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VAPOR LINE DESIGN

FOR VT-300 CO₂

VAPOR DISTRIBUTION SYSTEMS

 Vapor distribution systems are easily designed for almost any installation. Two factors must be considered. 1) The system must be designed to safely handle the 300 psi pressure of the CO₂ to be distributed. 2) There must be minimal drop in pressure due to vapor flow at the satellite to be served.

No Insulation: A CO₂ vapor distribution system does not have to be insulated because the CO₂ vapor rapidly attains ambient temperature after leaving the bulk CO₂ storage tank receiver. Uninsulated vapor distribution lines remain clear of condensation and icing whenever CO₂ vapor is being transferred to the satellite reservoir.

Line Material: Dry CO₂ vapor is inert so the choice of pipe or tubing is dependent only on pressure ratings. Copper tubing is readily available, inexpensive, light and very workable. For these reasons it is the primary choice in most VT-300 CO₂ vapor distribution systems.

Pressure Ratings: Copper tubing for refrigeration and air conditioning use has pressure ratings sufficiently high for bulk CO₂ vapor use. The table below gives the rated internal working pressure for several sizes of refrigeration tubing.

Nominal Size	Wall	1000F	2000F	3000F
3/8	(.030)	900 psi	880 psi	740 psi
1/2	(.032)	740 psi	730 psi	610 psi
5/8	(.035)	740 psi	720 psi	610 psi
3/4	(.042)	650 psi	630 psi	530 psi
7/8	(.045)	590 psi	570 psi	480 psi
1-1/8	(.050)	510 psi	490 psi	420 psi
1-3/8	(.055)	460 psi	440 psi	370 psi
1-5/8	(.060)	430 psi	410 psi	350 psi

Pressure Ratings and Burst Pressures: The pressure ratings above have been determined using an allowable stress that is a small fraction of copper's ultimate tensile strength. The actual burst pressure is many times higher than this, for example the measured burst pressure the 1/2" tube is 5900 psi.

Joining Tubing sections: The vapor system may be assembled using compression fittings, or permanently with solder-type fittings. If solder type fittings are used, use a brazing alloy or 95-5 Tin-Antimony solder do not use soft solder . A tin-lead 50-50 solder provides a joint with a pressure rating of only 200 psi.

Line Pressure Drop: Whenever CO₂ vapor flows through a pipeline a pressure drop occurs due to friction loss. The magnitude of the drop is dependent upon the velocity of the flow, and the size and physical properties of the pipe. The velocity of the CO₂ vapor is proportional to the rate of recondensing in the VT-300 units in pounds of CO₂/hour.

Allowance for Bends and Elbows and Tees: Flow disturbances that occur at tube fittings add to the pressure drop substantially. For instance, an elbow on a 1/2" line is equivalent to about one foot added to the length. For a one inch diameter, add two feet to the length. The proper factors must be considered when laying the system out.

Peak Flow Rate for Small Systems: In small systems with only a few VT-300 CO₂ Satellite reservoirs, the possibility that all satellites are recondensing at the same time is high. The peak flow rate should be used for design and calculated by adding the condensing rates of all of the satellite reservoirs. For example, a system with four VT-300-7 CO₂ Satellite reservoirs would have four times the 20 pounds/hr flow rate for a single unit or 80 pounds/hr.

Peak Flow Rate For Large Systems: In large systems the probability of all stations condensing CO₂ simultaneously is low enough to safely assume that only a fraction of the units will be recondensing at any time. If the maximum flowrate is assumed, the system will probably not need to be modified if more VT-300 CO₂ Satellite Reservoirs are added.

Acceptable Pressure Drop: A pressure drop of 10 or 15 psi is acceptable for satisfactory operation of the VT-300 satellites reservoirs and the environmental chamber solenoid valves. The pressure at any satellite is the source pressure less the pressure drops in all segments supplying it. Select segment pressure drops sufficiently low, so that when added together the sum is less than 15 psi at all satellites.

Calculating Pressure Drops: In a single piping system with satellites spaced along it, add the average recondensing rates of all satellites to determine the flowrate in the first segment of the piping. The next section of pipe will have the mass flowrate of the first section less the recovery rate of the first satellite. The last section of the piping system would only have the flow-rate of a single VT-300. The average condensing rates for VT-300 CO₂ Satellites are listed on the VT-300 price list.

Designing Trunk Lines: To determine the required pipe sizes for the system with branches start at the CO₂ receiver and select a tubing size to accommodate the total mass flow for the system. Add up the average recovery rates for all of the VT-300 Satellite reservoirs. Using the pressure drop table select a tubing size that gives a pressure drop of 5 psi or less.

Designing Branch Lines: Calculate the pressure drops of branch lines by adding the average recovery rates of all VT-300 units on the branch. Smaller diameter tubing may be used since there are fewer VT-300 CO₂ Satellite Reservoirs on a branch than a trunk. Generally two tubing sizes will satisfy the needs of an entire system. Each successive segment will have a lower pressure drop per 100 feet because the mass flow rate is reduced as each reservoir is passed. The total pressure drop at the end of any branch line should not be more than 15 psi under full flow condition.

Typical Calculation: Four VT-300-7 CO₂ Satellite reservoirs, each with an average condensing rate of 20 pounds of CO₂ per hour are fed from a line 500 feet long. The units are spaced 100 feet apart with the first VT-300-7 200 feet from the CO₂ receiver.

Diagram 1

The mass flowrate in the first segment is 80 pounds per hour because all four VT-300-7 CO₂ Satellites draw CO₂ vapor through the first line segment. Referring to the graph, it is apparent that 0.5" OD tubing with a 1.7 psi/100' pressure drop is the minimum pipe diameter. The next size smaller tubing shown (0.375") has an unacceptable pressure drop of almost 8 psi/100'. The total pressure drop in the 200' first segment is 3.4 psi.

The second segment will have a flow rate of 60 pounds per hour because the first VT-300-7 CO₂ Satellite has been passed. From the graph it is apparent that the tubing still must be 0.5" OD which has a pressure drop of about 1 psi for the 100' segment.

The third segment will have a flow rate of 40 pounds per hour with the load of the two remaining VT-300-7 CO₂ Satellites remaining. The next size smaller tubing now can be selected because 0.375" OD tubing only has a drop of 2.11 psi per hundred feet at 40 pounds per hour.

The last segment with only the flow rate of the last VT-300-7 CO₂ Satellite would have a pressure drop of less than 1 psi. for 0.375" OD tubing. The total pressure drop between the receiver and the last reservoirs would be the sum of the drops for each segment, 3.4 + 1.0 + 2.11 + 1.0 =7.51 psi. The total pressure drop of 7.51 psi is well within the 15 psi specified limit.

From a practical view point, it is likely that a single size of tubing will be used. This is a wise choice because it allows for future expansion of the vapor distribution system.

677 ANITA STREET, SUITE A, CHULA VISTA, CALIFORNIA 91911
phone: 619/423-4555 toll free 800/379-4231 fax: 619/423-4604

Copyright 1998, Va-Tran Systems Incorporated.

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