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Outside and inside design condition - Choice of inside design conditions

Technical information Air-Conditioning The choice of outside design conditions

Choice of external conditions on the design

There are three ways to create an outside summer design States:

(1) Use of average daily and monthly meteorological data as follows:

  • Choose the highest mean monthly maximum temperature as the design of the dry bulb value.
  • In the same month, used in (i) a couple of the average daily maximum temperature and the day of the entry relative humidity, optionally using equation (5.4)to adjust the average daily maximum coincides with the time value of humidity.
  • Temperature and humidity combined (ii) determine the appropriate moisture content of psychrometric tables or charts.
  • Use moisture (iii) with an average monthly maximum temperature install the appropriate screen or sling wet bulb temperature.
  • Select the mean monthly maximum temperature of (I) and the wet-bulb flows from (iv)as the outside summer structure of the state.

(2) Choose outside dry and wet significance in terms of their frequency of occurrence in the four summer months in the UK.

Table 5.3 and Figure. 5.4 should be referenced. In the past it has been common to select open summer design state on the basis of 1% of incidence in the four summer months, in the occupied for nine hours a day, and not 24 hours. This led to a 27C dry bulb and 19.5C W.B (screen), referred to as non-urban location, for example, in the former meteorological station in Kew. Both Table. 5.3 and Figure. 5.4 cm frequency of occurrence more than 24 hours, the Probability of warm temperatures arising at the occupied period of about eight or nine hours a day in three times the percentage cited within 24 hours. Therefore, we must seek about 0.33 percent, in the area of high-dry and wet bulb temperature, as a basis for our external structure of the state. Features: 0.31 per cent to 28C to 30C dry bulb, with 18C 20C W.B (screen), or 0.32% 26C to 28C dry bulb, with 20C to 22C W.B (screen).

The actual choice needs further consideration. Forecasts of global warming vary but something like 0.25 per decade To possible growth of external air temperature in the South of England over the next few years. Accountants withdrawn 60 years to the life of the office block, and it is very likely that the air conditioning system will be about the end of its useful life after 20-25 years, subject to normal wear and obsolescence. This suggests that half a degree, must be added to the dry bulb, perhaps, similar growth W.B, to meet in the future may increase the performance of the system over the life of the building. However, the choice remains open for further thought, choice below the outside temperature influences the cost of capital of the proposed design and this should be weighed against some dissatisfaction, which could result. Example 7.19 discusses some of the consequences of the choice of various internal and external design conditions.

The above leads to the notion that, after adjusting for the difference between the TV and throw the wet bulb, suitable outside the choice on an open area near London might be 29C dry bulb, 18.3C W.B (sling), or 27C dry bulb, 20.7C W.B (sling). These States have the appropriate moisture content 0.008616 kg kg-1 and 0.01264 kg kg"1. Additional 0.5 To should be done to cover global warming over the life of the plant, to give dry balls 29,5C and 27.5C. it is unlikely that the moisture content of the outside air to be permanent, because vegetation will dehydrate, as with the increase in air temperature. This decision a speculative but one can assume that wet bulb temperature may increase by around half of the rise in dry bulb temperatures because, as we can see on the psychrometric chart, the scale of wet-bulb temperature is about twice dry bulb thermometer temperature in the region of 30C dry bulb and 20C W.B. The proposal is that wet balls be increased by 0.3 To and suitable outside the design of the state on an open place near London can be 29.5C dry bulb 18,6C W.B (sling), or 27.5C dry bulb, with 21C W.B (sling), corresponding moisture content being 0.008 746-0,012-84 kg"1. The matter does not end here because, as we shall see, there is a building in London makes a further increase in the external design of the dry bulb temperature is necessary.

Take what is selected on the General custom, locally. For London, in the past, the value of 28C dry bulb, 19.5C W.B (sling) was widely adopted. This may not be satisfactory over the next few decades.

In the centers of mass of the building absorb radiant heat by day and emissions later. The city, therefore, are warmer areas than the surrounding countryside (often called " heat Islands'). According to Chandler (1965) influences at work: the climate of the region, local morphology, thermal properties of a collection of buildings and paved roads: apparently, every building in the city the complex has its own microclimate, which, due to local effects may be significantly different from their neighbors. In the middle of London, the average annual temperature is around 1K To 1.5 K warmer than the surrounding countryside. Dry bulb thermometer temperature after sunset in Central London can be 5 To above, than in remote country because of the heat emanating from the buildings in calm weather.

Given the above, two dry bulb thermometer temperature, suitable for an open area near London should be increased by 0.5 To 30C and 28C, to make them suitable for London itself. He argued that the above dry bulb is not likely to increase the moisture content of a much smaller amount of vegetation in the area, compared to the open countryside. Therefore, outside of the two States, which will be reviewed by 30C dry bulb, 0.008 746 kg kg"1, which is 18.7C W.B (sling) and 28C dry bulb, 0.01284 kg kg"1, 20.8C W.B (sling). It is assumed that the average arithmetic be adopted and suitable outside summer project of the decision, adopted on 29C dry bulb, 20C W.B (sling), for which the moisture content of 80 kg 0.010 kg enthalpy 56.76 kJ / kg 1 and the specific scope of 0.8705.m3 kg-1.

When determining the size of air-cooled condensers and cooling towers, we recommend that their choice based on at least 30C dry bulb and 20.5C W.B (sling), respectively. Choice outside of the winter design temperatures not made the same way, because most people at home, in bed during the period of low temperatures, approximately for an hour before sunrise. The building occupied in working time), the external design a dry ball -1C or -2C often choose and satisfactory, although it should be noted that for the purposes of determining the size of the heater battery, which handle 100% outside air temperature from -5C should be accepted due to the thermal inertia of the building plays no role in mitigating the effect on the heater battery temperature to drop below the designed value, as it does for comfort temperature inside the building. In the CIBSE Guide A2 (1999) recommends that you choose an outside design winter temperature in the conditions of the frequency of occurrence of low temperature and the thermal inertia of the building. Reference should be made to the Guidance on the selection of winter conditions on the design.

According to Chandler (1965) records Kew indicate that the earth temperature varies in London from the average monthly minimum of 0.6C in January to a maximum of 19.8C in July, with average annual values in these months 4.2C and 17.8C, 300 mm below the surface. The corresponding values at the depth of 1200 mm 5.4C, 17.0C (August) and 15.8C (August)...

 
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