Cooling Probe (Heat Transfer in Instrumentation)

I. INTRODUCTION

As Snr. Instrument and Control Engineer that supports the operation. In this case study, the writer supports the Corrosion and Inspection Team. On certain occasions, a study will be performed to support their objective. The case study is to provide cool fluid to the cooling probe in order to simulate the potential TLC in the subsea multiphase pipeline. This led to increasing the integrity of the subsea multiphase pipeline.

This is not a daily task for an instrumentation engineer. However, a variety of instrumentation devices or equipment includes electronic, pneumatic, and hydraulic therefore in certain cases the equipment is categorized as instrumentation. The writer provides options and schemes to solve their problem.

I. BACKGROUND

As part of the mitigation of ToLC in the pipeline, VCI (Volatile Corrosion Inhibitor) will be injected. To monitor the effectiveness of VCI injection, a cooling probe needs to be installed. The cooling probe will simulate the potential TLC in the subsea multiphase pipeline, which is achieved by cooling the probe which simulates the cooling effect of seawater. With this new method, can continuously monitor the ToLC rate and furthermore evaluate the chemical performance that has been applied.

II. Objective

Trial vortex tube performance to achieve cool down inside cooling probe with the temperature at 10-20 degC.

III. Methodology

The methodology to cool down inside the cooling probe could be a number of possibilities to cool down the probe. The writer proposes using a vortex cooling product from Vortec (https://www.vortec.com/en-us/vortex-coolers). The selected model vortex tube requires a continuous gas supply 11 SCFM with pressure inlet available. The cold fraction is expected 90%.

This is sample measured on manufactured laboratories for 208-15-HSS tube at 100 psig:

Cold fraction (%)	Valve open
100	0°-full closed
90	90° counter clockwise
80	210° CCW
70	380° CCW (one full turn plus 20°)
60	570° CCW (one and a half full turns plus 30°)
50	750° CCW (two full turns plus 30°)

The cold fraction is to predict the temperature drops and rises in the vortex tubes for various inlet temperatures. The cold fraction will determine the opening valve at the hot end.

Backpressure exceeding 5 psig (0.3 barg) will reduce the performance of the vortex tube.

The cooling fluid is using nitrogen from a bottle of nitrogen. As consequence, the instrument air is not available on the platform. The drawback of using a rack of bottled nitrogen on an offshore platform is logistic issues. Since it is not intended for continuous operation but rather for evaluation in certain periods of time.

To control the desired cold temperature, cold airflow and temperature are easily controlled by adjusting the slotted valve in the hot air outlet. Opening the valve reduces the cold airflow and the cold air temperature. Closing the valve increases the cold airflow and the cold air temperature. Adjusting the valve will alter the cold fraction; the maximum cold fraction could be achieved at 90% cold fraction. To achieve 90% cold fraction valve is turned 90° counter clockwise.

Since the supply is using a rack of bottle nitrogen, inlet pressure to vortex tube is not constant and it will decrease continuously. Thus it will affect the outlet temperature in both the cold and hot ends as per table 1. The temperature range is represented in table 1. The table is derived from absolute ratio table 3.

Cold Fraction		90
PSIG/BAR		OC
1.4	Temperature Cold End	25.106
Temperature Hot End	121.133
2.8	Temperature Cold End	18.122
Temperature Hot End	163.036
4.1	Temperature Cold End	18.122
Temperature Hot End	172.347
5.5	Temperature Cold End	16.958
Temperature Hot End	179.913
6.9	Temperature Cold End	15.795
Temperature Hot End	181.659
8.3	Temperature Cold End	15.212
Temperature Hot End	184.569
9.7	Temperature Cold End	14.631
Temperature Hot End	185.733

Pressure inlet is maintained by pressure regulator valve (set @6.9 barg). Outlet pressure regulators will fluctuate around 6.9 barg – 5.88 barg according to the flow rate curve. Hence outlet cold end will fluctuate at 14^oC-18^oC.

To protect vortex tube from excess pressure, Pressure Safety Valve (PSV) at the inlet line vortex tube is installed to avoid inlet pressure above 10.3 barg.

With selected vortex tube and cold fraction set 90% cooling capacity yield 640 BTU/Hr.

Inside the cooling probe, 2 heat transfer convection occur, namely natural convection and forced convection. However, in this cooling system, only force convection is considered. Natural convection is ignored since the focus is the cooling surface temperature of the element sensor.

 Force convection at the surface cooling probe is calculated using assumption 80^OC at surface temperature and flow cool air at 15^OC which resulted in loss heat rate 77.876 BTU/Hr. Thus using a cooling capacity 640 BTU/Hr is adequate to maintain 15 – 20^OC at the surface of the element sensor to create condensation

The performance of the vortex tube is depending on the stability of inlet pressure and flow rate. Replacement of bottle nitrogen is calculated based on the performance vortex tube. The performance of vortex tube is optimum when the flow rate is maintained at more or less 11 MMSCFD. To maintain the optimum flow rate range, bottle nitrogen should be replaced when pressure below 17 barg.

It is estimated that for 8 hours of sampling, it requires 6 bottles of nitrogen to supply the vortex tube at 11 SCFM.

The convection heat transfer of the cooling probe is complicated since it involves fluid motion as well as heat conduction. The convective heat transfer coefficient “h” strongly depends on the fluid properties and roughness of the solid surface, and the type of fluid flow (laminar or turbulent).

The case of cooling probe heat transfer is similar to flow over a flat plate. This analogy is adopted since heat transfer is focused on the bottom of the sensor surface.

It is found that the Nusselt number can be expressed as:

where L=D=Diameter

For isothermal surface plates, the local Nusselt number for turbulent is:

Assumptions:

It is assumed that the velocity of the fluid is zero at the surface; this assumption is called no-slip condition. As a result, the heat transfers from the solid surface to the fluid layer adjacent to the surface by pure conduction, since the fluid is motionless.

Which is valid for 5 x 10⁵ < Re < 10⁷ and 0.6 < Pr < 60. The fluid properties are evaluated at the free-steam temperature T∞, except for µs which is evaluated at surface temperature.

Heat Loss required (Please refer to Calculation Note – Attachment 1)

Re = 202110.81

Pr = 0.72055998

Nu_cyl = 582.412

h = 436.83 W/m2.K

Q = 77.876 BTU/Hr

Where nitrogen properties at 1.013 bar and 25^OC

ρ          1.1848 kg/m3

V          88.646 m/s

D         0.00635 m

µ          1.663E-05 kg/m.s

k          0.024001 W/m.K

Cp       1040 J/kg.K