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14THE BASICS OF HYDRAULIC CALCULATION OF HEAT SUPPLY
SYSTEMS
Zulaykho Ochilovna Tokhrova
Dildora Shuxratovna Akbarkxadjayeva
Dilnoza Shokirovna Salimova
Master's students of Tashkent institute of architecture and civil engineering
ABSTRACT
Water heating systems are complex hydraulic systems in which the work of individual links is mutually dependent. For proper control and regulation, it is necessary to know the hydraulic characteristics of the operating equipment — circulation pumps and networks.
Keywords: system, individual links, drop, circulation pumps and networks
INTRODUCTION
Hydraulic calculation is one of the most important sections of the design and operation of the heat network. The task of hydraulic calculation includes:
— Determination of pipeline diameters;
— Determination of the pressure drop (head);
— Setting the values of pressures (heads) at different points of the network;
— Linking all points of the system in static and dynamic modes in order to ensure the permissible pressures and required pressures in the network and subscriber systems.
METHODOLOGY
In some cases, the task of determining the throughput capacity of pipelines with a known diameter and a given pressure loss can also be set.
The results of the hydraulic calculation provide the initial material for solving the following problems:
1. Determination of capital investments, consumption of metal (pipes) and the main volume of work on the construction of the heat network;
2. Establishing the characteristics of circulation and filing pumps, the number of pumps and their placement;
3. Clarification of the operating conditions of the heat network and subscriber systems and the choice of schemes for connecting subscriber installations to the heat network;
4. Selection of autoregulators for the heat network and subscriber inputs;
5. Development of operating modes.
The Bernoulli equation for the steady motion of an incompressible fluid through a pipeline, which expresses the specific energy balance of this fluid, attributed to a unit of mass, without taking into account its enthalpy, can be written as the expression
Ap
^l + A
2 P
r, vv; p2
2 p p
And Z2 — the geometric height of the center of gravity of the pipeline with respect to the horizontal reference plane, m;
wi And w2 — the velocity of the liquid, m/s
P1 u p2 — the pressure of the liquid, Pa;
ap — The pressure drop in the section, Pa;
P — The density of the liquid, kg/'m3;
g-the acceleration of a free-falling body, 9.81 m/s2.
The value of Zg - is the specific energy of the height in this section, attributed to the unit mass of the liquid, J/kg. The value w
2/2 is the specific kinetic energy of a liquid
in a given cross-section, related to the unit mass of the liquid, J/kg. The value p - is the specific potential energy of a liquid in a given cross-section, related to the unit mass of the liquid, J/kg.
Value v — loss of potential energy of 1 kg of liquid due to friction and local resistances in the pipeline section, J/kg
The lost potential energy of the liquid p is converted into heat, which leads to an increase in the specific enthalpy of the liquid during its movement through the pipeline.
In the hydraulic calculation of heat networks, the value w /2g, which is the velocity head of the flow in the pipeline, is usually not taken into account, so. as a rule, this value is a relatively small fraction of the total head and varies slightly along the length of the network. Usually take:
That is, the total head is considered equal to the sum of the piezometric head and the height of the pipeline axis above the plane. Piezometric pressure refers to the pressure in the pipeline, expressed in linear units (usually meters) the column of the
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ISSN: 2181-1601
liquid that is transmitted through the pipeline. It follows from equation that H = H0—Z. The piezometric head is equal to the difference between the total head and the geometric height of the pipeline axis above the reference plane.
The pressure drop in the pipeline can represent as the sum of two terms: the linear drop and the drop in local resistances:
Ap> Linear pressure drop; Ap><— pressure drop in local resistances.
Linear drop Ap < - represents the pressure drop on straight sections of the pipeline. Pressure drop in local resistances Ap»< - this is the pressure drop in valves (valves, valves, taps, etc.) and other equipment elements that are not evenly spaced along the length of the pipeline (bends, washers, transitions, etc.).
The linear pressure drop in pipelines transporting liquid or gases is determined by the formula:
Ap»- Linear pressure drop on the section, Pa; - specific linear pressure drop, i.e. pressure drop per unit length of the pipeline, Pa/m; 1 - length of the pipeline, m.
The initial dependence for determining the specific linear pressure drop in the pipeline is the Darsi equation
R" (5)
^ — Hydraulic friction coefficient (dimensionless value); w — medium speed, m/s; P — medium density, kg/m3; d — internal diameter of the pipeline, m;
The coefficient of hydraulic friction ^ depends on the nature of the pipe wall (smooth or rough) and the shape of the fluid movement (laminar or turbulent).
DISCUSSION AND RESULTS
You can imagine two types of roughness — uniform and uneven. With a uniform roughness, all projections have the same height and pitch. With uneven roughness, the height and pitch of the protrusions are not constant. Steel, cast iron, be-ton and other pipes used in engineering have, as a rule, uneven roughness.
With sufficient accuracy for practical calculations, it is assumed that in the so-called transition region, i.e., when 2300<Re<Repr the coefficient of hydraulic friction
depends on both equivalent relative roughness d , so it depends on the value of the criterion Re, and when Re>Repr
The coefficient of hydraulic friction depends only on d and does not depend on the criterion re. ^ ' d ^
The value of the roughness of the pipes, taking into account corrosion, is assumed to be=:
The coefficient of hydraulic friction, depending on the flow mode: (Corrosion Me., Precipitation of particles): For hydraulically rough pipes at the value of
If there are a number of local resistances in the pipeline section, the total pressure
A t-f™1
ApM =<?--
drop in all local resistances is determined by the formula 2 ; Pa
%— Coefficients of local resistances installed on the site (a dimensionless value depending on the nature of the resistance)
If we imagine a straight-line pipeline with a diameter d, the linear pressure drop on which is equal to the pressure drop in the local resistances, then the length of such a section of the pipeline, called the equivalent length of the local resistances, can obviously be found from the equality
aPm =rm x/3 or " 2 p 2 d Where from J ^ , m
The ratio of the pressure drop in the local resistances of the pipeline to the linear drop in this pipeline is a fraction of the local losses. It is not difficult to see that the proportion of local losses is equal to the ratio of the equivalent length of local resistances to the length pipelines
In preliminary calculations, when the nature and location of local resistances on the pipeline are not known, the average share of local pressure losses can be determined by the formula of prof.
G — flow rate of the heat carrier in the pipeline, kg/s; z— constant coefficient, depending on the type of heat carrier.
For water Z = 0,03h - 0,05; For water vapor Z = 0,24-0,4.
CONCLUSION
In the final calculation, the hydraulic resistances on all sections of the t are specified. Since the actual values are rounded to the standard values when selecting the pipe diameter, it is necessary to determine the actual specific losses in the length and
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ISSN: 2181-1601
speed of the pipe supply chain. And determine the equivalent lengths of local resistances in the sections.
The obtained values are compared with the available difference P at the starting point of the network. The calculation is considered satisfactory if the hydraulic resistance of the network does not exceed the available differential and differs from it by no more than 10%.
REFERENCES
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3. 6.Ионин А. А. и др. «Теплоснабжение». Москва, «Стройиздат» 1982 год.
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5. Ioan Sarbu, Calin Sebarchievici, in Solar Heating and Cooling Systems, 2017
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