DHW calculations, BKN. We find the volume, power of the hot water supply, power of the BKN (snake), warm-up time, etc.

In this article, we will consider practical problems for finding the volume of hot water accumulation and DHW heating power. Heating equipment power. Hot water readiness time for various equipment and the like.

Let's look at examples of tasks:

Task 1. Find the power of an instantaneous water heater

Instantaneous water heater- This is a water heater, the volume of water in which can be so small that its existence is useless for storing water. Therefore, it is believed that an instantaneous water heater is not intended to accumulate hot water. And we do not take this into account in our calculations.

Given: Water consumption is 0.2 l/sec. Temperature cold water 15 degrees Celsius.

Find: The power of an instantaneous water heater, provided that it heats the water to 45 degrees.

Solution

Answer: The power of the instantaneous water heater will be 25120 W = 25 kW.

It is practically not advisable to consume a large number of electricity. Therefore, it is necessary to accumulate (accumulate hot water) and reduce the load on electrical wires.

Instantaneous water heaters have unstable heating of hot water. The hot water temperature will depend on the water flow through the instantaneous water heater. Power or temperature switching sensors do not allow for good temperature stabilization.

If you want to find the output temperature of an existing instantaneous water heater at a certain flow rate.

Task 2. Electric water heater (boiler) heating time

We have an electric water heater with a capacity of 200 liters. The power of electric heating elements is 3 kW. It is necessary to find the time for heating water from 10 degrees to 90 degrees Celsius.

Given:

Wt = 3 kW = 3000 W.

Find: The time it takes for the volume of water in the water heater tank to heat up from 10 to 90 degrees.

Solution

The power consumption of heating elements does not change depending on the temperature of the water in the tank. (We will consider how power changes in heat exchangers in another problem.)

It is necessary to find the power of heating elements, as for an instantaneous water heater. And this power will be enough to heat water in 1 hour.

If it is known that with a heating element power of 18.6 kW, the tank will heat water in 1 hour, then it is not difficult to calculate the time with a heating element power of 3 kW.

Answer: The time for heating water from 10 to 90 degrees with a capacity of 200 liters will be 6 hours 12 minutes.

Task 3. Indirect heating boiler heating time

Let's take an example of an indirect heating boiler: Buderus Logalux SU200

Rated power: 31.5 kW. It is not clear for what reasons this was found. But look at the table below.

Volume 200 liters

The snake is made from steel pipe DN25. Inner diameter 25 mm. Outer 32 mm.

Hydraulic losses in the snake pipe indicate 190 mbar at a flow rate of 2 m3/hour. Which corresponds to 4.6.

Of course, this resistance is high for water and new pipe. Most likely, there were risks associated with pipeline overgrowth, high-viscosity coolant and resistance at connections. It is better to indicate obviously large losses so that someone does not miscalculate.

Heat exchange area 0.9 m2.

Fits 6 liters of water in a snake pipe.

The length of this snake pipe is approximately 12 meters.

Warm-up time is written as 25 minutes. It is not clear how this was calculated. Let's look at the table.

BKN snake power table

Consider the table for determining the power of the snake

Consider the SU200 snake heat dissipation power of 32.8 kW

At the same time, in the circuit DHW consumption 805 l/hour. Flows in 10 degrees comes out 45 degrees

Another variant

Consider the SU200 snake heat dissipation power of 27.5 kW

A coolant with a temperature of 80 degrees flows into the snake at a flow rate of 2 m3/hour.

At the same time, the flow rate in the DHW circuit is 475 l/hour. Flows in 10 degrees comes out 60 degrees

Other characteristics

Unfortunately, I will not provide you with a calculation of the heating time for an indirect heating boiler. Because this is not one formula. There are many intertwined meanings here: Starting from the heat transfer coefficient formulas, correction factors for different heat exchangers (since water convection also introduces its own deviations), and this ends with an iteration of calculations based on changed temperatures over time. Here, most likely in the future I will make a calculation calculator.

You will have to be content with what the manufacturer of the BKN (Indirect Heating Boiler) tells us.

And the manufacturer tells us the following:

That the water will be ready in 25 minutes. Provided that the flow into the snake will be 80 degrees with a flow rate of 2 m3/hour. The power of the boiler producing heated coolant should not be lower than 31.5 kW. Ready-to-drink water is considered to be 45-60 degrees. 45 degrees wash in the shower. 60 is very hot water, for example for washing dishes.

Task 4. How much hot water does it take to take a 30-minute shower?

Let's calculate for an example with an electric water heater. Since the electric heating element has a constant output of thermal energy. The power of the heating elements is 3 kW.

Given:

Cold water 10 degrees

Minimum tap temperature 45 degrees

The maximum temperature of water heating in the tank is 80 degrees

Comfortable flow rate of flowing water from the tap is 0.25 l/sec.

Solution

First, let's find the power that will provide this water flow

Answer: 0.45 m3 = 450 liters of water will be needed to wash off the accumulated hot water. Provided that the heating elements do not heat the water at the time of hot water consumption.

It may seem to many that there is no accounting for the entry of cold water into the tank. How to calculate the loss of thermal energy when water temperature of 10 degrees enters 80 degree water. There will obviously be a loss of thermal energy.

This is proven as follows:

Energy spent on heating the tank from 10 to 80:

That is, a tank with a volume of 450 liters and a temperature of 80 degrees already contains 36 kW of thermal energy.

From this tank we take energy: 450 liters of water with a temperature of 45 degrees (through the tap). Thermal energy of water with a volume of 450 liters at a temperature of 45 degrees = 18 kW.

This is proven by the law of conservation of energy. Initially, there was 36 kW of energy in the tank, they took 18 kW, leaving 18 kW. This 18 kW of energy contains water at a temperature of 45 degrees. That is, 70 degrees divided in half gives 35 degrees. 35 degrees + 10 degrees cold water we get a temperature of 45 degrees.

The main thing here is to understand what the law of conservation of energy is. This energy from the tank cannot escape to no one knows where! We know that 18 kW came out of the tap, and there was initially 36 kW in the tank. Taking 18 kW from the tank, we will lower the temperature in the tank to 45 degrees (to the average temperature (80+10)/2=45).

Let's now try to find the volume of the tank when the boiler is heated to 90 degrees.

Used energy consumption of hot water at the outlet of the tap 18317 W

Answer: Tank volume 350 liters. An increase of just 10 degrees reduced the tank volume by 100 liters.

This may seem unrealistic to many. This can be explained as follows: 100/450 = 0.22 is not that much. Stored temperature difference (80-45)

Let's prove that this is a valid formula in another way:

Of course this is a rough theoretical calculation! In the theoretical calculation, we take into account that the temperature in the tank between the upper and lower layers is instantly mixed. If we take into account the fact that the water is hotter at the top and colder at the bottom, then the volume of the tank can be reduced by the temperature difference. It is not for nothing that vertical tanks are considered more efficient in storing thermal energy. Since the greater the height of the tank, the higher the temperature difference between the top and bottom layers. When hot water is consumed quickly, this temperature difference is higher. When there is no water flow, very slowly the temperature in the tank becomes uniform.

We will simply lower 45 degrees to 10 degrees lower. For place 45 it will be 35 degrees.

Answer: Due to the temperature shift, we reduced the volume of the tank by another 0.35-0.286 = 64 liters.

We calculated on the condition that at the time of hot water consumption the heating elements were not working and did not heat the water.

Let's now calculate under the condition that the tank begins to heat the water at the moment of hot water consumption.

Let's add another power of 3 kW.

In 30 minutes of operation we will get half the power of 1.5 kW.

Then you need to subtract this power.

Answer: The tank volume will be 410 liters.

Task 5. Calculation of additional power for hot water supply

Let's consider a private house area 200 m2. The maximum power consumption for heating the house is 15 kW.

4 people live in the house.

Find: Additional power for domestic hot water

That is, we need to find the boiler power taking into account: House heating power + hot water heating.

For this purpose it is better to use scheme No. 4:

Solution

It is necessary to find how many liters of hot water a person consumes per day:

SNiP 2.04.01-85* states that, according to statistics, 300 liters per day are consumed per person. Of these, 120 liters are for hot water at a temperature of 60 degrees. These city statistics are mixed with people who are not used to using so much water per day. I can offer my consumption statistics: If you like to take hot baths every day, you can spend 300-500 liters of hot water per day for just one person.

Volume of water per day for 4 people:

That is, to the heating power of a house of 15 kW, you need to add 930 W = 15930 W.

But if we take into account the fact that at night (from 23:00 to 7:00) you do not consume hot water, you get 16 hours when you consume hot water:

Answer: Boiler power = 15 kW + 1.4 kW for hot water supply. = 16.4 kW.

But in this calculation there is a risk that at the time of high consumption of hot water at certain hours, you will stop heating the house for a long time.

If you want to have a good flow of hot water for a private home, then choose a BKN of at least 30 kW. This will allow you to have an unlimited flow rate of 0.22 l/sec. with a temperature of at least 45 degrees. The boiler power should not be less than 30 kW.

In general, the objectives of this article were focused on energy conservation. We did not consider what was happening at a particular moment, but took a different route to calculate. We followed the undisputed method of energy conservation. The energy expended at the outlet of the tap will then be equal to the energy coming from the boiler equipment. Knowing the power in two different places, you can find the time spent.

Once we discussed the calculation of hot water supply on the forum: http://santeh-baza.ru/viewtopic.php?f=7&t=78

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A series of video tutorials on a private home
Part 1. Where to drill a well?
Part 2. Construction of a water well
Part 3. Laying a pipeline from the well to the house
Part 4. Automatic water supply
Water supply
Water supply for a private home. Principle of operation. Connection diagram
Self-priming surface pumps. Principle of operation. Connection diagram
Self-priming pump calculation
Calculation of diameters from central water supply
Water supply pumping station
How to choose a pump for a well?
Setting up the pressure switch
Pressure switch electrical diagram
Operating principle of a hydraulic accumulator
Sewage slope per 1 meter SNIP
Heating schemes
Hydraulic calculation of a two-pipe heating system
Hydraulic calculation of a two-pipe associated heating system Tichelman loop
Hydraulic calculation of a single-pipe heating system
Hydraulic calculation of radial distribution of a heating system
Scheme with a heat pump and solid fuel boiler - operating logic
Three-way valve from valtec + thermal head with remote sensor
Why the heating radiator in an apartment building does not heat well
How to connect a boiler to a boiler? Connection options and diagrams
DHW recirculation. Operating principle and calculation
You are not calculating the hydraulic arrows and collectors correctly
Manual hydraulic heating calculation
Calculation of warm water floors and mixing units
Three-way valve with servo drive for domestic hot water
Calculations of hot water supply, BKN. We find the volume, power of the snake, warm-up time, etc.
Water supply and heating designer
Bernoulli's equation
Calculation of water supply for apartment buildings
Automation
How servos and three-way valves work
Three-way valve to redirect the flow of coolant
Heating
Calculation of thermal power of heating radiators
Radiator section

Introduction

1. Determination of the thermal loads of the microdistrict for heating, ventilation, hot water supply

2. Selecting a scheme for connecting the hot water heater to the heating network and temperature chart TsKR

Thermal hydraulic calculation of a shell-and-tube heater

Calculation of two-stage sequential circuit connection of DHW water heaters

Thermal and hydraulic calculation of plate hot water heaters

List of sources used

INTRODUCTION

In this work, the thermal loads of the microdistrict for heating and hot water supply are calculated, a scheme for switching on hot water heaters is selected, and thermal and hydraulic calculations of two heat exchanger options are performed. Only residential buildings of the same type, 5-10 storeys, will be considered. The coolant system is closed, 4-pipe with installation of a hot water heater in the central heating substation. All calculations are carried out using aggregated indicators. We accept residential buildings without ventilation.

Calculation and graphic work is carried out in accordance with the current standard norms and rules, technical. conditions and basic provisions for the design, installation and operation of heat supply systems for residential buildings.

1. Determination of the thermal loads of the microdistrict for heating, ventilation, and hot water supply.

Maximum heat flow for heating residential buildings in the microdistrict:

where is the aggregated indicator of the maximum heat flow for m²;

A - total area of ​​the residential building, m²;

The coefficient of heat flow for heating residential buildings (share of residential buildings)

80 W/m² Astrakhan

A= 16400 m² - as specified

0, because Only residential buildings are considered.

Maximum heat flow for hot water supply

where is the coefficient of hourly uneven consumption of the number of FGPs

The aggregated indicator of the average heat flow for hot water supply is 376 W/ml;

U - the number of residents in the microdistrict, according to the assignment, is equal to 560 people;

376 W/ml;

Thermal loads on ventilation for a residential building are zero.

2. Selecting a scheme for connecting the hot water heater to the heating network and the temperature schedule of the central heating system

Selecting a heater connection diagram

where - from formula (2)

From formula (1)

When accepted two-stage scheme, when a single-stage parallel circuit is adopted

Conclusion: there is only one heater, therefore one common heater located in the central heating station is connected in 2 ways step scheme.

According to the instructions of the TsKR, heat supply is carried out according to the household heating schedule of 130/700C, therefore the parameters of the break point, which are calculated, are known and amount to;

Maximum consumption - average heat flow for hot water supply (DHW)

where is the maximum heat flow to the hot water supply from formula (2)

Coefficient of hourly unevenness in FGP consumption

3. Thermal hydraulic calculation of a shell-and-tube heater

Outside air temperature at the "breaking point"

where is the indoor air temperature,

Design air temperature for heating design,

water temperature in the falling pipeline at the “break point”,

The water temperature in the return pipeline is approximately at the “breaking point”, with the estimated coolant temperature in the falling pipeline being 1300C.

Estimated water temperature difference in the heating network, determined by the formula

Where - design temperature network water in the supply pipeline,

Estimated temperature of network water in the return pipeline,

4. Calculation of a two-stage sequential connection scheme for DHW water heaters

heating ventilation shell and tube heater

Select and calculate a water heating installation for DHW central heating station equipped with a water heater consisting of shell-and-tube type sections with a pipe system of straight smooth pipes with a block of supporting partitions in accordance with GOST 27590. The heating system of the microdistrict is connected to the main heating network according to a dependent circuit. The central heating station has storage tanks.

Initial data:

The temperature of the coolant (heating water) in accordance with the calculated increased schedule is accepted:

At the design outdoor temperature for heating design;

in the supply line ? 1 = 130 0С, in reverse - ? 2 = 700C;

at the break point of the temperature graph t` n= -2.02 0С;

in the supply line ? 1 n= 70 0С, reverse ? 2 n= 44.9 0C.

Cold temperature tap water tc=5 0 WITH.

The temperature of the hot water entering the SGV is th=60 0 WITH.

Maximum heat flow for heating buildings Qo max= 1312000 W.

Estimated thermal performance of water heaters Qsph=Qhm=QhT=210560 W .

6 Heat loss by pipelines Qht=0.

Take water density ?= 1000 kg/m3.

Maximum calculated second water consumption for DHW qh= 2.5 l/s.

Calculation procedure:

Maximum calculation of water for heating:

Temperature of heated water behind the 1st stage water heater:

Consumption of heating network water for DHW:

4 Consumption of heated water for DHW:

Heat flow to stage II of the SGV water heater:

Heat flow for heating at the break point of the network water temperature graph at outside air temperature t`n:

Heating water flow through the first stage of the water heater:

Estimated thermal performance of the first stage of the water heater:

Estimated thermal performance of the second stage of the water heater:

Temperature of heating network water at the outlet of the second stage water heater:

The temperature of the heating network water at the outlet of the first stage water heater, subject to equality:

12 Average logarithmic temperature difference between heating and heated water for stage 1:

The same for stage II:

The required cross-section of the water heater tubes at the water speed in the tubes and with single-flow operation:

From the table adj. 3, based on the obtained value, we select the type of water heater section with the following characteristics: , .

Water speed in tubes:

Speed ​​of network water in the annulus:

Calculation of the 1st stage of the DHW water heater:

e) heat transfer coefficient at:

e) required heating surface of stage 1:

g) number of sections of the 1st stage water heater:

We accept 2 sections; actual heating surface F1tr=0.65*2=1.3 m2.

Calculation of the second stage of the SGV water heater:

a) average temperature of heating water:

b) average temperature of heated water:

c) heat transfer coefficient from heating water to the walls of the tubes:

d) heat transfer coefficient from the walls of the tubes to the heated water:

e) heat transfer coefficient at

f) required heating surface of stage II:

g) number of sections of the second stage water heater:

We accept 6 sections.

As a result of the calculation, we got 2 sections in the 1st stage heater and 6 sections in the 2nd stage heater with a total heating surface of 5.55 m2.

Pressure loss in water heaters (6 consecutive sections 2 m long) for water passing in tubes taking into account? = 2:

Stage I: PV 76*2-1.0-RG-2-UZ GOST 27590-88

II stage: PV 76*2-1.0-RG-6-UZ GOST 27590-88

5. Thermal and hydraulic calculation of plate hot water heaters

Select and calculate the water heating installation of a plate heat exchanger assembled from 0.3p plates for the SGW of the same central heating station as in the example with shell-and-tube sectional heaters. Consequently, the initial data, flow rates and temperatures of coolants at the inlet and outlet of each stage of the water heater are taken to be the same as in the appendix. 3.

We check the ratio of strokes in the heat exchanger of the first stage, first taking the pressure loss for the heated water? Рн = 100 kPa, for the heating water? Рgr = 40 kPa.

The stroke ratio does not exceed 2, but the flow rate of heating water is much greater than the flow rate of heated water, therefore, an asymmetrical arrangement of the heat exchanger is adopted.

By optimal speed water and the open cross-section of one interplate channel, we determine the required number of channels for heated water and heating water:

General live section channels in the package along the flow of heated and heating water (taken equal to 2, =15):

Actual speeds of heating and heated water:

Calculation of the 1st stage water heater:

a) from Table 1, Appendix 4; we obtain the heat transfer coefficient from the heating water to the plate wall:

b) heat absorption coefficient from the plate wall to the heated water:

d) required heating surface of the 1st stage water heater:

e) according to Table 1, Appendix 4, heating surface of one plate, number of strokes through heating and heated water in the heat exchanger:

f) actual heating surface of the first stage water heater:

g) stage 1 pressure losses for heating and heated water:

Calculation of the second stage water heater:

a) heat transfer coefficient from the heating water to the plate wall:

b) heat absorption coefficient from the plate to the heated water:

c) , heat transfer coefficient:

d) required heating surface of the second stage water heater:

e) number of strokes through heating and heated water in the heat exchanger:

We accept by heating water, by heated water.

f) actual heating surface of the second stage water heater:

g) pressure loss of stage II for heating and heated water:

As a result of the calculation, as a hot water heater we accept two heat exchangers (stages I and II) of a collapsible design (p) with plates of type 0.3p, 1 mm thick, made of steel 12×18N10T (version 01), on a cantilever frame (version 1k), With sealing gaskets made of rubber brand 51-1481 (symbol 12). The heating surface of stage I is 8.7 m2, stage II is 8.7 m2. Specifications plate heat exchangers are given in Table 1-3 app. 4.

Symbol heat exchangers:

Steps: P 0.3r-1-8.7-1k-0.1-12 CX1=

II Stage: P 0.3r-1-8.7-1k-0.1-12 CX2=

LIST OF SOURCES USED

1. SNiP 2.04.01-85. Internal water supply and sewerage of buildings.

Lipovka Yu.L., Tselishchev A.V., Misyutina I.V. Hot water supply: method. instructions for course work. Krasnoyarsk: SFU, 2011. 36 p.

GOST 27590-88. Water-to-water heaters for heat supply systems. Are common technical specifications.

SNiP 2.04.07-89*. Heating network.

5. SNiP 23-01-99. Construction climatology.

6. STO 4.2 - 07 - 2012 Quality management system. General requirements to the construction, presentation and execution of documents of educational activities. Instead of STO 4.2 - 07 - 2010; date entered 02/27/2012. Krasnoyarsk: IPK SFU. 2012. 57 p.

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Calculation of hot water supply systems consists of determining the diameters of the supply and circulation pipelines, selecting water heaters (heat exchangers), generators and heat accumulators (if necessary), determining the required pressure at the inlet, selecting booster and circulation pumps, if they are necessary.

Calculation of a hot water supply system consists of the following sections:

The estimated costs of water and heat are determined and, on the basis of this, the power and dimensions of water heaters are determined.

The supply (distribution) network is calculated in water collection mode.

The hot water supply network is calculated in circulation mode; opportunities for use are determined natural circulation, and if necessary, parameters are determined and circulation pumps are selected.

In accordance with the individual assignment for coursework and diploma design, calculations of storage tanks and coolant networks can be made.

2.2.1. Determination of estimated consumption of hot water and heat. Selection of water heaters

To determine the heating surface and further selection of water heaters, hourly consumption of hot water and heat is required; to calculate pipelines, second consumption of hot water is required.

In accordance with paragraph 3 of SNiP 2.04.01-85, the second and hourly consumption of hot water are determined using the same formulas as for cold water supply.

The maximum second consumption of hot water at any calculated section of the network is determined by the formula:

- second consumption of hot water by one device, which is determined by:

a separate device - in accordance with the mandatory Appendix 2;

different devices serving the same consumers - according to Appendix 3;

various devices serving different water consumers - according to the formula:

, (2.2)

- second consumption of hot water, l/s, by one water tap for each group of consumers: taken according to Appendix 3;

N i – number of water taps for each type of water consumer;

- probability of operation of devices determined for each group of water consumers;

a is the coefficient determined according to Appendix 4 depending on the total number of devices N in the network section and the probability of their action P, which is determined by the formulas:

a) with identical water consumers in buildings or structures

, (2.3)

Where
- maximum hourly consumption of hot water of 1 liter by one water consumer, taken according to Appendix 3;

U – number of hot water consumers in a building or structure;

N – number of devices served by the hot water supply system;

b) with different groups of water consumers in buildings for various purposes

, (2.4)

and N i - values ​​​​related to each group of hot water consumers.

The maximum hourly consumption of hot water, m 3 / h, is determined by the formula:

, (2.5)

- hourly consumption of hot water by one device, which is determined by:

a) with identical consumers - according to Appendix 3;

b) for different consumers - according to the formula

, l/s (2.6)

And
- values ​​related to each type of hot water consumer;

magnitude determined by the formula:

, (2.7)

- coefficient determined according to Appendix 4 depending on the total number of devices N in the hot water supply system and the probability of their operation P.

Average hourly hot water consumption , m 3 / h, for the period (day, shift) of maximum water consumption, incl., is determined by the formula:

, (2.8)

- maximum daily hot water consumption of 1 liter by one water consumer, taken according to Appendix 3;

U – number of hot water consumers.

Amount of heat (heat flow) per period (day, shift) maximum water consumption for the needs of hot water supply, taking into account heat loss, is determined by the formulas:

a) within a maximum hour

b) during the average hour

And - maximum and average hourly consumption of hot water in m 3 / h, determined by formulas (2.5) and (2.8);

t с – design temperature of cold water; in the absence of data in the building, t is taken equal to +5ºС;

Q ht – heat losses from supply and circulation pipelines, kW, which are determined by calculation depending on the lengths of pipeline sections, outer diameters of pipes, the difference in temperature of hot water and the environment surrounding the pipeline and the heat transfer coefficient through the walls of the pipes; In this case, the efficiency of pipe thermal insulation is taken into account. Depending on these values, heat loss is given in various reference books.

When calculating in course projects, heat loss Q ht by supply and circulation pipes can be taken in the amount of 0.2-0.3 of the amount of heat required for preparing hot water.

In this case, formulas (2.9) and (2.10) will take the form:

a) , kW (2.11)

b) , kW (2.12)

A smaller percentage of heat loss is accepted for systems without circulation. Most civil buildings use high-speed sectional water heaters with variable output, i.e. with adjustable coolant consumer. Such water heaters do not require heat storage tanks and are designed for maximum hourly heat flow
.

The selection of water heaters consists of determining the heating surface of the coils using the formula:

, m 3 (2.13)

K – heat transfer coefficient of the water heater, taken according to table 11.2; for high-speed water-water heaters with brass heating tubes, the value of k can be taken in the range of 1200-3000 W/m sq., ºC, with a smaller one accepted for devices with smaller section diameters;

µ - coefficient of reduction in heat transfer through the heat exchange surface due to deposits on the walls (µ = 0.7);

- calculated temperature difference between the coolant and heated water; for counterflow high-speed water heaters
º is determined by the formula:

, ºС (2.14)

Δt b and Δt m – greater and lesser temperature difference between the coolant and heated water at the ends of the water heater.

The parameters of the coolant during the winter calculation period, when the heating networks of buildings are operating, are assumed to be 110-130 ºC in the supply pipeline and -70 in the return pipeline, the parameters of the heated water during this period are t c = 5ºC and t c = 60...70 ºC. IN summer period the heating network only works for preparing hot water; The parameters of the coolant during this period in the supply pipeline are 70...80 ºC and in the return pipeline 30...40 ºC, the parameters of the heated water are t c = 10...20 ºC and t c = 60...70 ºC.

When calculating the heating surface of a water heater, it may happen that the determining period will be the summer period, when the temperature of the coolant is lower.

For cylinder water heaters, the calculation for the temperature difference is determined by the formula:

, ºC (2.15)

t n and t k – initial and final temperature of the coolant;

t h and t c – temperature of hot and cold water.

However, DHW water heaters are used for industrial buildings. They take up a lot of space and in these cases can be installed outdoors.

The heat transfer coefficient for such water heaters, according to table 11.2, is 348 W/m 2 ºC.

The required number of standard sections of water heaters is determined:

, pcs (2.16)

F – design heating surface of the water heater, m2;

f – heating surface of one section of the water heater, adopted according to Appendix 8.

The pressure loss in a high-speed water heater can be determined by the formula:

, m (2.17)

n – coefficient taking into account the overgrowth of pipes, is taken according to experimental data: in their absence, with one cleaning of the water heater per year n=4;

m – coefficient of hydraulic resistance of one section of the water heater: with a section length of 4 m m=0.75, with a section length of 2 m m=0.4;

n in – number of sections of the water heater;

v is the speed of movement of heated water in the water heater tubes without taking into account their overgrowth.

, m/s (2.18)

q h – maximum second water flow through the water heater, m/s;

W total - the total open cross-sectional area of ​​the water heater tubes is determined by the number of tubes, taken according to Appendix 8, and the diameter of the tubes, taken as 14 mm.

Water consumption for hot water supply needs should be determined according to hot water consumption standards, taking into account the likelihood of using water taps. Determine the load on DHW system according to the maximum flow rate of hot water and take it into account when choosing a heat source. Hello, dear friends! We are used to using hot water every day and can hardly imagine comfortable life, if you cannot take a warm bath or you have to wash dishes under a tap from which a cold stream flows. Water desired temperature and in the right quantity - this is what the owner of every private home dreams of. Today we will determine the estimated water and heat consumption for hot water supply to our home. You must understand that at this stage it is not particularly important to us where we get this heat. Perhaps we will take it into account when choosing the power of the heat supply source and will heat water for the needs of hot water supply in the boiler. Perhaps we will heat water in a separate electric boiler or a gas water heater, or perhaps they will bring it to us.

Well, what if there are no technical capabilities to install a hot water system at home, then we will go to our own or the village bathhouse. Our parents mostly went to the city baths, and now the mobile Russian bathhouse under your window has rung. Of course, life does not stand still and having a bath and shower in the house today is no longer a luxury, but a simple necessity. Therefore, we will provide a hot water supply system in the house. The correct calculation of the hot water supply will determine the load on the domestic hot water system and, ultimately, the choice of the power of the heat source. Therefore, this calculation must be approached very seriously. Before choosing the design and equipment of a domestic hot water system, we need to calculate the main parameter of any system - the maximum consumption of hot water per hour of maximum water consumption (Q g.v max, kg/h).

In practice, using a stopwatch and a measuring container, we determine the consumption of hot water, l/min when filling the bathtub

Calculation of the hourly maximum hot water flow rate per hour of maximum water consumption

To calculate this consumption, let's turn to the hot water consumption standards (according to chapter SNiP 2-34-76), see table 1.

Hot water consumption standards (according to chapter SNiP 2-34-76)

Table 1

g и.с – average for heating season, l/day;

g and – maximum water consumption, l/day;

g i.h – highest water consumption, l/h.

Dear friends, I want to warn you against one common mistake. Many developers, and even young inexperienced designers, calculate the hourly maximum hot water flow using the formula

G max =g i.h *U, kg/h

g i.h – rate of hot water consumption, l/h, maximum water consumption, taken according to table 1; U – number of hot water consumers, U=4 people.

G max = 10 * 4 = 40 kg/h or 0.67 l/min

Q year max = 40 * 1 * (55 – 5) = 2000 kcal/h or 2.326 kW

Having calculated the water flow in this way and selected the power of the heat source to heat this flow, you have calmed down. But when you get into the shower, you will be surprised to find that only 3 drops of water per second are dripping onto your dirty and sweaty bald head. Neither washing your hands, nor rinsing the dishes, not to mention taking a bath is out of the question. So what's the deal? And the mistake is that the maximum hourly water consumption for the day of greatest water consumption was not correctly determined. It turns out that all hot water consumption rates according to Table 1 should be used only to calculate the flow rate through individual devices and the likelihood of using their action. These standards are not applicable for determining costs based on the number of consumers, by multiplying the number of consumers by specific consumption! This is precisely the main mistake made by many calculators when determining the heat load on the hot water supply system.

If we need to determine the performance of heat generators (boilers) or heaters when subscribers do not have hot water storage tanks (our case), then design load for the DHW system should be determined by the maximum hourly consumption of hot water (heat) for the day of greatest water consumption according to the formula

Q g.v max =G max * s * (t g.wed –t x), kcal/h

G max – maximum hourly consumption of hot water, kg/h. The maximum hourly consumption of hot water, G max, taking into account the likelihood of using water taps, should be determined by the formula

G max = 18 *g * K and * α h * 10 3, kg/h

g – hot water consumption rate, l/with water taps. In our case: for a washbasin g y = 0.07 l/s; for washing g m = 0.14 l/s; for shower g d = 0.1 l/s; for a bath g in = 0.2 l/s. We choose a larger value, that is, g = g in = 0.2 l/s; K and – dimensionless coefficient of use of a water-folding device for 1 hour of maximum water consumption. For a bathtub with a characteristic (highest) hot water flow g x = 200 l/h, this coefficient will be equal to K u = 0.28; α h is a dimensionless value determined depending on the total number N of water-folding devices and the probability of using them R h for 1 hour of greatest water consumption. In turn, the probability of using water-folding devices can be determined by the formula

R h =g i.h *U/3600*K and*g*N

g i.h – rate of hot water consumption per hour of greatest water consumption, l/h. It is taken according to table 1, g and.h = 10 l/h; N – total number of water taps installed in the house, N = 4.

R h = 10 * 4 / 3600 * 0.28 * 0.2 * 4 = 0.0496. At R h< 0,1 и любом N по таблице (N * Р ч = 0,198) определяем α ч = 0,44

G max = 18 * 0.2 * 0.28 * 0.44 * 10 3 = 444 kg/h or 7.4 l/min.

Q year max = 444 * 1 * (55 – 5) = 22200 kcal/h or 25.8 kW

No, neither the desired temperature nor the proper flow of hot water - discomfort

As you can see, dear friends, the consumption of water and, accordingly, heat has increased approximately 10 times. In addition, the heat consumption for hot water supply (25.8 kW) is 2 times greater than the total heat consumption for heating and ventilation of the house (11.85 + 1.46 = 13.31 kW). If this data is presented to the “Customer”, then his hair will stand on end and he will demand that they explain to him - what’s the matter? So let's help him. Tables 2 and 3 below will help us with this. Now let's turn to Table 2 and calculate the highest hourly water consumption when loading all water consumers at the same time. Adding up all the typical costs, we get 530 l/h. As you can see, the total characteristic consumption turned out to be 86 l/h more than the calculated one (444 l/h). And this is not surprising, since the likelihood that all water taps will work at the same time is very small. Our maximum requirement for hot water is already 84%. In reality, this value is even less – about 50%. Let's try to get the real value, for this we use table 3. Do not forget that hot water consumption standards are developed for consumers at t g.av = 55 o C, but from the table we will find costs at t g. av = 40 o C.

The minimum total consumption of hot water, with an average water temperature equal to t g.v = 40 o C and the simultaneous operation of all water intake devices with a probability of this consumption of 84%, will be equal to G min =[ (5 * 1.5) + (20 * 5) + (30 * 6) +(120 * 10) ] * 0.84 = 342.3 l/h (239.6 l/h at t g.v = 55 o C)

The maximum total consumption of hot water, with an average water temperature of 40 o C and the simultaneous operation of all water intake devices with a probability of this consumption of 84%, will be equal to G max = [ (15 * 3) + (30 * 5) + (90 * 6 ) +(200 * 15) ] * 0.84 = 869.4 l/h (608.6 l/h at t g.v = 55 o C)

The average flow rate at t g.v = 55 o C will be equal to G avg = (G min + G max)/2 = (239.6 + 608.6)/2 = 424.1 l./h. So we got what we were looking for - 424.1 l/h instead of 444 l/h according to calculations.

Hot water consumption standards for water taps (chapter SNiP 2-34-76)

table 2

Hot water consumption standards for various water intake devices

Table 3

Collection point	Sink	Kitchen sink	Economical shower	Shower standard	Shower comfort.	Bath
DHW temperature, o C	35-40	55	40	40	40	40
Consumption time, min	1,5-3	5	6	6	6	10-15
Hot water consumption for domestic needs, l	5-15	20-30	30	50	90	120-200

Thus, when calculating hot water supply, it is imperative to take into account the following nuances: the number of residents; frequency of use of the bath, shower; number of bathrooms where hot water is used; technical characteristics of plumbing elements (for example, the volume of the bathroom); the expected temperature of the heated water, as well as the likelihood of using water taps at the same time. IN next posts We will take a closer look at three common hot water supply systems. Depending on the method of heating water, these systems, for private country house, subdivided: DHW with storage water heater(boiler); DHW with instantaneous water heater; DHW with double-circuit boiler.

What do you think I’m doing?!!!

The obtained values ​​of water and heat consumption for DHW needs – G max = 444 kg/h or 7.4 l/min and Q g.v max = 22200 kcal/h or 25.8 kW we accept, with subsequent clarification, when choosing a heat source. Today we completed the 4th point of our home plan - we calculated the maximum hourly hot water consumption for a private house. Who hasn't joined yet, join us!

Best regards, Gregory

Published: 05.12.2010 | |

Throughout 2004, our organization received applications for the development of technical proposals for boiler houses for heat supply to residential and public buildings, in which the loads on hot water supply were very different (to a lesser extent) from those previously requested for identical consumers. This was the reason for analyzing the methods for determining the loads on hot water supply (DHW), which are given in the current SNiPs, and possible errors arising when they are used in practice.
E.O. SIBIRKO

Currently, the procedure for determining heat loads on hot water supply is regulated normative document SNiP 2.04.01–85* “Internal water supply and sewerage of buildings.”

The methodology for determining the estimated flow rates of hot water (maximum second, maximum hourly and average hourly) and heat flows (heat power) per hour at average and maximum water consumption in accordance with section 3 of SNiP 2.04.01–85* is based on the calculation of the corresponding costs through water-folding devices (or groups of similar devices with subsequent averaging) and determining the probability of their simultaneous use.

All service tables with data on various specific consumption rates, etc., given in SNiP, are used only for calculating the flow rate through individual devices and the probability of their operation. They are not applicable for determining costs based on the number of consumers, by multiplying the number of consumers by specific consumption! This is precisely the main mistake made by many calculators when determining the heat load on the hot water supply.

The presentation of the calculation methodology in section 3 of SNiP 2.04.01–85* is not simple. Introduction of numerous superscript and subscript Latin indices (derived from the corresponding terms in English language) makes it even more difficult to understand the meaning of the calculation. It is not entirely clear why this was done in the Russian SNiP - after all, not everyone speaks English and easily associates the index “ h"(from English hot- hot), index " c"(from English cold- cold) and " tot"(from English total- result) with corresponding Russian concepts.

To illustrate the standard error encountered in calculations of heat and fuel needs, I will give a simple example. Need to determine DHW load for a 45-apartment residential building with a population of 114 people. The water temperature in the DHW supply pipeline is 55°C, the cold water temperature is winter period-5°C. For clarity, let’s assume that each apartment has two similar water points (sink in the kitchen and washbasin in the bathroom).

Option I of calculation is incorrect (we have repeatedly encountered this method of calculation):

According to the table “Rates of water consumption by consumers” of the mandatory Appendix 3 of SNiP 2.04.01–85*, we determine for “Apartment-type residential buildings: with bathtubs from 1500 to 1700 mm long, equipped with showers” ​​the hot water consumption per inhabitant at the hour of greatest water consumption is equal to q hhr, u = 10 l/h. Then everything seems to be quite simple. Total consumption hot water per house at the hour of greatest water consumption based on the number of residents 114 people: 10. 114 = 1140 l/h.

Then, the heat consumption per hour of greatest water consumption will be equal to:

Where U- number of residents in the house; g - density of water, 1 kg/l; With- heat capacity of water, 1 kcal/(kg °C); t h - hot water temperature, 55°C; t c - cold water temperature, 5°C.

The boiler room, actually built on the basis of this calculation, clearly could not cope with the load of hot water supply at the moments of peak hot water supply, as evidenced by numerous complaints from the residents of this house. Where is the mistake here? It lies in the fact that if you carefully read section 3 of SNiP 2.04.01–85*, it turns out that the indicator q hhr, u, given in Appendix 3, is used in the calculation method only to determine the probability of operation of sanitary fixtures, and the maximum hourly flow of hot water is determined completely differently.

Calculation option II - in strict accordance with the SNiP methodology:

1. Determine the probability of the device operating.

,

Where q hhr,u = 10 l - according to Appendix 3 for this type of water consumer; U= 114 people - the number of residents in the house; q h0 = 0.2 l/s - in accordance with clause 3.2 for residential and public buildings, it is allowed to take this value in the absence technical characteristics devices; N- the number of sanitary fixtures with hot water, based on the two water points we have adopted in each apartment:

N= 45. 2 = 90 devices.

Thus we get:

R= (10 x 114)/(0.2 x 90 x 3600) = 0.017.

2. Now let’s determine the probability of using sanitary appliances (the ability of the appliance to supply normalized hourly water flow) during the estimated hour:

,
Where P- the probability of the device action determined in the previous paragraph, - P= 0,017; q h0 = 0.2 l/s - second water flow rate related to one device (also already used in the previous paragraph); q h0,hr - hourly water consumption by the device, in accordance with clause 3.6, in the absence of technical characteristics of specific devices, it is allowed to take q h0,hr = 200 l/h, then:

.

3. Since P h is less than 0.1, we further use the table. 2 of Appendix 4, according to which we determine:

at .

4. Now we can determine the maximum hourly hot water flow:

.

5. And finally, we determine the maximum thermal load DHW (heat flow during the period of maximum water consumption during the hour of maximum consumption):

,

Where Q ht- heat losses.

Let's take into account heat losses, taking them as 5% of the design load.

.

We got a result more than twice the result of the first calculation! As practical experience shows, this result is much closer to the real needs for hot water for a 45-apartment residential building.

You can give for comparison the result of the calculation using the old method, which is given in most reference literature.

Option III. Calculation using the old method. Maximum hourly heat consumption for hot water supply needs for residential buildings, hotels and hospitals general type by the number of consumers (in accordance with SNiP IIG.8–62) was determined as follows:

,

Where k h - coefficient of hourly unevenness of hot water consumption, taken, for example, according to table. 1.14 reference book “Adjustment and operation of water heating networks” (see Table 1); n 1 - estimated number of consumers; b - the rate of hot water consumption per consumer, adopted according to the relevant tables of SNiPa IIG.8–62 and for apartment-type residential buildings equipped with bathrooms from 1500 to 1700 mm in length, is 110–130 l/day; 65 - hot water temperature, °C; t x - cold water temperature, °C, we accept t x = 5°C.

Thus, the maximum hourly heat consumption for hot water supply will be equal to:

.

It is easy to see that this result almost coincides with the result obtained using the current method.

Application of the hot water consumption rate per inhabitant per hour of greatest water consumption (for example, for “Apartment-type residential buildings with bathtubs from 1500 to 1700 mm long” q hhr == 10 l/h), given in the mandatory Appendix 3 SNiP 2.04.01–85* “Internal water supply and sewerage of buildings”, is illegal for determining the heat consumption for the needs of hot water supply by multiplying it by the number of inhabitants and the temperature difference (enthalpies) of hot water and cold water. This conclusion is confirmed both by the given calculation example and by a direct indication of this in the educational literature. For example, in the textbook for universities “Heat supply”, ed. A.A. Ionin (M.: Stroyizdat, 1982) on page 14 we read: “...Maximum hourly water consumption G h. max cannot be mixed with the water consumption given in the standards at the hour of greatest water consumption G i.ch. The latter, as a certain limit, is used to determine the probability of operation of water-folding devices and becomes equal to G h. max only with an infinitely large number of water taps.” Calculation using the old method gives a much more accurate result, provided that daily hot water consumption rates are used at the lower limit of the ranges given in the corresponding tables of the old SNiP than the “simplified” calculation that many calculators perform using current SNiP.
The data from the table in Appendix 3SNiP 2.04.01–85* must be used specifically to calculate the probability of operation of water-folding devices, as required by the methodology outlined in Section 3 of this SNiP, and then determine bhr and calculate the heat consumption for the needs of hot water supply. In accordance with the note in paragraph 3.8 of SNiP 2.04.01–85*, for auxiliary buildings of industrial enterprises the value q hr can be determined as the sum of water costs for using a shower and household and drinking needs, taken according to the mandatory Appendix 3 according to the number of water consumers in the most numerous shift.