case of the prior art) and after (as in the case of the present invention) air for combustion has been heated and moistened, and Figure 3 shows a much simplified schematic arrangement of domestic space heating system including the embodiment.

Figure 1 shows heating apparatus comprising a pre-heater 1 which is connected to a flue passageway 2 and a heat exchanger3 disposed in the flue passageway. Figure 3 shows a domestic space heating system including radiators R1, R2, R3 and water is pumped around the radiators by pump P3 after being heated by the heat exchanger 3.

As shown in Figure 1 the pre-heater 1 has a vertical column portion 4 and in use, air for combustion is blown by a fan into the bottom of the pre-heater.

Loose packing material 5 such as coke or pumice is arranged in the column portion 4 on a grid 6 which extends across the column portion. The packing material 5 may be any material of low thermal mass and may comprise plastics balls, preferably of approximately 1 cm in diameter. The pre-heater 1 has a water inlet 7 and a water outlet 8. The inlet 7 is at the top end of the pre-heater 1 and above the packing material 5 and is provided with five ports 7a to introduce water on top of the packing material.

The pre-heater 1 is connected to a drain 9 and is provided with an upper lateral extension 10 in which fuel such as methane can be introduced into the air for combustion through fuel pipe 11. A burner 12 is provided at one end of the lateral extension 10 and extends into the flue passageway 2 which carries gaseous combustion products from the burner 12 to the heat exchanger 3 and then out through the flue passageway exit 2a at the top of the passageway 2.

The flue passageway 2 is U-shaped and has a column portion 13 which is similarto column portion 4 of the pre-heater 1 and has loose packing material 14 such as carbon arranged on a grid 15 in a similar manner as in the pre-heater. The flue passageway 2 has an inlet 16 provided with five ports 17 at the top end of the column portion 13, and above the packing material 14. The inlet 16 is connected to the preheater outlet 8 by pipeway 18. The branch portion 19 of the U-shaped flue passageway 2 is provided with a water outlet 20 which is connected to the inlet 7 of the pre-heater 1 by pipeway 21.A pump P1 is provided in the pipeway 21 to pump water from the bottom of the flue passageway 2 up to the top of the column portion 4 of the pre-heater 1, and a pump P2 is provided in pipeway 18 to pump water from the bottom of the pre-heater to the top of the column portion 13 of the flue passageway. The flue passageway 2 has a second column portion 22 in which is housed the heat exchanger 3. The exchanger 3 comprises a water pipe 3a which, in this example, leads to an arrangement of domestic hot water radiators (not shown). Surrounding the pipe 3a is a heat exchange arrangement (shown schematically at 3b in Figure 1) which may be of any suitable form for absorbing heat from the combustion products in the flue passageway 2, for example, an arrangement of fins of relatively large surface area as known per se.

The second column portion 22 and holes therethrough which support the pipe 3a comprise means to a co-operate with the heat exchanger. In addition, a hollow cowl or combustion chamber 23 surrounds the burner 12 and lies in between the pre-heater 1 and the flue passageway 2. The cowl 23 has an inlet 23a and an outlet 23b and air orwater can be passed through the cowl (which acts as an ancillary heat exchanger) to an ancillary heating system, for exam ple, an ancillary boiler. Alternatively, pipe 3a may be connected directly to inlet 23a; the exchanger 3 and cowl 23 then comprise a compact heat exchanger system with low stray heat losses.

In operation, cooled water may be pumped by pump P2through pipeway 18to inlet 16atthetop of the column portion 13 of the flue passageway 2. The cooled water is trickled through ports 17 onto the loose packing material 14 where it pemeates downwardly as hot gaseous combustion products rise upwardly thereby loosing heat to the water. The gaseous products are cooled to below the dew point so that water vapour present therein condenses and joins the water permeating downwardly in the column 13. Some condensation may occur around the heat exchanger 3 and so the gaseous products may be at a temperature below the raised dew point before entering column 13. The water at the bottom of column 13 has been heated by the gaseous products and this water is pumped to the top of column 4 of the pre-heater 1, where it is trickled onto packing material 5 through parts 7a.The heated water looses its heat and some moisture content to the incoming, upwardly moving air for combustion as it permeates downwardly to the bottom of the column 4. This cooled water is now recycled by pump P2 to the top of column 13 again to cool the gaseous products of combustion. Since condensation occurs in the flue passageway 2 and heat exchanger 3 which is not entirely offset by the evaporation in column 4there is a net build up of water in the system and so any excess is drawn off through the drain 9.

Advantages of the described embodiment can be seen with reference to Figure 2. Line A shows the relationship between enthalpy (plotted on the vertical axis) and temperature (plotted on the horizontal axis) of the gaseous products in a flue gas passageway of a typical prior art heating apparatus. Enthalpy is calculated from the equation H = U + pv where H is enthalpy, U is internal energy, p is pressure and V is volume. In this case the fuel is methane burnt at 9% carbon dioxide in dry flue gas. The usual equation associated with methane fuel burnt in air is CH4 + 202 = 2H 20 + CO2 + heat.Point A, on line A represents the usual dew point (55"C) below which the gaseous products must be cooled to extract the latent heat ofvapourisation. At temperatures below point A1 on line A the water vapour in the gases is progressively condensed, releasing the latent heat.

Line B shows the relationship between the enthalpy of combustion air and temperature in a typical prior art apparatus required to give 9% Carbon dioxide in the dry flue gases. As shown this air cannot absorb more than 1/4to 3 1/aofthe enthalpy in the flue gases for any given temperature in the temperature range.

Lines C and D show the operating characteristics of the described embodiment of the invention. Line D shows that the dew point has been raised to 62"C at D1 but otherwise follows the same line as line A, and this raising of the dew point is a consequence of the pressure of the additional water vapour. Line C represents the enthalpy of combustion air with added water vapour and can be compared to line B of the prior art. Therefore, in this case, the combustion air is able to absorb the enthalpy of the flue gases for any given temperature in the temperature range thereby utilising all of the available heat of the gases. In practice, most of the available heat in the flue gases is re-cycled back into the system.

It is to be appreciated that heating apparatus in accordance with the invention may be used in a domestic space heating system which may also include a domestic hot water supply tap-off system.

Alternative arrangements of the heating apparatus in accordance with the invention may of course be envisaged by persons skilled in the art.

Such an alternative arrangement may, for example, consist of stacking the two columns, 4 and 13, one on top of the other, thus utilising gravity as a means for circulating the water therethrough, so that only one pump instead of two is required to pump the water from the bottom of the stacked columns to the top thereof.

Claims (7)

1. Heating apparatus comprising gas supply means to a gas burner, means for moistening in-coming air for combustion, heat exchange means for conveying heat from the gas burner to a location requiring said heat, means for exhausting combustion products from the gas burner to atmosphere, and fluid meansfortransferring heat from the combustion products, before being exhausted, to the in-coming airforcombustion.

2. Heating apparatus according to Claim 1 wherein said fluid means consists of water, the water also being used as the means for moistening the in-coming air.

3. Heating apparatus according to Claim 2 including a pre-heaterthrough which the in-coming air for combustion passes before mixing with gas from the gas supply means, the pre-heater having packing material arranged therein through which water heated by the combustion products permeates, to provide heating and moistening of said in-coming air.

4. Heating apparatus according to Claim 2 or 3 wherein said means for exhausting the combustion products comprises a flue passageway having packing material arranged therein through which water permeates, said water being heated by the combustion products.

5. Heating apparatus according to any preceding claim including at least one pump to provide circulation of the fluid means between the combustion products and the in-coming air for combustion.

6. Heating apparatus substantially as herein described with reference to the accompanying drawings.

7. A space heating system including heating apparatus according to any preceding claim.