Refrigeration Pressure Vessel Basics

Transcription

1 Refrigeration Pressure Vessel Basics Presented by John Murdaugh Apex Refrigeration & Boiler Co. Phoenix Arizona

2 Pressure Vessel: A container designed to hold liquid and or vapor at a pressure greater than the ambient pressure.

3 Refrigeration Pressure Vessels Purpose: 1. Storage of liquid refrigerant. 2. Separation of liquid and vapor. 3. Separation of oil and refrigerant (vapor or liquid). Page 3

4 Types of Refrigeration Pressure Vessels: 1. High Pressure Receivers including Thermosyphon Receivers, Pilot Receivers, and Controlled Pressure Receivers. 2. Suction Accumulators including simple vessels for gas separation, low pressure receivers, accumulator/subcoolers, intercoolers, flash tanks, surge drums and liquid recirculation units. 3. Heat Exchangers (which also separate liquid and vapor). 4. Oil separation such as oil pots and oil separators on compressors. Page 4

5 Design Standards: IMC-2006 with local edits ANSI/ASHRAE ASME Boiler and Pressure Vessel Code, Section VIII, Division 1 ANSI/IIAR 2008 Page 5

6 Materials: Courtesy of Frick Page 6

7 Manufacturing: 1. Plate steel for the shell is cut to the circumference length. 2. Plasma arc cutters remove all nozzle diameters in a computer controlled CNC station. 3. Plate is rolled into a circulator shape and seam welded to form the shell. 4. Cold formed heads are plasma arc cut for any nozzles on the heads. 5. The heads are welded onto the shell per a engineered weld spec. 6. Nozzles and pipe fittings added. 7. Vessel openings are plated off and filled with water or oil and hydro-static tested. 8. Vessel evacuated to be completely dry, then filled to a dry nitrogen charge to 5 PSIG. Page 7

8 Summary of IIAR Design Guidelines: Pressure vessel maximum allowable working pressure shall not be less than: High side service utilizing water cooled or evaporative condensing: 250 PSIG (industry practices are moving to 300 PSIG) High side service utilizing air cooled condensing: 300 PSIG (rarely used in low ambient climates) Low side service: 150 PSIG (industry practices are moving to 250 PSIG). EXCEPTION: When ammonia liquid is to be transferred from pressure vessels by pressurized ammonia gas, the pressure vessel design pressure shall accommodate the maximum possible transfer pressure and take into account the lowest possible coincident metal temperature. Page 8

9 Summary of IIAR Design Guidelines (continued): Pressure vessels exceeding 6 in inside diameter shall comply with the governing edition of Section VII, Division 1, ASME Boiler and Pressure Vessel Code Pressure vessels shall be provided with adequate opening(s) for the attachment of pressure relief device(s) as required in Heads of pressure vessels shall be hot formed or stress relieved after cold forming. NOTE: It is recommended that high-side vessels receive post-weld heat treatment per Appendix J A vessel shall be designed and stamped with a MDMT no higher than its lowest expected operating temperature In applications where the vessels are subject to external corrosion, the vessels shall be designed and specified with a minimum of 1/16 corrosion allowance. Page 9

10 Dynamics: Tangential Stress Longitudinal Stress Page 10

11 High Side Vessels: High Pressure Receiver (HPR): Sized to accept the full pumpdown volume of refrigerant in the entire system at 80% full. On large systems, it is sized to supply ½ hour of liquid supply buffer to the low side (Stoecker). Thermosyphon Receiver (TSR): Sized to provide enough liquid refrigerant to reject total oil cooler heat for 5 minutes of running time if the condenser liquid supply is interrupted. Pilot Receiver (PR): Provides a small storage vessel to receive liquid from the condenser providing a liquid only transfer with float controls to the CPR. Also may provide a small liquid supply for LIOC. Controlled Pressure Receiver (CPR): Same purpose as the HPR, but is maintained at a constant pressure that does not float with SCP, providing a constant pressure drop across the liquid metering device. Pressure maintained by a suction and hot gas regulator. Page 11

12 Low Side Vessels: Suction Accumulator (SA): Sized to allow for the full flow of the suction line capacity with a free area to reduce the gas velocity to a point where any entrained liquid droplets fall out of suspension. Can be coupled with a liquid subcooling coil to aid in separation. Intercooler (IC): Maintains a liquid level or liquid flash injection to accept the discharge from the low stage compressors and desuperheats the gas prior to going to the high stage suction. Often combined duty of being the SA for the higher temp loads. Liquid Recirculators (Various): Maintains a liquid level in the SA portion of the unit to provide a liquid head to recirculator pumps. The pumps supply pressure subcooled liquid to the evaporators. Page 12

13 Low Side Vessels (continued): Transfer Drum (TD): Accepts liquid carryover from the SA at a piped overflow point. When near full, hot gas is injected to the drum stopping overflow and pushes the liquid to a CPR or an HPR with a pump assist. Flash Subcooler/Subcooler (SC): Subcools a liquid feed from a higher temp source by flashing a separate liquid feed in an open vessel or into a heat exchanger. Surge Drums (SD): Maintains a liquid level to provide a constant head of liquid to feed a heat exchanger then accepts a wet suction and separates the vapor from the liquid with a decrease in velocity. Simply a specialized SA. Low Pressure Receiver (LPR): Provides a ballast supply of liquid to the low side and provides storage space for pump down in addition to the HPR. Often combined with a pumping system being a defacto liquid recirculation unit. Page 13

14 Other Vessels: Oil Pots: Mounted at a low point (NH3) or skim from a liquid level allows for the removal of oil accumulations to minimize oil fouling. Oil Separators: Remove oil from the discharge gas and returns it to the oil sump. Flash Economizers: Provide liquid subcooling at an intermediate suction pressure injected during the compression cycle. Heat Exchangers: Endless variety of purposes and configurations. Page 14

15 Chiller/Transfer Page 15

16 Chiller/Transfer Page 16

17 Chiller/Transfer Page 17

18 Chiller/Transfer Page 18

19 Chiller/Transfer Page 19

20 Thermosyphon Page 20

21 Horizontal vs Vertical Separation Cross area decreases/velocity Increases as liquid level rises Cross area stays the same, velocity stays the same most of the height of the vessel. Page 21

22 Pressure Relief All closed circuit ammonia refrigeration system shall be protected by a pressure relief device per ASHRAE-15. All pressure vessels shall be protected per ASME B&PV Code. All relief valves must be installed at the highest point in the vessel where possible and required to be above the highest anticipated liquid level. Stop valves down stream of the relief device must the locked open and the pressure drop associated with the valve must be calculated into the to pressure drop of the piping system. Pressure vessel of 10cf or greater to have dual relief assemblies. Install PRV as close to the vessel being served as possible. The maximum length of pipe on a PRV discharge allowable determined by the method in Appendix A (IIAR 2). Discharge of a PRV device to atmosphere must be at least 20 away from any window, ventilation intake, or personnel door. PRVs, currently, must be replaced or recertified every 5 years. Page 22

23 Coolware Demo Page 23

24 Questions? Page 24