When designing heating and ventilation systems in multi-apartment and low-rise suburban buildings, such an indicator as the calculated outdoor temperature is mandatory taken into account. Provided that this parameter is correctly determined, home engineering systems subsequently work efficiently even on the most frosty or, on the contrary, hot days and, moreover, turn out to be quite economical.
Definition
Since the climate in Russia is cold, we are especially careful in our country regarding the design of heating systems. Therefore, under the concept of "calculated outdoor temperature" most often in Russia falls the average t ° of the coldest five-day days in this particular area over the 8 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 minimum t ° for the same region.
For example, for Moscow, the calculated outdoor temperature is -26 ° C. The absolute minimum winter t ° for the capital is already -41 ° C. For St. Petersburg, these indicators are -24 and -36 ° C, respectively.
Despite the fact that the calculated outdoor temperature is always above the absolute minimum, heating systems designed with it in mind are quite effective. The fact is that in most cases, building envelopes 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 a minimum of -41 ° C, will not lead to any significant cooling of the air in the premises of the houses.
The use of the design indicator of the calculated outdoor temperature, thus, can significantly save on the installation of heating systems. This is especially noticeable in mass buildings.
Interesting fact
Various kinds of standards governing the design of engineering systems were developed in our country in Soviet times. And of course, at the moment, many of these documents may even be considered obsolete. 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 people's buildings.
For example, in our time, design companies usually take the coldest five-day period for the 8 coldest winters of the last not 50, but not more than 20 years, for the calculated outdoor temperature. In addition, such firms can lay in automation projects of various kinds, switching on / off equipment as necessary. For example, ventilation in modern buildings can only function when people are in them.
Determination methods for heating systems
Thus, we found out what the calculated winter temperature of the outside air is. In most cases in Russia, when designing heating systems, it is taken into account precisely the t ° of the coldest five-day days of 8 winters. However, sometimes this indicator can be calculated differently. When designing heating networks, engineers take into account, among other things, the degree of massiveness of the building envelope.
The design temperature for building structures of very thick (for example, log walls) is exactly t ° five days for 8 years. If the building envelope is light, the design of heating systems takes into account the average temperature for the coldest days of the year (that is, for Moscow, for example, it can be -46 ° C). For houses with not too thick and not particularly thin walls, the arithmetic mean between these two indicators is calculated.
How to determine the degree of massiveness of building envelopes
This indicator for walls and floors is determined primarily by the value of their thermal inertia. The latter is calculated by the formula:
Here R1, R2, R3, Rn - thermal resistance of the layers of the fence in m 2 * h * deg / kcal. S1, S2, S3, Sn in the formula is the coefficient of heat absorption of materials during the period of temperature fluctuations over 24 hours in kcal / m 3 * h * deg.
The parameter S is calculated for each layer according to the following formula:
In this formula, λ is the coefficient of thermal conductivity of the material in kcal / m * h * deg, s is its specific heat capacity kcal / kg * deg, y is the bulk weight in kg / m 3 .
The calculated outdoor temperature for heating, depending on the massiveness of the walls (indicator D), is determined by the table.
Residential Temperature Standards
The design of the heating system, including taking into account the outside temperature, should be such that in the premises for various purposes in the house or apartment, a microclimate is subsequently created in accordance with the current legislation. For residential buildings in our country in this regard, the following standards are provided:
Moreover, if the estimated outdoor temperature in this particular area is below -31 ° C, for residential ordinary and corner rooms, the indicators increase to +20 and +22 ° C, respectively.
Standards for public buildings
Of course, air temperature standards are also provided for various public premises.
According to the regulations, in those buildings in which 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. Moreover, the minimum t ° in winter in such buildings is considered to be -13 ° C.
Design temperature for ventilation
Such engineering systems in apartment buildings are mounted without fail. Provide ventilation networks in many suburban residential buildings. In the design of such systems also, as already mentioned, the calculated winter outdoor temperature is taken into account. Using this indicator, the optimum 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 walls of low massiveness - the average outside temperature of the coldest days;
for medium-sized fences - the arithmetic mean between the readings t ° of the coldest day and the coldest five-day day;
for massive walls - the temperature of five days.
Estimated air temperature and air exchange
In terms of power, ventilation equipment for residential and all kinds of public buildings is selected in such a way as to ensure the greatest comfort for the people 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 outside.
For rooms for various purposes, an unequal air exchange rate is also provided.
Design temperature for refrigeration units
When designing such equipment, engineers take into account primarily such indicators as processing time and safety factor. Also, when calculating refrigeration units, as well as heating systems, the calculated outdoor temperature is taken into account. However, in this case, of course, not the lowest t ° in the year, but the highest are taken into account.
Unfortunately, in our country at the moment there is practically no reference literature that would indicate the indicators of the calculated external temperatures for refrigeration units in a particular area. However, programs with bases of the highest t ° 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 in such lists.
Formulas for Refrigeration Units
Finding one hundred percent reliable tables and lists of cities with an indication of the calculated outside temperatures for such equipment today is thus quite difficult. But in Russia, fortunately, from Soviet times, documents with formulas for calculating such indicators have been preserved. 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:
Here tCP MES is the average temperature of the hottest month in the region, tAM is the temperature of the absolute maximum.
Also, the selection of the estimated outdoor temperature during the design of refrigeration units can be carried out, for example, using the following formula:
Here Ts.m is the average temperature at 13 hours of the hottest month, Ta.m is the maximum according to climatic data.
Among other things, according to the standards, the heat transfer coefficient of the fences of the refrigeration unit is determined taking into account the climatic zone, of which there are three in Russia:
southern - with an average annual temperature of 9 ° C;
medium - with a temperature of 1-8 ° C;
northern - with an outside temperature of 0 ° C or lower.
Sometimes in the winter season, refrigerator chambers have to be heated using special devices. Calculation of the power of the latter, of course, is also made taking into account the outside temperature. But in this case, winter rates are taken into account.
Selection of normalized outdoor parameters
Designing heating, ventilation and air conditioning systems in buildings for various purposes, therefore, should take into account changes in weather on the street. In our country, it regulates the numerical values of the external air parameters SNiP 23-01-99.
According to this document, when designing engineering systems responsible for the indoor climate, the following can be taken into account:
In this case, the parameters for the transition periods (spring / autumn) are taken as follows:
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 in summer.
When performing thermal engineering calculations, determining the humidity regime and air permeability of building envelopes, the following basic design parameters of outdoor air can be taken into account, among other things:
for objects of mass construction - the average t ° of the coldest five-day security of 0.92 for concrete structures and 0.98 for steel;
average outside temperature of the coldest month;
average temperature and duration of the heating period with a stable average daily outdoor temperature t ° of 8 and 10 ° C (necessary to determine the operating conditions of walls and ceilings of buildings);
Estimated wind speed (equal to the maximum of the average for January in the rhombuses with a repeatability of 16% at a height of 10 m from the surface of the earth).
Also, when designing building envelopes of buildings and structures, of course, the humidity zone of the design area is also taken into account.
What is security?
When designing engineering systems, the estimated temperature in our time, companies often choose based on security. In this case, this term is understood to mean the probability that the temperature will not subsequently exceed a predetermined value. For example, a security of 0.92 means that the outdoor t ° will not go beyond certain limits within 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 of concrete buildings according to the standards in our country, the calculated outdoor temperature with a security of 0.92 is taken into account. However, at the same time, 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 services, large indicators of this parameter can be used in the design of communications. This approach guarantees the creation of the most comfortable living or working conditions for people in the building.