Design Of A Cooling System For A Hydraulic Fracturing Equipment

Open Access
Author:
Tiwari, Ankit
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 01, 2015
Committee Members:
  • Savas Yavuzkurt, Thesis Advisor
Keywords:
  • CFD
  • Conjugate Heat Transfer
  • 3-D flow separation
  • Heat Transfer enhancement
  • induction motor cooling
Abstract:
The objective of this thesis is to design a cooling system for a skid-mounted hydraulic fracturing equipment. Hydraulic fracturing, popularly known as “fracking”, is a process by which natural gas and oil are extracted by fracturing shale rock formations using a high pressure water-jet mixed with sand, abrasives and chemicals. A fracking equipment typically consists of a reciprocating pump driven by an electric drill motor. The motor itself is cooled by air from a centrifugal blower while the lubricating oil from the pump is cooled separately by an air-cooled radiator. The newly proposed design aims to remove the radiator fan and cool both the motor and the radiator via cooling air supplied by a centrifugal blower through a duct system. The scope of this thesis is limited to the design of the drill pump motor cooling tubes and the transition duct connecting the exit motor plenum to the radiator. The total heat load of the system is 139.5 kW. Out of this, 64.6 kW & 38.9 kW are due to internal heat generation in the stator and the rotor respectively. The remaining 36kW is due to the heat exchange in the oil cooler/radiator. The design mass flow rate is 4000 SCFM. Pressure drop through the system and heat load of the system are the two most important parameters affecting the design of rotor and stator cooling tubes & transition duct. This is because these parameters are the potential bottlenecks to the overall performance of the system. Baseline CFD & Conjugate Heat Transfer analysis of the drill pump motor revealed that at the maximum operating load of 103.5 kW, the maximum steady state temperature in the motor was 203.50C which exceeded the upper limit of 2000C for the electric insulation of the motor windings. This observation was consistent with GE-Transportation’s experimental data. Above 2000C, the electric insulation of the windings would melt causing short circuit in the motor. To alleviate this problem, a number of heat transfer enhancement techniques like longitudinal fins, transverse grooves and transverse ribs of various shapes were studied. Out of these, circular grooves were chosen as the heat transfer enhancement solution due to their simplicity with respect to manufacturability and reasonable effectiveness. Parametric analysis of the design of the rotor/stator tubes with circular grooves was also done. It was concluded that the recommended heat transfer enhancement solution could bring down the working temperatures of the rotor and the stator by 43.10C at full load at operating point. The transition duct connecting the exit motor plenum to the radiator supplies cooling air from the blower to the radiator after it has passed through the inlet plenum, rotor & stator cooling tubes and exit plenum. Design this transition duct for optimal performance is the second major objective of this thesis. Due to the dimensional constraints, the transition duct has a high aspect ratio at the inlet (3.5:1) and high divergence angles (78.40 in the top view and 37.90 in the front view). Consequently, without any flow control mechanism, this duct would have a lot of flow separation & recirculating flow which would reduce the cooling efficiency of the radiator. After many design iterations, an optimum perforated flow distributor plate was designed which had a solidity ratio of 58.6%. Further, it was concluded that the best results were obtained if the plate was located at the center of the transition duct. The percentage forward-flow area at the outlet (where the radiator is placed), was compared with that in case of fan. The best perforated plate design had a forward flow area of approximately 75-80% while the forward flow area in case of a fan was approximately 71.3%.