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Typical errors at designing of independent systems of a heat supply
The purpose of the given publication is consideration of separate concrete errors in technical decisions of basic thermal schemes AIT with comments and recommendations for developers of systems of a heat supply of low power.                      

The great bulk of errors at designing is caused by simple carrying over of out-of-date technical decisions of the elementary thermal schemes AIT for one – two-functional systems of consumption of warmth on the modern, difficult systems of a heat supply including some consumers of systems of heating.

Objective preconditions of introduction of the independent (decentralised) systems of a heat supply is:
- Absence in some cases free capacities on the centralised sources;
- Consolidation of building of city areas by objects of habitation;
- Besides, the considerable part of building is necessary on district with not developed engineering infrastructure;
- Lower capital investments and possibility of a stage-by-stage covering of thermal loadings;
- Possibility of maintenance of comfortable conditions in apartment at own desire that in turn is more attractive in comparison with apartments at the centralised heat supply the temperature in which depends on the directive decision on the beginning and the termination of the heating period;
- Occurrence in the market of a considerable quantity of various updatings domestic and import (foreign) heat generators low power.

Let's reduce characteristics of the centralised and decentralised heating in the table.

Indicators Central heat supply Decentral heat supply
1. Economic 1) Resulted expenses RE=En*K+I:
a) a source thermal and the electric power +
b) a copper + +
c) thermal and electric networks +
d) CHS +
e) pumps + +
f) heat exchangers +
g) tanks accumulators + +
h) local systems of a heat supply + +
i) a gas network +
2) Working costs
a) annual deductions on amortisation, operating repair and vehicle safety. +
b) the fuel expense + +
c) thermal losses +
d) heat-carrier swapping +
e) the heat-carrier (water) + +
f) water treatment make-up water + +
g) service of systems of a heat supply + +
h) earth alienation +
i) the salary to workers +
2. Ecological
1) Quantity of harmful substances (CO2, NO2, ashes) deleted in atmosphere + +
2)-//- deleted in a reservoir + +
3) requirement in not renewed (natural) energy resources + +
3. The reliability
1) Reliability of systems + +
4. Comfortable
1) Fluctuations of internal temperature of air within days +
2) Daily average expenses of time of the population for service +
3) Safety (a consequence of failures) + +
5. Metal consumption + +
6. The building area occupied with the equipment + +
7. Term of building of systems of a heat supply before commissioning + +
8. Service life of the capital equipment before full replacement + +
9. The annual expense of fuel + +
10. The electric power expense
On heat-carrier swapping in a thermal network +

The heat hydraulic scheme of an independent source of warmth represents a difficult complex of the functional interconnected equipment, matching a mode of development of warmth heat generators and modes of consumption of warmth taking into account features of initial conditions: the kind of used fuel, type and number heat generators, quality of initial water, a design of systems of heating (central, floor, etc.), a water consumption condition on the purposes of hot water supply, an operating mode of systems of ventilation, etc. Technical decisions demand a careful substantiation of a choice the heat hydraulic scheme, the analysis of operating modes, maintenance of reliability of functioning and protection of the equipment against not settlement and inadmissible service conditions.

1. First of all it is necessary to notice that in projects AIT often enough selection of number and capacity of the established equipment is carried out only on as much as possible winter mode, for temperature of a cold five-day week, i.e. on the maximum capacity without calculation of the basic operating modes:
- For average temperature of cold month; – for average temperature of the heating period.
Often enough the settlement estimation is not carried out “summer” an operating mode. There are technical decisions without a necessary settlement substantiation of operational modes and quantity of the established equipment. For example, with installation only one copper which has been picked up for the maximum capacity which on partial loadings works in modes of item regulation “it is included-is switched off” with considerable amplitude of fluctuation of temperature of the heat-carrier and unprofitable modes of operation of the equipment that reduces efficiency of use of fuel and reduces equipment service life.

2. use of the modern effective forced coppers with high degree of an intensification of processes of burning of fuel and heat exchange in heating surfaces most sharply puts following requirements:
- maintenance of a constancy of the expense of the heat-carrier through a copper (according to requirements of the manufacturer of a copper);
- inadmissibility of decrease in temperature of the heat-carrier on an input in a copper “return” waters below the level excluding low-temperature corrosion (for no condensation coppers).
Operating experience of local systems of heating with application of thermostatic valves on heating devices shows, what even at use “weather-compensated” regulations (not to mention “thermostatic” regulation) holidays of warmth work of system of heating is characterised by considerable fluctuations of the expense of the heat-carrier. Hour and daily non-uniformity of consumption of warmth by the purposes of hot water supply and periodicity of work of some systems even more increase fluctuations of the expense and heat-carrier temperature in heat supply system. Application of the elementary thermal schemes will directly transfer process of change of the expense of the heat-carrier in local systems on the heat-carrier expense through coppers. In the thermal scheme there is no basic possibility to influence temperature of the return heat-carrier and to provide protection of a copper from “cold” return water. For these reasons application of thermal schemes AIT with absence of circulating pumps of coppers is irrational.

3. The considerable group of errors is caused by carrying over of principles of qualitative regulation of holiday of warmth of otopitelno-ventilating loadings on systems of a heat supply with elements of quantitative regulation in local systems of heating, and especially at a priority of holiday of warmth on hot water supply in traditionally used schemes that leads to essential fluctuations of the expense of the heat-carrier through a copper as it was marked earlier. Therefore management of work of the thermal scheme to organise so that even is necessary at essential changes of holiday of warmth (maxima of consumption of hot water, cases of mass operation, for example closings of thermostatic valves, warmth selection on heating of water in pool, etc.) At use “thermostatic” regulations on a water temperature t1 in the submitting pipeline of coppers in variable parametre there should be not a water expense through a copper, and temperature of return water t2. It will provide value of difference of temperatures corresponding to selected capacity in a source of warmth Dt = t1 – t2 = f (Qк), up to water rise in temperature in a return highway, in the absence of consumption of warmth to values t2, relatives, or even equal, to value t1. Specified to the full concerns and source operating modes at “weather-compensated” regulation. At the same time, for AIT, considering presence of loading of hot water supply, “thermostatic” regulation at two seasonal modes, for example during the warm period t1 = 70°C, and during the cold period t1 = 95°C, is more rational under condition of maintenance of a constancy of the expense of the heat-carrier through a copper and its protection from “cold” return water.

4. To lift temperature of return water and to provide the demanded expense of the heat-carrier through a copper it is possible by installation recirculation pump, however in this case the submitting collector and submitting pipeline A-V appear sites of the least hydrostatic pressure in system. It is necessary for considering at the feed organisation (on hydrostatic pressure upon site А-В) and selection of pumps of local systems and the recirculation pump.

5. Installation of the circulating pump of a copper can be applied to protection of a copper against not settlement operating modes in the submitting or return pipeline of system with the organisation recirculation lines, for example line А-В, or if it is supposed by a design of local systems, with the device of a closing site of small pressure difference, for example “a hydraulic regulator”. With other things being equal in the submitting pipeline behind a copper it is necessary to consider installation of the circulating pump irrational as it appears in a zone of the maximum temperatures of the heat-carrier that worsens service conditions of the pump and increases occurrence threat cavitation in it (especially at the maximum loadings), in comparison with installation of the circulating pump in a return line. Therefore installation of the circulating pump in a submitting line is necessary only when the big losses of pressure in planting (external) pipelines and knots of local systems take place (at dependent hydraulic connection), thus installation of the circulating pump in a return line will demand a high pressure on an input in a copper which value can exceed working pressure of a copper. At dependent (hydraulically) connection of local systems for stabilisation of the expense of water through a copper in various operating modes it is required to mount closing site А-В with installation of balancing crane S

6. At use as local sources of warmth of two-planimetric coppers with the built in heat exchangers of a hot water supply and the circulating pump of system of heating established in a copper (almost all two-planimetric flowing wall thermoblocks) often enough selection of coppers is made on heating loading. It can be an error, since. Flowing two-planimetric wall coppers of all without an exception of manufacturers of this equipment are designed and work with a priority of loading of hot water supply. The basic area of their application are multiroom residential buildings in which settlement heating loading averages 5-6 kw, and peak loading of hot water supply – about 20-25 kw. The circulating pump established in two-planimetric coppers is not intended for use in systems of heating with heat consumption, corresponding to a copper total power, and cannot provide necessary giving and a pressure in the extended and branched out system of heating, therefore the copper does not leave on a total power, being disconnected on a heat-carrier overheat. To exclude noted circumstance at use of one or several two-functional coppers application of collectors of small pressure difference (will allow “a hydraulic regulator” a short-circuited collector) with use of circulating pumps of local systems.

7. Often enough for protection of system of heating and a source of warmth from “defrosting” it is recommended to use “nonfreezing” heat-carriers, and it is considered that transition can be carried out simple replacement of water on containing special water mixes. However it is in most cases inadmissible, as physical properties “nonfreezing” liquids on heat conductivity, viscosity, density and thermal capacities differ from water, in system and a copper heavy expenses of the heat-carrier and circulating pressures of pumps be required. In some cases, for example at use of flowing coppers, application special mixes is not admissible.


Sources:
1. Khavanov P.A., Barynin, K. P. Some errors by working out thermomechanical parts of independent sources of warmth//AVOK, #8, 2004 [http://www.stroinauka.ru/d26dr3533m123.html]
2. Gila V., Markevich U. Analysis of perspective systems of a heat supply//The climate world, #21 [http://www.mir-klimata.com/archive/number21/article/article11/]

The author: Челябэнергопроект
Date: 07/22/2009

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