What is "calculated outdoor temperature": definition, calculation features and interesting facts

2024 Author: Henry Conors | [email protected]. Last modified: 2024-02-12 02:41

When designing heating and ventilation systems in multi-apartment and low-rise suburban buildings, such an indicator as the design outdoor temperature is necessarily taken into account. Provided that this parameter is correctly determined, home engineering systems subsequently work efficiently even on the coldest or, conversely, hot days and, moreover, turn out to be quite economical.

Definition

Since the climate in Russia is cold, in our country we are especially attentive to the design of heating systems. Therefore, the concept of "estimated outdoor temperature" most often in the Russian Federation falls on the average t ° of the coldest five-day days in this particular area for 8 of the most frosty winters of the last 50 years. It is this parameter that is taken into account when designing heating and ventilation systems. It can be considered interesting that this indicator is always much higher than the absolute minimumt° for the same area.

For example, for Moscow, the calculated outdoor temperature is -26 °С. The absolute minimum winter t° for the capital is already -41°С. For St. Petersburg, these figures are -24 and -36 °С, respectively.

Despite the fact that the calculated outdoor temperature is always higher than the absolute minimum, heating systems designed with this in mind are quite efficient. The fact is that in most cases the building enclosing structures of various kinds of buildings are distinguished by a sufficiently large thermal inertia. As a result, a short-term decrease in the outside temperature, for example, in the capital, even to the minimum of -41 ° C, will not lead to any significant cooling of the air in the premises of houses.

The use of the indicator of the calculated outdoor air temperature in the design, thus, allows significant savings on the installation of heating systems. This is especially noticeable during mass developments.

Interesting fact

Different kinds of standards governing the design of engineering systems were developed in our country back in Soviet times. And of course, at the moment, many of these documents can even be considered outdated. Today, construction companies, developed in Soviet times, often do not comply with the rules on some points. However, in most cases, this does not affect the comfort of living and working in buildings for people.

For example, in our time for the settlementoutside temperature, design companies usually take the coldest five-day period for the 8 most frosty winters of the last not 50, but not more than 20 years. Additionally, such firms can include various kinds of automation in communication projects, turning equipment on / off as needed. For example, ventilation in modern buildings can only function when there are people in them.

Determination methods for heating systems

Thus, we found out what the estimated winter outdoor temperature is. In most cases, in Russia, when designing heating systems, it is precisely the temperature of the coldest five-day periods of 8 winters that is taken into account. However, sometimes this indicator can be calculated in a different way. When designing heating networks, engineers take into account, among other things, the degree of massiveness of building envelopes.

The design temperature for very thick building structures (for example, log walls) is exactly the t° of a five-day period for 8 years. If the building envelope is light, when designing heating systems, the average temperature for the coldest day of the year is taken into account (that is, for Moscow, for example, it can be -46 ° C). For houses with not too thick and not especially thin walls, the arithmetic average between these two indicators is calculated.

How to determine the degree of massiveness of enclosing structures

This indicator for walls and ceilings is determined primarily by the value of their thermalinertia. The latter is calculated by the formula:

D=R1S1 + R2S2 + R3S3 + … + RnSn.

Here R1, R2, R3, Rn - thermal resistance of the fence layers in m²hdeg/kcal. S1, S2, S3, Sn in the formula - the coefficient of heat absorption of materials during the period of temperature fluctuations for 24 hours in kcal/m³hdeg.

The S parameter is calculated for each layer using the following formula:

S=√0.51λcy.

In this formula, λ is the thermal conductivity of the material in kcal/mhdeg, c is its specific heat capacity kcal/kgdeg, y is the bulk density in kg/m^3.

The calculated outdoor temperature for heating, depending on the massiveness of the walls (indicator D), is determined from the table.

The massiveness of the enclosing structures

Temperature standards for residential premises

The heating system should be designed, including taking into account the outside temperature, in such a way that in the premises for various purposes in the house or apartment, a microclimate is subsequently created that complies with current legislation. For residential buildings in our country, this plan provides for the following standards:

living rooms - the temperature in winter is not lower than +18 °С;
corner living rooms - +20 °С;
bathrooms - +25 °С.

At the same time, if the calculated outdoor air temperature in this particular area is below -31 ° С, for residential ordinary and corner premises, the indicatorsincrease to +20 and +22 °С respectively.

Regulations for public buildings

Of course, air temperature standards are also provided for various types of public premises.

According to the regulations, in buildings where people are always in street clothes, the air temperature should not fall below 14 °C. The permissible maximum temperature in administrative and office premises in the summer is 30 °C. At the same time, the minimum t ° in winter in such buildings is -13 ° С.

Design temperature for ventilation

Such engineering systems in apartment buildings are installed without fail. Ventilation networks are also provided in many suburban residential buildings. When designing such systems, as already mentioned, the design winter outdoor temperature is also taken into account. Using this indicator, the optimal power of ventilation equipment is subsequently calculated.

In this case, the design is carried out using approximately the same rules as for heating systems. That is, most often:

for light fences, the minimum temperature of the coldest month is taken into account;
for low-mass walls - the average outside temperature of the coldest days;
for fences of medium massiveness - the arithmetic mean between the t ° readings of the coldest day and the mostfrosty five-day period;
for massive walls - five-day temperature.

Calculated air temperature and air exchange

In terms of power, ventilation equipment for residential and various public premises is selected in such a way as to ensure the greatest comfort for the people living in them. That is, such systems should be designed in such a way as to provide the desired air exchange rate. This indicator, in turn, depends on the calculated outdoor temperature on the street.

For premises for different purposes, unequal air exchange rates are also provided.

Design temperature for refrigeration units

When designing such equipment, engineers primarily take into account such indicators as processing time and safety factor. Also, when calculating refrigeration units, as well as heating systems, the estimated outdoor temperature is taken into account. However, in this case, of course, the indicators are taken into account not the lowest t ° in the year, but the highest.

Unfortunately, in our country at the moment there is practically no reference literature that would indicate the calculated outdoor temperatures for refrigeration units in a particular area. However, programs with the highest t° bases for different cities can be found on the Internet. The only thing is that an engineer who needs such information is unlikely to be able to verify the primary sources and the reliability of the information insuch lists.

Formulas for refrigeration units

Finding 100% reliable tables and lists of cities indicating the estimated outdoor temperatures for such equipment today is therefore quite difficult. But in Russia, fortunately, documents with formulas for calculating such indicators have been preserved since Soviet times. If it is not possible to find the necessary information about the outside temperature in this particular area, the calculation can be made, for example, using the following formula:

tH=tCP MON + 0.25 × tAM (4.3).

Here tCP MON is the average temperature of the hottest month in the region, tAM is the temperature of the absolute maximum.

Also, the selection of the calculated outdoor air temperature when designing refrigeration units can be carried out, for example, using the following formula:

Тnar=0.4Ta.m.+ 0.6Ts.m.

Here, Ts.m is the average temperature at 13:00 of the hottest month, T.m is the maximum according to climate data.

Among other things, according to the regulations, the heat transfer coefficient of the refrigeration plant enclosures is determined taking into account the climatic zone, of which there are three in Russia:

southern - with an average annual temperature of 9 °С;
medium - with a temperature of 1-8 °C;
northern - with outside temperature from 0 ^oC and below.

Sometimes in the winter season refrigerators have to be heated using special devices. Power calculationthe latter, of course, is also produced taking into account the outside temperature. But in this case, winter indicators are taken into account.

Selection of normalized outdoor air parameters

Heating, ventilation and air conditioning systems in buildings for various purposes should be designed, therefore, taking into account changes in the weather outside. Regulates in our country the numerical values of outdoor air parameters SNiP 23-01-99.

According to this document, when designing engineering systems responsible for indoor microclimate, the following can be taken into account:

parameters of group A - for the warm season;
group B - for cold.

In this case, the parameters for transition periods (spring/autumn) are as follows:

for temperature - 10 °С;
for enthalpy - 26.5 kJ/kg.

According to SNiP 23-01-99, while designing structures and buildings, it is allowed to take into account lower outdoor air parameters in winter and higher ones in summer.

When performing heat engineering calculations, determining the humidity regime and the air permeability resistance of enclosing structures, among other things, the following main design parameters of outdoor air can be taken into account:

for mass construction facilities - the average t° of the coldest five-day period with a security of 0.92 for concrete structures and 0.98 for steel;
average outdoor temperatureair of the coldest month;
average temperature and duration of the heating period with a stable average daily t° of outdoor air 8 and 10 °C (necessary to determine the operating conditions of walls and ceilings of buildings);
calculated wind speed (equal to the maximum of the average for January in points with a frequency of 16% at a height of 10 m from the earth's surface).

Also, when designing the enclosing structures of buildings and structures, of course, the humidity zone of the design area is also taken into account.

What is endowment

When designing engineering systems, the design temperature in our time is often chosen by companies taking into account availability. This term is understood here as the probability that the temperature will not exceed a given value in the future. For example, a security of 0.92 means that the outdoor t° will not go beyond certain limits for 92% of the time. In the remaining period, the engineering system can perform its functions, unfortunately, not efficiently enough.

As already mentioned, when designing communications for concrete buildings according to the standards in our country, the estimated outside temperature is taken into account with a security of 0.92. However, in practice, companies install engineering systems in buildings for various purposes in most cases with a security of 0.95. Sometimes, by agreement with customers, when increasing the payment for a service, when designing communications, large indicators of this parameter can also be used. This approach guarantees the creation of the most comfortable living or working conditions for people in the premises of the building.