【 Weak current College 】 chilled water system pressure control valve selection calculation

Preface

For chilled water system of the chiller flow Basic constant; frozen water pump operation stable, generally used by load side designed for variable flow, control the water flow at the end of the device, which uses electric 2-way valve as terminal equipment of the adjusting device to control the flow of frozen water of terminal equipment. In the cold source side of the set pressure differential bypass control device to guarantee cold source part frozen water flow remains constant, however in a real project, since designers tend to ignore the importance of regulating valve selection calculation, in the design process, generally only is simple in cold water unit and a user-side set the by-pass pipe, the by-pass pipe diameter of and bypass valve selection without any detailed calculations done in actual operation of chiller flow stability often and design a large gap, by-pass generally cannot achieve the desired effect for future run management creates unnecessary trouble, this article on differential pressure regulating valve selection calculation methods and practical engineering conduct a brief analysis.

1, pressure regulating device works

Pressure adjustment by pressure controller, electric actuators, control valves, pressure pipe and bypass pipeline components, its working principle is pressure controller through the piezometric on air conditioning systems for the return pipe for detection of differential pressure, according to its results and comparison of set pressure value, output control signal from electric actuator by controlling the valve stem of the itinerary or corner to change the control valve to open degree, thereby controlling the return pipe for water supply pipe and conduit between by-pass, final freezing water flow guarantees the system pressure is constant at set pressure value. When the system pressure is higher than the set pressure, differential pressure controller output signal, so that the electric control valve to open or to open degree, bypass pipeline volume increase, so that the system pressure tends to set value; when the system pressure is lower than the set pressure, electric control valve to open degree decrease, bypass flow decreases so that the system pressure is maintained at the set value.

2. Select control valve should be taken into account

Control valve diameter is selected when one of the most important factors, control valve selection if it is too small, the maximum load time may not provide sufficient flow, if too big and can often be a small opening status, control valve opened through small causes the plug of frequent oscillation and transition wear and system instability and increased the cost of project.

Computed control valve should be 10%-90% of opening interval adjustment, and also avoid using less than 10 per cent.

In addition, the installation should be considered when regulating their valve capacity of PV (i.e. control valve fully open when the differential pressure of the valve covers pipe segment, the proportion of the total pressure difference), from regulating valve pressure drop situation analysis, select control valve control valve must be combined with tubes before and after, when the PV value is less than 0.3, linear flow rate characteristic of valve's flow characteristic curve is serious deviations from the ideal flow characteristics, approximate quick closure properties, not suitable for valve adjustment.

Third, the control valve selection calculation

Control valve size from its distribution capacity, flow capacity refers to when a control valve, the valve fully open at both ends of the pressure drop for 105Pa, fluid density is 1g/cm3, hourly flow through valves fluid cubic meters. Import control valve flow capacity of representation usually have two cv and kv, kv = c, and cv is a fully open when the control valve, the flow of water, valve 60oF at both ends of the pressure drop to 1B/in2, flow through valves per minute flow, cv = 1.167kv.

Pressure differential bypass adjusting device map (brief)

(1) determine the control valve pressure difference (¨s P)

As shown, role in regulating the pressure difference is between E and F, because the pressure value C-D bypass pipeline and after the end of the user's line of resistance equivalent D-U-C, so the pressure difference between E-F should equal D-U-C pipeline pressure differential (means end users the most negative loop pressure difference) minus C-E segment and F-D pipe pressure value.

(2) calculation control valve needs by-pass of maximum and minimum flow

For stand-alone unit air conditioner system, according to the end user's actual use of the minimum load can determine the minimum load of traffic, you need to determine the maximum bypass flow, the formula is:

G=(Q-Qmin)*3.6/CP*⊿T(1)

Formula, G-flow units (m3/h), Q-chiller cooling capacity (KW), the smallest Qmin air-conditioning system load (KW), specific heat of water CP, CP = 4.187kJ/kg.oC, ¨s t is the temperature of the chilled water, backwater generally 5oC

According to the actual adjustable ratio RS = 10 (PV) 1/2 (2)

You can calculate the control valve bypass minimum flow

(3) calculation of the pressure regulating valve of the desired flow capacity C

C=316G*(⊿P/ρ)-1/2(3)

The formula for the density ρ,, units (g/cm3), G-flow, (m3/h), P-valve ends ¨s differential pressure units (Pa). According to the calculated value for the select control valve C makes it flow capacity greater than and closest to the computed value.

(4) control valve to open degree, as well as adjustable ratio calculation. According to the selected control valve control valve C value is calculated when the minimum opening and maximum aperture, its adjustable ratio meets the requirements, according to the calculated adjustable ratio for maximum flow and minimum flow and control valve to open degree at minimum and maximum aperture of flow, repeatedly checking until qualified.

Chilled water system pressure control valve selection calculation

IV. control valve selection example

A total of 12 storey office buildings, a building area of about 11000 square meters, floor 3.6 meters high,Adopt a York screw chillers, cooling capacity as 1122KW.

(1) determination of the differential pressure

The hydraulic calculation, the system minimum load (bypass tube at maximum load), the total resistance losses H about 235KPa in system chilled water for backwater trunk service set pressure differential bypass control device, bypass the service cold source side water pipeline pressure loss is the most disadvantageous end 80KPa, loop resistance loss to 155KPa.

(2) valve calculation:

After calculation, the air conditioning system in its minimum branch loop, its load to a minimum load of about 35 per cent of the total load, use the formula (1) G = (Q-Qmin) * 3.6/CP * ¨s T, considered the by-pass the largest flow 125.4m3/h, then the most negative loop 155KPa pressure difference.

(3) flow capacity calculation

According to the formula (2) C = 316G * (¨s P/ρ)-1/2 considered C = 100.6

(4) control valve selection

(5) the control valve to open degree and adjustable ratio check

Bypass pipe length, for enquiries on table 6m when C = 110, by the formula (4) ¨s P = ρ (316G/C) 2 get ¨s P = 129.8KPa, when adopting the by-pass pipe and valve of the same diameter, when the by-pass pipe maximum 125.4m3/h of water, through hydraulic calculation, the total loss to 42.8KPa along, Department of General Administration, control valve for 23KPa loss at both ends of the pressure differential 129.8-42.8-23 = 64KPa < 129.8KPa, valve capacity of PV = 64/129.8 = 0.49, then control valve flow characteristic curve for equal percentage characteristic, this time dealing with the actual maximum by-pass 88.1m3/h water, < 125.4m3/h, and its flow only system requirements of maximum 70% of the by-pass flow, by the formula (2) you can find the actual adjustable ratio Rs = 7, namely, the actual minimum flow 88.1/7 = 12.6m3/h, maximum flow and minimum flow apparently are not able to meet the actual requirements, bypass pipe diameter selection DN80 inappropriate.

In accordance with the above calculation method, to continue the trial balance, elected by the by-pass pipe DN125, calculated control valve pressure to both ends, PV = 0.95 123.2KPa, treatment of water for most bypass 122.1m3/h, the relative opening is 90%, relative flow 97.3%, by the formula (2) you can find the actual adjustable ratio Rs = 9.7, i.e. minimum bypass water 122.1/9.7 = 12.6m3/h and regulating work in 10 per cent of the opening of traffic compared to 12.21m3/h have been very close. At this point the valve's flow characteristics is close to ideal flow characteristic curve, has been able to satisfy the system requirements.

5. conclusions

From the above analysis, it is possible to draw the following conclusions:

(1) control valve flow capacity c determine selection control valve crucial step, only the circulation capacity C calculation is correct, the control valve would make it possible to meet the technological requirements.

(2) valve valve capacity of PV is select one of the important indicators of control valve, in principle possible select large Pv value.

(3) exceeds the actual adjustable ratio Rs is regulating valve can meet the technological requirements of one of the arguments. Actual adjustable ratio often far less than ideal adjustable ratio, but when the select control valve should, as far as possible, the actual adjustable ratio close to the maximum.

(4) control valve can flow through the maximum and minimum flow is the key to select the calculation, the two value should be the actual adjustable ratio and process requirements.

(5) through the project can beseen the by-pass pipe diameter calculation is also very important, if not calculated to select and control the same caliber is unable to meet the technological requirements.

Through the above 5 points you can see the pressure regulating valve in the air-chilled water systems regulation control occupies a relatively important position, and only after careful calculations to make the choice of pressure regulating valves meet the technological requirements.