Design principles and calculations of air-conditioning water and air systems

The general comfort air-conditioning cold water supply/return temperature is 7℃/12℃. Hot water supply / return water temperature of 60 ℃ / 50 ℃. Storage of cold large temperature difference between low-temperature water supply cold water temperature is generally 1 to 5 ℃. The temperature of supply/return water in cold storage is 2℃/13℃. The temperature of supply/return water for district chilled water supply/return water is 5℃/13℃.

1. The forms of commonly used air-conditioning water systems and their design principles:

Open and closed systems.

Co-travelling and asynchronous systems.

The air-conditioned water system can be divided into zones and heights.

Two control and four control systems.

Constant-flow and variable-flow systems

Primary and secondary pumping systems

2. Hydraulic calculations for air-conditioned water systems   

3. Air-conditioning air system and its hydraulic calculation

4. Design calculation of duct system   

In the duct system design calculations, must first determine the location of the air supply (back) and its air volume, piping system, the arrangement of related equipment, duct materials. The purpose of the design calculation is: to determine the pipe diameter (or section size) and pressure loss of each pipe section, to ensure that the system to achieve the required distribution of air volume, and for the fan selection and drawing of construction drawings to provide a basis.

1) Method of hydraulic calculation of duct system

Assumed flow rate method, pressure loss averaging method, equivalent pressure loss method, static pressure recovery method. In general duct design calculations, the more common method is the assumed flow rate method and pressure loss averaging method.

2) Basic Design Calculation Procedure

System Pipe Section Number

Generally from the furthest away from the fan section, from far to near the order of the number; usually to the air volume and wind speed of the duct for the same section; local pipe fittings (such as elbows, tees, air supply, return air, etc.) contained in the pipe section.

Selection of a reasonable air flow rate

Calculation of pipeline pressure loss

Pressure loss calculations should be made from the most unfavourable loop (furthest point from the fan).

Piping pressure loss balance calculation. A typical air-conditioning and ventilation system requires that the difference in pressure loss between the two branches does not exceed 15 per cent.

When the difference in pressure loss of the parallel branch pipe exceeds the above, it may be passed:

① Adjust the branch pipe diameter;

② Increase the flow rate of the section of the branch pipe with small pressure loss;

③Adjust the opening of the valve and increase the pressure loss in the section of the branch pipe where the pressure loss is small, etc. to balance the pressure.

Fan Selection

To choose low-noise fan, consider fan muffling at the same time, not only to meet the indoor noise standards, but also outdoor air inlet and exhaust at the noise to meet the requirements of environmental protection; selection of fans, air volume, air pressure affluence should not be too large; according to the analysis of the operating conditions, to determine the number of economically reasonable units; conditions can be used in frequency conversion fan, in order to reduce the operating costs. The air volume of the fan additional;

In addition to meeting the calculated air volume, the air volume of the fan should be increased by a certain amount of duct leakage, with an additional leakage rate of less than 10 per cent. In the calculation of the pipe network, the air leakage of the duct is not considered;

Pressure addition to the fan. The full pressure of the fan is the total pressure loss of the system network, and usually the total pressure loss of the network of the air-conditioning and ventilation system is considered to be an additional value of about l0 per cent.

Design selection of major equipment

1. Calculation of fan coil unit sizing

2. Selection and calculation of fresh air conditioning box

3. Selection and calculation of combined air-handling units

Cooling and insulation of air-conditioning equipment and piping, sound deadening and vibration isolation

1. Cooling and insulation of air-conditioning equipment and pipelines

2. Muffling and vibration isolation of air-conditioning equipment and piping

By Sammi