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Do You Know The Power Of Vacuum Condensers?

Do You Know The Power Of Vacuum Condensers
Process condensers and precondensers should be considered when large condensable loads are identified in a vacuum pumping system. Imagine one cubic foot of water being heated; turning into steam that one cubic foot of water now occupies 1700cu/ft of space. The proper use of condensers will allow the use of a smaller vacuum pump, reducing total energy and capital costs. Condensers will also recover valuable process materials. Reducing the condensable pump load to a liquid ring vacuum pump translates into a smaller heat exchanger; less temperature rise due to condensation inside the liquid ring means a deeper vacuum level and higher capacity pump. Reducing the condensable load to oil sealed or dry vacuum pump (pumps with a low tolerance to vapor loads) will mean the difference between consistent catastrophic pump failures and normal pump operation.
The use of a condenser is not always technically possible. If the dew point of a gas stream is less than the available cooling medium temperature, the condenser can't be used to remove process vapors. In a direct contact condenser, a device that uses direct contact of the cooling medium to condense the vapor load, condensing will not be possible at pressures below the vapor pressure of the cooling medium. If the use of a condenser proves possible the question becomes one of economics. Will the potential of reduced capital and operating costs for the vacuum pump offset the new costs of the condenser, associated components, and utilities. Valuable product recovery, an increase in pumping system reliability, smaller vacuum pump and related components, reduced operating costs and a number of other factors must be considered.
The proper sizing of both the condenser and the vacuum pump depends on an accurate estimation of the condensable vapor load. Process gasses from many vacuum applications will contain 95 to 100 percent condensable vapors. When process condensers are not in use, the vapor load entering the precondenser or the vacuum pump can be considered large. This load should be treated as evaporated vapor because the flow rate of this type of stream is directly related to the evaporation rate in the process vessel.
Air leaking or intentional release of noncondensables into a process vacuum system will become saturated with vapors as they contact the liquid process stream. Condensable vapors that saturate noncondensable gases are called "vapors of saturation". If these vapors are present they must also be accounted for in order to calculate the load to the condenser or vacuum pump.
In the ideal process conditions the condenser can be the most energy efficient, cost effective vacuum pump ever developed.
Example - Process Condenser using Simple Water Customer requirements: P1 - 760mm P2 - 125mm Pd - 760mm Tg - 135f. Tw - 65F. Condensable pump load: 500lbs/hr water vapor Noncondensable pump load: 50lbs/hr air
Step 1: Determine pump size without the implementation of a process condenser. Step 1a. Determine volumetric requirement by using the ideal gas law PV=nRT V= (n) (R))( 1/P)(T) V= 1,368acfm @ 125mm
The type of mechanical pump selected must be a liquid ring vacuum pump due to the large condensable pump load. The volumetric requirement calculated would indicate an initial pump size using a 125Hp motor. After performing a mass balance on the discharge side of the liquid ring it was found that the temperature rise exceeds 20f. and the next larger pump size was selected with a greater service liquid flow. The pump size now requires a 150Hp motor and carries an estimated system capital cost of $75,000.00.
Step 2: Determine pump size when implementing a process condenser. Step 2a. Determine Process conditions at the inlet of the condenser Inlet process conditions: P2 - 125mm Tg - 135f Condensable pump load: 500lbs/hr water vapor Noncondensable pump load: 50lbs/hr air
Step 3: Determine new process conditions at the vacuum pump inlet P1 - 760mm P2 - 120mm Pd - 760mm Tg - 80f. Tw - 65F. Condensable pump load: To be determined Noncondensable pump load: 50lbs/hr air
Step 4: Determine amount of water in the air by using Dalton's law of partial pressures. Assume 60f cooling water into the condenser and 70f out (70f +10f approach = 80f gas temp) Wi = Wair Mi/29 (Xi Yi Poi/P-pc) Where Wi = mass flow rate of vapor Wair = mass flow rate of air Mi = Molecular weight of vapor Wi = 10lbs/hr water vapor
The power of condensing becomes clear as we are able to reduce the 500lbs/hr of water vapor to 10lbs/hr.
Step 5: Determine volumetric requirement by using the ideal gas law PV=nRT V = 95.5acfm @ 120mm
The volumetric requirement calculated would indicate an initial pump size using a 7.5Hp motor and carries an estimated system capital cost of $5,500.00.
Estimated capital cost of pump alone: $75,000.00 Estimated capital cost of pump when using process condenser: $5500.00 Estimated Vacuum Pump Capital Savings...$69,500.00
The example using simple water is very common on applications such as Uranium drying where very large condensable loads are encountered and the implementation of a process condenser saves thousands of dollars in capital and operating costs.

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