Diffusion pumps pumping speed

The maximum permissible backing pressure (the critical backing pressure, pcrit) for a diffusion pump (pumping speed = Seff Dp) is 2 x 10 l mbar. If the pump is used only at inlet pressures (/ in) of 10 3mbar or below, calculate the speed of the backing pump (Sback) in terms of Seff DP. 

Compute the diffusion pump speed. The diffnsion pnmp rednces the system pressure from the... 

An optimized relationship is obtained between the beU jar, 60° swing-leaf valve, LN trap, baffle for the oil, and the plane of action for the diffusion pump (DP) top jet. The valve open area equals 0.38 of the cross-sectional area of the inside diameter of the furnace. The volumetric speed factor for water vapor is thus 0.38 x 0.9 crr 0.34, where 0.9 is the Clausing factor. 

Sodium borate (borax) [1330-43-4] M 201.2, m 741 , d 2.37. Most of the water of hydration was removed from the decahydrate by evacuation at 25° for three days, followed by heating to 100° and evacuation with a high-speed diffusion pump. The dried sample was then heated gradually to fusion (above 966°), allowed to cool gradually to 200°, then tranferred to a desiccator containing P2O5 [Grenier and Westrum J Am Chem Soc 78 6226 1956]. 

The two BCs of the TAP reactor model (1) the reactor inlet BC of the idealization of the pulse input to tiie delta function and (2) the assumption of an infinitely large pumping speed at the reactor outlet BC, are discussed. Gleaves et al. [1] first gave a TAP reactor model for extracting rate parameters, which was extended by Zou et al. [6] and Constales et al. [7]. The reactor equation used here is an equivalent form fi om Wang et al. [8] that is written to be also applicable to reactors with a variable cross-sectional area and diffusivity. The reactor model is based on Knudsen flow in a tube, and the reactor equation is the diffusion equation ... 

In a diffusion pump, the dense oil vapour produced by the boiler (see Fig. 1.12) is ejected into the vacuum at high (or even supersonic) speed through the nozzles. 

A cold cap is usually mounted on the top of the pump assembly to prevent vapour from reaching the vacuum chamber. A thermal protection switch is often used. The maximum working pressure of a diffusion pump is about 10-3 torr. The ultimate pressure of a diffusion pump can be around 10-9 torr and heavily depends on the oil vapour pressure (p < 10-8 torr at room temperature for very good oils). Pumps with very large pumping speed (up to 104l/s) are commercially available. 

A drawback of oil diffusion pumps is the so-called back-steaming. It is the flow of a small quantity of oil vapour towards the inlet of the pump. A water-cooled baffle like that shown in Fig. 1.13 can be put above the inlet. Baffles are made up of arrays of optically dense fins cooled by a continuous water flow. A baffle always reduces the pumping speed. 

Example 1.2. A diffusion pump with constant speed Sx = 301/s below 10-3 torr has a critical forepressure pK = 0.12 torr. It is connected to a rotary pump by a tube of length L = 150cm (see Fig. 1.15). 

The pumping speed S2 at the exhaust port of the diffusion pump must be at least ... 

With metal gaskets and moderate bakeout, turbo pumps can reach pressure below 10-9 torr without traps. They can be started at a pressure up to 1 torr. The time required to reach full pumping speed ( lmin) is much shorter than for diffusion pumps. Also these pumps must be backed by a primary pump. 

Inert material of the sampling device (bags, tubes, valves), no losses by diffusion Preflushing Prediction equipment Volume of the sample Sampling time.pump speed Storage time(<24h) Inert tube material (PTFE. stainless steel, glass) Preflushing Predilution equipment Heated probe jf Dust filters required... 

To characterize the effectiveness of a diffusion pump, the so called HO factor is defined. This is the ratio of the actually obtained specific pumping speed to the theoretical maximum possible specific pumping speed. In the case of diffusion pumps from LEYBOLD optimum values are attained (of 0.3 for the smallest and up to 0.55 for the larger pumps). 

A cold cap baffle is constructed so that it can be mounted immediately above the high vacuum nozzle. The cold cap baffle is made of metal of high thermal conductivity in good thermal contact with the cooled pump wall, so that in practice it is maintained at the cooling-water temperature or, vi/ith aircooled diffusion pumps, at ambient temperature. In larger types of pumps, the cold cap baffle is water cooled and permanently attached to the pump body. The effective pumping speed of a diffusion pump is reduced by about 10 % on installation of the cold cap baffle, buf the oil backsfreaming is reduced by about 90 to 95 %. 

A pumping speed of 100 l/s alone is required to continuously pump away the quantity of gas flowing in through the leaks or evolving from the chamber walls. Here the evacuation process is similar to the examples given in Sections 2.3.1.1. However, in the case of a diffusion pump the pumping process does not begin at atmospheric pressure but at the... 

In order to attain an ultimate pressure of 1 10 mbar within 5 minutes after starting to pump with the diffusion pump an effective pumping speed of... 

The pumping speed characteristic of a pump is easily derived from the measured pumping speed (volume flow rate) characteristic of the pump as shown for a 6000 l/s diffusion pump (see curve S in Fig. 2.76). To arrive at the throughput characteristic (curve Q in Fig. 2.76), one must multiply each ordinate value of S by its corresponding pA value and plotted against this value. If it is assumed that the inlet pressure of the diffusion pump does not exceed 10 mbar, the maximum throughput is 9.5 mbar l/s... 

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