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The new approach to vacuum pump design

November 1, 2005 02:46 PM

Results of special testing used in manufacture of new oil-free (dry) rotary vane vacuum pump with reduced noise level and less overheating.

DuraVane models RVD040/060/075/100L in capacities from 42 – 100 CFM.

DEKKER’s supplier of DuraVane rotary van vacuum pumps has initiated a new approach to the design of vacuum pumps and compressors to reduce the "time to market” and obtain more and more reliable products. This is the result of a close collaboration with the University of Bologna, Faculty of Engineering, utilizing special evaluation and simulation software (Computational Fluid Dynamics or CFD). Computer simulation allows us to identify common problems and arrive at the correct solutions to them, and also to implement the right technical improvements. Below is a description of the application of this new approach and a specific case. In order to reduce the noise level and limit over-heating of the DuraVane RVD0100L rotary vane vacuum pump, a complex survey has been conducted on the thermal-fluid-dynamic factors of the pump, with the aid of a computer. This survey involved both the cooling air coming into contact with the pump body on the outside, and generated by the cooling fan, and the airflow emanating from the exhaust outlets.

The principles of the survey are:

  1. Finding the simplest way of extracting heat from the pump and increasing the supply of cooling air coming into contact with it.
  2. The actual flow rate coming from the fan was lower than that of other models available on the market; therefore there were some intrinsic limitations to the efficiency of the former.
  3. By appropriately shaping the exhaust ducts, the general dynamics of the backpressure phenomenon (which cannot be avoided in vacuum pumps operating at lower pressure than atmosphere) were improved.

By looking at the limits in terms of the shape and construction of the fan casing of the pump examined (RVD0100L), a double spiral has been developed around the fan. The purpose was to channel the airflow towards two exhaust sections only, instead of letting the fan exhaust the entire airflow through the holes drilled on the casing. A fan with a lower diameter (from 130 mm to 100 mm) has been chosen and the distance between fan and pump has been increased, in order to reduce the pressure loss of the fan and increase efficiency. This solution has resulted in the fan being able to operate with a greater capacity and head.


Figure 1 Old configuration (motor side) 



Figure 2 New configuration (motor side)



Figure 3 Old configuration (pump side)             



Figure 4 New configuration (pump side)


In order to validate and evaluate the advantages offered by the modifications, we conducted two tests using the fan casing of the former version and that designed on the basis of the above premise. Apart from changing the shape of the fan casing (and reducing the weight of the fan), the gaskets on the exhaust ducts were removed to improve the heat exchange between the pump body made of cast iron and the outlets made of aluminum. The studies and computer simulation results have been corroborated by field tests (the pump's temperature had been reduced by over 40°C).Studying the shape of the outlets and the area covered by the flow of air cooling the pump proved to be more troublesome than expected, owing to the fact that it was impossible to actually examine the reactions since they take place in an enclosed area. Once again, computer-aided simulations have allowed us to design a shape of the ducts, which is compatible with the construction and operation limits of the pump. Here below are two views illustrating

the flow field of air generated by the pump in its initial version (on the left) and in its modified version (on the right) (intermediate solutions have been omitted for sake of brevity). One can easily see that the flow of air inside the pump in its modified version is much smoother. In addition, a higher speed of the airflow is noted: this increases the heat exchange coefficient and reduces the air stagnation phenomenon downstream to the outlet.


Figure 6



Figure 7

Figure 6/7: Airflow field inside the pump near the exhaust outlets: high turbulence in the initial solution (Figure 6)



Figure 8



Figure 9

Figure 8/9 Flow field, cross-section: note the higher speed of the airflow near the exhaust outlets (Figure 9)

Once the efficiency of the fan was improved (shape of the spirals and dimensions) and its airflow rate established, to increase the heat removed from the pump, the next step was to improve the heat exchange conditions. As is known, the thermal power exchanged between two bodies (in our case the pump and air) is a result of the temperatures involved, the relative flow and the surfaces connecting the two bodies. In particular (since it is impossible to control ambient temperature), after increasing the airflow around the pump by means of the previously described modifications, it is sufficient to increase the heat exchange surfaces: the best and most familiar solutions can be found in fin surfaces theory.

We now had to assess whether the presence of fins could offer advantages in terms of heat reduction, bearing in mind the greater costs implied by this solution. Still using a computer, we simulated air/pump heat exchanges with and without the presence of fins on the pump body and with parameters differing in terms of shape, position and number of fins. After analyzing the various solutions, we chose the one offering more thermodynamic advantages with respect to the costs of development: six fins on the top section of the body resulting in a temperature reduction of approximately 20 degrees.

We therefore designed a prototype with this new shape and during the test runs we noticed an exceptional correspondence with the results obtained during computer simulation.

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