Final

Principle and Operation of Pulse Tube Refrigeration

1. Principle And Operation of Pulse Tube Refrigeration University of the South Pacific School of Engineering and Physics MM321- Refrigeration and Air conditioning 1

2. Objective To demonstrate the principle and operation of the pulse tube refrigeration system. To describe the processes involved and the governing equations in the basic pulse tube refrigerator To briefly describe the modifications made to the basic pulse tube refrigerator. 2

3. Introduction A pulse tube refrigerator is a C ryocooler capable of reaching temperatures of a few tens of Kelvin in a single stage and a few kelvin in two stage. Unlike ordinary VCR cycle, a pulse tube refrigeration system implements the oscillatory compression and expansion of gas within a closed volume to achieve the desired refrigeration. In other words, generation of low temperature is achieved due to compression and expansion of a gas. 3

4. Brief History Pulse tube refrigeration is a recent innovation. They were first reported by Prof. W. Gifford and his graduate student, R Longworth, of Syracuse University at around 1960 [1]. They noticed that a closed end of a pipe became very hot when there was a pressure oscillation inside, whereas the open end towards the compressor was cool. Connecting such a line to a compressor through a regenerator produced cooling at one end and heating at the other. 4

5. Literature Review It is not generally realized that delivery of a constant temperature gas into a closed volume, thus increasing the pressure, will result in large temperature gradients. By suitable arrangement of a thermal regenerator, heat exchangers it is possible to preserve this temperature gradient in an essentially static state even though there is a rapid flow of gas in and out of the volume, pressure variation is great, and pressure oscillation from a maximum to a minimum occurs many times per minute [2]. 5

6. Literature Review These temperature gradients are maintained by pulsating gas. Therefore, if the hot end of the gradient is cooled to room temperature, the cold end will descend to a lower temperature. Pulse tube refrigeration is a method which uses this principle to achieve low temperature refrigeration in small compact tubes [3]. 6

7. Literature Review Pulse tube refrigerator units operate as closed systems where no mass is exchanged between the C ryocooler and the environment. The only moving component is the piston ( and the rotary valve) which oscillates back and forth to generate periodic pressure oscillation of the working fluid. Mostly helium is chosen as the working fluid because it offers the lowest critical temperature compared to other available gases. 7

8. Literature Review Working Fluid The most common working fluid for the pulse tube refrigerator is Helium. Helium non toxic and environment friendly. 8

9. Literature Review Working Fluid Helium was a choice of coolant as its properties allow components to be kept cool over long distances. At atmospheric pressure gaseous helium becomes liquid at around 4.2 K (-269.0°C). However, if cooled below 2.17 K (-271.0°C), it passes from the fluid to the superfluid state. 9

10. Literature Review Basic Pulse Tube R efrigerator: Its basic components include a pulse tube, regenerator, a pressure wave generator and two heat exchangers and an after cooler as shown in the figure below. Fig 1 10

11. Literature Review The piston, compressor or similar pressure wave generator is attached to the warm end of the regenerator and provides the pressure oscillations that drives the refrigeration. The regenerator is a periodic flow heat exchanger. It absorbs heat from the gas pumped into the pulse tube precooling it, and stores the heat for half a cycle then transfers it back to outgoing cold gas in the second half of the cycle cooling the regenerator. The interior of the regenerator tube is filled with either stacked fine mesh screens or packed spheres to increase its heat capacity. 11

12. Literature Review The pulse tube is a simple tube, with one open end and closed end. The closed end is the hot end and is capped with a heat exchanger that cools it to the ambient temperature. Fig 2 Closed end 12

13. Literature Review The open end is the cold end. It is connected to the regenerator and a cold stage by a second heat exchanger Fig 3 Open end 13

14. Literature Review Components Fig 4 14

15. Literature Review The pulse tube works by transporting heat against a temperature gradient in a process called surface heat pumping [4]. It occurs in many systems subjected to pressure oscillations. The piston compresses the working gas and this high pressure gas in turn compresses the gas already in the tube acting as a gas piston. At the same time, the temperature of the gas rises as they undergo adiabatic compression. 15

16. Literature Review All the gas that was initially in the tube will be compressed to the hot end. The extra gas that flows in from the regenerator has a pressure gradient. The pressure is highest closest to the hot end and lowest at the bottom of the pulse tube. Fig 5 16

17. Literature Review The pressure gradient directly results in a temperature gradient. At the hot end of the pulse tube, the gas conducts its heat to the heat exchanger and the temperature falls. The piston then retracts and the gas undergoes adiabatic expansion cooling it even more. As the expanding gases passes from the pulse tube into the regenerator, it absorbs heat from the regenerator and pulse tube walls cooling them. 17

18. Literature Review The next cycle starts by compressing the gas back through the precooled regenerator. The gas begins at a lower temperature and it therefore reaches an even lower temperature after finishing its compression and expansion cycle. Record temperatures of 74K have been achieved with the basic pulse tube refrigerator. 18

19. Literature Review Analysis of Pulse Tube Refrigerator If a mass of gas with specific heat is compressed to a temperature just before it enters the hot end heat exchanger at a temperature , then when it enters this heat exchanger it will give up an amount of heat equal to the refrigeration effect . During the exhaust phase of the cycle, gas leaves the hot end heat exchanger at temperature and expands to an exhaust temperature which is less than creating a refrigeration effect .   19

20. Literature Review The relation for the temperature at any point x in the tube, Tx in terms of the volume of the hot end heat exchanger, , and the total volume from the top of the tube point to point x , for a gas whose ratio of specific heat is , is given by:   20

21. Literature Review Orifice Pulse Tube Refrigeration The basic pulse tube and more generally the surface heat pumping technique is of limited use when very low temperatures are required. Modifications made to the basic design involved adding an orifice outside the heat exchanger and a reservoir closing the orifice [5]. With the improvements, a low temperature of 60K was achieved. 21

22. Literature Review Fig 6 (Orifice Pulse Tube Refrigerator) 22

23. Literature Review The hot end heat exchanger is equivalent to the condenser and the cold end heat exchanger is equivalent to the evaporator in the vapor compression cycle. During the PTR operation, most of the heat generated due to the compression is rejected through the after cooler. The rest of the energy that is not rejected is carried to the regenerator by enthalpy flow The regenerator enthalpy flow and the additional refrigeration load are absorbed at the cold heat exchanger.   23

24. Literature Review The enthalpy flow enters the pulse tube, and travels down the tube, reaches the HHX and part of this enthalpy is rejected to the environment. The portion of the enthalpy that has not been rejected through the heat exchanger flows to the reservoir through the orifice . 24

25. Literature Review Applications Military Environmental Transportation Energy Problems Reliability Efficiency Cost 25

26. Conclusion Pulse tube refrigeration is still a developing technology in the field of refrigeration We have briefly studied the basic pulse tube and the modifications made to it by the addition of the orifice tube and the reservoir. Although they are very much similar, the method of analysis is somewhat different. Pulse tube refrigeration is very important in cryogenics 26

27. Referencing 1. Gifford, W.E. and Longsworth, R.C. Pulse tube refrigeration, Trans ASME B J Eng Industry 86(1964), pp.264-267. 2. Gifford, W.E. and Longsworth, R.C. Pulse tube refrigeration progress, Advances in cryogenic engineering 3B (1964), pp.69-79. 3. Gifford, W.E. and Longs worth, R.C. Surface heat pumping, Advances in cryogenic engineering 11(1966), pp.171-179. 4. Gifford, W.E. and Kyanka, G.H. Reversible pulse tube refrigerator, Advances in cryogenic engineering 12(1967), pp.619-630. 5. de Boer, P. C. T., Thermodynamic analysis of the basic pulse-tube refrigerator, Cryogenics34(1994 ) ,pp. 699-711 . 27