Contact surface heaters (CSH) are contact type appliances where the products of fuel combustion in such CSH directly contact the heated water. The contact takes place in the layer of special attachment sprayed by water through distribution device. Such attachment layer creates maximum contact area between gases and water. The appliances operate with positive pressure (forced supply) of air supplied by the fan.
CSH are mainly used as heat generators for standalone heating and water supply. Thus, CSH are a good alternative to traditional water boilers which are widely used at heating plants.
CSH possess a number of clear advantages compared to boilers. These are primarily efficiency, compact design and reliability..
Under contact method of water heating there is no need to use heater tubes system, which makes the installation smaller and less metal containing. On the other hand, the contact method ensures deep cooling of smoke fumes even to temperature below dew point of moisture contained therein. This is attained without making the construction more complicated. This is really important, because such performance ensures much higher efficiency of the installation (see Fig. 2).
It may be explained as follows. During combustion, hydrogen contained in the fuel reacts with oxygen giving water steam.1 Latent heat of vaporization of this steam quantity is approx. 12% of Qol – the fuel lowest calorific efficiency.2 Conventional heating boilers completely loose this heat and also the heat of vaporization of moisture arriving to the boiler together with the air, because the temperature of leaving gases is 115-1250С. But these losses are ignored in boiler efficiency calculation. Only "evident losses" are taken into account. Such losses are due to quantity, temperature and heat capacity of the leaving gases and also heat transfer from the boiler surface to the ambient air. They total to approx. 7% of Qol. The actual amount of the heat lost with the leaving gases would be equal to the sum of "evident" and "hidden" losses, i.e. approx. 19%. Decreasing of the leaving gases temperature in CSH, eg. to 500С , would cause partial condensation of the moisture contained therein (see Fig. 2) and therefore return of 4% of fuel heat, i.e. return of condensation (vaporization) heat of this moisture quantity..
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Total increase in efficiency of CSH3 compared to conventional boilers is the sum of the following components (see Figs. 2, 3):
- decrease of heat release (losses) to environment due to special design and smaller dimensions of the installation – 0,5%;
- decrease of heat losses due to lower temperature of leaving gases and decreasing their quantity – 3,5%;
- decrease of heat losses due to partial condensation of moisture contained in the smoke fumes – 4%.
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Thus, total increase in efficiency would total: 0,5+3,5+4= 8%.
CSH reliability is based on the simple design and the fact that water is heated in the apparatus under atmospheric pressure.4
There are a number of other advantages:
- natural deaeration of the heated water in CSH;
- possibility to use tap water or even artesian water (without further processing);
- possibility of self-cleaning of working surfaces with scale removal;
- replenishment of inevitable water losses in the installation by condensed moisture;
- low inertia (due to small quantity of metal used) allows a user to bring the heater from cold condition to nominal heat generation mode within several minutes. It also provides more flexibility in responding to load changes in the heat network;
- reducing of harmful emissions to the atmosphere.
The above advantages allow decreasing of the total cost of the heating plant based on CSH (as compared to boilers) with simultaneous improvement in reliability, safety and efficiency of operation.
CSH efficient operation conditions
Unlike water boilers, CSH efficiency practically does not depend on heat load, although depends on the temperature of water supplied to the apparatus. The lower is the water initial temperature, the lower would be the temperature of the leaving gases and the higher would be the installation efficiency. Thus, with supplied water temperature 45, 35, 250С and, accordingly, the temperature of the leaving gases equaling to 50, 40, 300С, the CSH efficiency calculated based on Qol, would be equal 101; 105,5; 108%.5
CSH operated the most efficiently and stably in hot water supply system because input water has quite low temperature. In case of heating system, CSH efficiency would mostly depend on return water temperature. When implementing the CSH into existing heating network, it is desirable to perform reconstruction and readjustment of the network prior to CSH installation in order to attain the best results.
CSH TECHNICAL DATA:
| 1. Installation power, MW |
0,5 |
0,9 |
2,6 |
| 2. Fuel |
natural gas |
| 3. Fuel consumption per 1 GCal of heat, m3 (under Qol=8300 KCal/m3) |
115 |
| 4. Heated water flow rate, m3/hour |
12 |
22 |
60 |
| 5. Installed electrical power, KW |
3 |
5 |
12 |
| 6. Maximum temperature of the hot water, 0С |
85 |
| 7. Return water temperature, 0С |
45-50 |
| 8. Leaving gases temperature, 0С |
50-55 |
| 9. Operation mode |
Automatic |
| 10. Overall dimensions, m: |
length |
1,6 |
2,2 |
3,0 |
| width |
0,8 |
1,1 |
1,8 |
| height |
2,0 |
2,3 |
2,6 |
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1 Combustion of 1 m3 of the natural gas generates 1.6 kg of steam on the average.
2 Back in the USSR period, in boilers efficiency calculations 100% is equal to Qol. The highest fuel calorific capacity Qoh takes latent heat of vaporization into account. For natural gas Qoh is equal approx 1,12 Qol.
3 Given comparison example is more characteristic for CSH operation in the heating network.
4 Therefore, the CSH is not subject to registration in boiler control bodies.
5 Efficiency of a modern water boiler is equal to approx 93%.
