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Reducing the Ownership Cost of a Rotary Screw Gas Compressor

 

Rotary screw compressors have become more popular than ever with the increased vapor recovery market. Whereas rotary screw compressors have a much lower initial cost of investment that a reciprocating compressor, the “number one factor” in the cost of ownership is managing the amount of lubricating oil used by the rotary screw compressor. The lube oil is directly affected by the compressor system’s ability to regulate the temperature of the oil and to do so without high energy costs.

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Air cooled Heat Exchanger

 

Air cooled Heat Exchanger

 

Air cooled Heat Exchanger

 

Lube Cooling Example (before new turbulator component): Lube oil cooling systems for rotary screw gas compressors are usually designed for the highest possible ambient temperature at the highest possible load on the system (generally at 100 deg F). An electric driven rotary screw compressor rated at 350 horsepower would use a fan driven by a 15 horsepower electric motor running continuously 365 days a year. Out of the 365 days a year, the worst case scenario may only occur for 14 days, for just a few hours, or maybe never. The average ambient temperatures for north Texas, one of the higher temperature states in the United States, as actually only 67 deg F.. Using 10 cts/KW, a typical fin/fan air cooler for lube oil costs $14,000 a year to run, even though the compressor thermostat bypasses the oil cooler completely, most of the time.

Due to the advent of a revolutionary new turbulator, owners of rotary screw gas compressors now have the option of using traditional fin/fin air coolers that are the size and use 10% of the electricity on an annual basis. This reduced cooler size and energy efficiency give a payback of less than 24 months on air cooler purchases - a return on investment very difficult to achieve in today's market. The new turbulator is a patent pending tube insert called SpiralXTM. The new Spiral XTM slides inside a traditional fin tube and provides the ability to cool lube oil at a rate of 250% over the 19th century twisted ribbon. The SpiralXTM is shown in Figure A, on the right side.

How SpiralX works

First, a fluid flowing in a turbulent fashion is easier to cool than one that is flowing in a “laminar” fashion. “Laminar” comes from the latin phrase “flowing in layers”, which is exactly what lube oil does. Lube oil flows in layers, the thickness of the layers defined by the viscosity of the oil. Higher viscosities have larger layers. The amount of energy required to mix the laminar layers can be defined by Newton's Law of Friction as in the following figure: 

               

Second, to increase the ability to transfer heat from the laminar layers to the air cooler tube wall, energy must be added to the system in the form of pressure drop, or Delta P. This ability to transfer heat is known as the “inside film coefficient” of the system, or commonly seen as “Hi” The rate of increase of Hi divided by the increased Delta P is the “thermal efficiency” of the system.

The third key thermodynamic principle is that air coolers are designed by balancing the amount of heat rejection required, or Qreg, to the system’s heat rejection capacityor Qact. Qreg is calculated as the flow rate multiplied by the specific heat of the fluid, multiplied by the difference between the inlet temperature, T2 and the desired outlet temperature T1, or:

Qreg = M*Cp*(T2-T1)

Equation for Heat Rejection Required

Qact is calculated as :

Qact = U*A*DeltaT

Equation for System’s Heat Rejection Capacity

U is the overall film coefficient of the system, A is the required surface area, and DeltaT is the difference between the average temperature and the ambient temperature, corrected for type of unit [this last phrase is awkward]. Thus to properly size an air cooler, the following equation is solved:

M*Cp*(T2-T1) = U*A*DeltaT

Equation to Properly Size an Air Cooler

As shown in the above equation, if the overall film coefficient U is increased, the required surface area, A, of the cooler unit can be proportionally decreased. The result of decreasing the required surface area of the cooler unit is significant savings - lower investment costs, smaller space, and less energy used.

The SpiralXTM inserts, patented by Heat Exchanger Technology, LLC, have been tested in a very controlled laboratory environment, which included double pipe heat exchangers where the SpiralXTM could be compared the twisted ribbon . This test facility is shown in Figure E.

Figure E

In over 40 different tests of different turbulator designs, the SpiralXTM inserts were the only design to show a significant increase in the thermal efficiency of the system by dramatically increasing the overall film coefficient, U, of the system, thus reducing the required surface area to cool laminar fluids, such as lube oil.

The results of the SpiralXTM test were significant and are outlined in Figure F.

Figure F

By increasing the overall U of the system, as shown in the Equation to Properly Size an Air Cooler, the amount of surface area required to do the same amount of cooling for laminar fluids can be reduced by between 140% and 150%.  A comparison of the relative number of required finned tubes for lube oil heat transfer with SprialX inserts versus twisted ribbon inserts is shown below:

This illustration shows that for lube oil cooling services in air coolers, four (4) 1" SpiralX finned tubes will transfer the same amoutn of heat as twelve (12) 5/8" finned tubes with twisted ribbons.  Comprehensive test data verifying the comparison is available upon request. 

With SpiralX TM inserts, the reduction in size of the air coolers also allows packagers to provide units that meet the requirements of the A.S.M.E. Code for U fired pressure vessels at an affordable price. This “U” stamp is increasingly demanded by end users, similar to the way reciprocating compressor packages have been packaged for decades. With SpiralX TM inserts, there is no longer a need for using aluminum radiators or soldered joints in an environment they were never designed for in the first place.

To meet the requirements necessary to label the SpiralXTM air coolers as “green”, Heat Exchanger Technology added another design feature that is new to the natural gas industry but proven technology for HVAC systems, water chillers, and refrigeration systems. Instead of one enormous fan blowing air across the coils 24/7, 365 days a year, multiple, small efficient fans are used which are turned on and off based on the load required by the system.




 
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