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The Analyze of Problems in Civil Construction HVAC Design

  • source:metmac;
  • Time:03/23/2021

Abstract: For input data selection in the cooling and heating load calculation, output form of calculation collusion, the limit for of pump, ventilation in highland and ventilation of emergency generator in the civil air defense works, this paper describes the analyses for these problem's reason and risk, and proposes the solution, such as calculation, the method of seeking table and modification the design and so on.

 

HVAC DUCT MANUFACTURE AUTO LINE MACHINE

Civil Construction HVAC Design

At present, the civil construction market is large in scale and rapidly developing. There are many novice designers, short design cycles, lack of responsibilities for professional leaders and proofreaders, and the design quality is generally unsatisfactory. In view of the current situation, this article analyzes and summarizes a number of frequently-occurring problems, hoping to attract the attention of young designers and avoid similar problems.

1 Cooling and heating load calculation book

1.1 Raising the question

According to the design specification, the design of construction drawings should perform heat load calculation and time-by-hour cooling load calculation for each room. But in actual work, there are the following problems:

①The thermal parameters of the building envelope entered in the calculation of the cooling and heating load are inconsistent with the data adopted by the building major after the energy-saving calculation;

②Increase the value of lighting power density without following the requirements of the specification;

③The calculation results are not output by the system, and it is difficult to check the air-conditioning pipe diameters of each system and the corresponding air-conditioning terminal equipment, which increases the workload of proofreading and auditing.

1.2 Analysis and measures

Before the final result of the cooling and heating load calculation book is determined, it should be checked with the architectural profession whether it is consistent with the thermal parameters of the building envelope; the lighting power density of the room should be consistent with the electrical professional, if there is no definite value, please refer to the "Architectural Lighting Design Standard "GB50034 selection.

The cooling load calculation results can be output in different ways according to different air-conditioning systems.

 

SPIRAL TUBE FORMER 1602

 

Civil Construction HVAC Design2

1) Central air-conditioning cold and hot water system

①Output the hourly and itemized cooling load of a typical room for analysis purposes; ②Output the calculation results of the cooling and heating load of each room, thereby selecting the terminal equipment;

③According to the water supply and return system set up by the manifold, output the corresponding cooling and heating loads of the system to determine the pipe diameter of each system;

④ Output the fresh air load according to the fresh air system, as the basis for the selection of fresh air treatment equipment;

⑤Total cold and heat load, select cold and heat source equipment based on this value. 2) Multi-line air conditioning system

① Output the hourly and itemized cooling load of a typical room for analysis purposes;

②Output the calculation results of the cooling and heating load of each room, which is used to select the terminal equipment;

③According to each outdoor unit system, output the corresponding cooling and heating load of the system to determine the capacity of each outdoor unit;

④ Output the fresh air load according to the fresh air system, as the basis for the selection of fresh air treatment equipment.

2 The minimum flow and maximum flow of the pump in continuous operation

2.1 Raising the question

When choosing cold and hot water pumps, the minimum and maximum continuous flow rates of the pump were not considered. The rotation speed of the variable frequency water pump is too low or too high, which exceeds the limit working range of the water pump, causing the water pump to produce vibration, noise, temperature rise, cavitation and overload, so that it cannot operate normally.

2.2 Analysis and measures

1) Determine the limit working range of the pump according to the performance curve

The limit working range of the centrifugal pump refers to the area EFGH surrounded by curves 1, 2, 3, and 6 in the diagram. Curve 1 represents the H-Q curve under the maximum impeller diameter D; curve 2 represents the H-Q curve under the minimum impeller diameter Dmin; curve 3 represents the similar parabola of the minimum continuous flow Q; curve 6 represents the similar parabola determined by the maximum continuous flow Qm. The pump can run continuously within the limit working range. And ABCD is the best working range of the pump.

Civil Construction HVAC Designmin_2

2) Minimum continuous flow Qmn

The minimum continuous flow rate refers to the minimum flow rate that the pump can work normally without exceeding the noise, vibration, and temperature rise specified in the standard. It is generally provided by the pump factory and can also be estimated by the following methods:

① When the shaft power is less than 100kW, Qm takes 25%~35% of the flow at the best efficiency point of the pump;

②When the shaft power is "100kW, it shall be determined according to Figure 2.

The specific speed n of the pump in Figure 2 is calculated according to formula (1).

n,=3.65n O;(1) H

Where: n is the number of revolutions of the pump in the design working condition, r/min; Q is the flow in the design working condition of the pump, m? /s; H is the design head of the pump, m.

3) Maximum limit flow Qmx

As the flow rate of the pump rises, NPSHr also rises, and the possibility of cavitation may increase at any time. At the same time, the shaft power rises quickly, and the pump may be overloaded. Therefore, the pump is generally not allowed to run at 125%~135% of the flow rate Q at the best efficiency point.

The maximum limit flow rate of the pump is: Qm "(125%~135%) Qv

3 Selection of ventilation equipment in plateau areas

3.1 Raising the question

In the design of the building ventilation system in plateau areas, the shaft power of the ventilation equipment motor was not checked, and the exhaust (supply) air volume during operation did not meet the design requirements.

3.2 Analysis and measures

The working parameters of the ventilator are designed according to the standard working conditions, that is, running under the working condition of atmospheric pressure of 101325Pa and temperature of 209C. Since the atmospheric pressure in the plateau area is less than the standard atmospheric pressure, the influence of atmospheric pressure changes on the operating parameters of the ventilator should be considered in the ventilation design, and the shaft power of the ventilation equipment should be checked and calculated.

The shaft power N (kW) required by the fan is calculated according to formula (2):

Q.P Nz = 3600.1000.7.7m (2)

Where: Q is the air volume delivered by the fan, m3/h; P is the air pressure generated by the fan, Pa; n is the efficiency of the fan; n is the transmission efficiency of the fan, see Table 1 [2].

The motor power N is determined by formula (3): N=K.Nz (3)

In the formula: K is the safety factor of motor capacity, see Table 2 [2.

When the atmospheric pressure P and its temperature t change, the conversion formula of fan wind pressure and power:

Q=Qo (4)

P=Po P272+20 (5)

Pbo273+t

Pb273+20

N=No Pbo273+t (6)

n=no (7)

Where: Qo, P, No, 0, Po are the air volume, wind pressure, power, efficiency and atmospheric pressure in the standard state or performance table respectively; Q, P, N, n, P are the air volume under actual working conditions, respectively , Wind pressure, power, efficiency and atmospheric pressure and temperature.

Take a place in the northwest as an example. The altitude of this area is 2295.2m, and the outdoor atmospheric pressure is 77290Pa. The working state parameters and converted standard state parameters of the ventilation system of a certain project are shown in Table 3.

 

DUCT PLASMA CUTTING MACHINE

--4-min_2

--5-min_1

According to the calculation results in Table 3, the parameters for selecting the fan according to the standard state should be: air volume L=25000m3/h, total pressure P=787Pa, motor power N=11kW.

 

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