Basic design steps for air-conditioning system design and its main design procedures-part 2

Step 11: Design of air-conditioning equipment and its piping for cold-keeping and heat preservation, muffling and vibration isolation

Step 12: Engineering drawings, collation of design and calculation instructions Air conditioning heat and humidity load calculation

Air-conditioning load can be divided into air-conditioning room or area load and system load two kinds: air-conditioning room or area load that is directly occurring in the air-conditioning room or area load; there are also a number of loads occurring in the air-conditioning room or area outside the load, such as the fresh air load (new air state and indoor air state of the load generated by different), the pipeline temperature rise (drop) load (ducts or water pipes caused by the heat transfer of loads), fan temperature load (air through the ventilator temperature rise), pump temperature load (liquid through the pump temperature rise) and so on, these loads do not act directly on the room, but ultimately also by the air-conditioning load. (load caused by heat transfer from air ducts or water pipes), fan temperature rise load (temperature rise after air passes through the ventilator), pump temperature rise load (temperature rise after liquid passes through the pump), etc. These loads do not act directly on the room, but are ultimately borne by the air-conditioning system. The above loads that occur directly in the air-conditioned room or area and the additional loads that do not act directly in the air-conditioned room or area are called system loads.

Usually, according to the heat and humidity load of the air-conditioning room or area to determine the air supply or air supply parameters of the air-conditioning system; according to the load of the system to select the fan coiler, fresh air unit, air processor and other air-handling equipment and chillers, boilers and other cold and heat source equipment. Therefore, the first step in designing an air-conditioning system is to calculate the heat and humidity load of the air-conditioned room or area.

Calculation of additional loads inside and outside an air-conditioned room or zone:

1) Fan temperature rise load: When the motor is installed in the snail casing of the ventilator, after the air passes through the fan, due to the heat generated by the mechanical friction of the motor, it will result in the temperature of the air rising after it passes through the ventilator, which will lead to an increase in the cold load.

2) Temperature rise load of water pump: the temperature of air-conditioning chilled water rises after passing through the water pump, causing the cold load to increase.

3) Air duct temperature load: air through the supply and return ducts, due to the supply and return ducts by the duct insulation, internal and external temperature difference, air flow rate, duct area and other factors, will be through the duct wall heat or cold loss, resulting in air temperature through the duct temperature drop (or temperature rise). Insulated cold water (or hot water) ducts will also cause a temperature rise (or temperature drop) in the liquid through the ducts due to heat transfer through the duct walls, causing an increase in the cold (or heat) load.

4) Fresh air load: In order to ensure that the air-conditioned rooms or areas of health conditions, the need for outdoor fresh air into the room, due to the impact of the temperature difference between indoor and outdoor, this part of the new air to cause the cold (or heat) load increase.

System cooling load:

The summer system cooling load for an air-conditioned zone shall be determined by the combined maximum of the hourly cooling loads for each air-conditioned zone or the cumulative summer cooling loads for each air-conditioned zone based on the simultaneous use of the air-conditioned zones served, the type of air-conditioning system and the mode of regulation, taking into account the additional loads involved.

The combined maximum value of the hourly cooling load of each air-conditioning zone is the maximum value of the series obtained by adding the hourly loads of the air-conditioning zones in use at the same time.

The so-called cumulative value of summer cooling loads in air-conditioned zones is a direct addition of the maximum value of the hour-by-hour cooling loads in each air-conditioned zone, irrespective of whether they are used at the same time or not.

Obviously, the results calculated by the method of "cumulative value of summer cold load in the air-conditioning area" are greater than the results calculated by the method of "integrated maximum value of hour-by-hour cold load in each air-conditioning area". Usually, when the use of variable air volume centralised air-conditioning system, because the system itself has the ability to adapt to changes in the air-conditioning zone cold load adjustment, you can use the former method of calculation; when the use of fixed-air volume centralised air-conditioning system or end equipment room temperature control device fan coil system, because the system itself can not adapt to changes in the air-conditioning zone cold load, you can use the latter method of calculation.

By Sammi