Design Load Components for Vacuum Systems

To produce an effective and reliable vacuum system the suction load components specified must simulate the actual load conditions as closely as possible. The design suction load to a vacuum producing system will be the sum of the predictable components coming from the vacuum user (reactor, tower, condenser, etc.). Fundamental guidelines for choosing the design mass flow rate of each component type are as follows:

  1. Air Leakage - Air leakage in a typical chemical process equipment system will occur. These systems are designed, installed, and maintained based on the required profitability and rate of return on the investment needed to create and operate them in a competitive environment. Air leakage will be a function of the following conditions:

    A.     Flanged Joint Design. Typical process equipment uses ANSI B16.5 type flanges (as well as threaded pipe connections as required). ANSI flanges have gaskets that require a certain seating stress and flange face finish to be leak tight. These conditions change over time with temperature cycles and corrosion. O-Ring seals can be used for near zero leak rates. However, the O-Ring material can be adversely affected by some chemicals used in the industry, as well as the temperatures experienced during service, causing leaks to occur. O-Ring seal are not typical in Chemical Service.

    B.    Shaft Incursions. The number of rotating shafts that are installed from the atmosphere into the vacuum space, along with the quality of seals used, will affect air leak rates. These shafts are used for mixing, creating thin films, component control, and other uses.

    C.    Maintenance. The frequency of tightening joints, maintaining joint and seal components, and leak checking (hydro, halogen detectors, etc.) will effect the air leak rate.

    D.    Volume Under Vacuum. The number of connection joints, shaft seals, and weld joints is typically a function of the total volume under vacuum. As the number of possible leak paths increases so does the total air leak rate.

    E.    Operating Vacuum Level. The air leak rate through any leak path will be constant if critical flow conditions exist around the path i.e. the upstream absolute pressure (typically atmospheric) is more than approximately twice the down stream pressure (the absolute pressure in the vacuum space - see the absolute pressure chart in this section). Systems operating at less than approximately ½ an atmosphere or about 380 mm HgA should therefore have constant leak rates as pressure varies (except if some flanged joints are not already tight and benefit from the increased pressure differential on the flange area). Systems operating above ½ an atmosphere will not have constant leak rates and are harder to predict. This type, however, is not typical for chemical process equipment. Typical process systems will operate in the 1 to 50 mm HgA range for most processing requirements (filtration, drying, material transfer, and other unit operations will have different vacuum needs). This range tends to create the same number and type of connections and shaft seals (per unit volume) and the same degree of maintenance. Systems that operate in this range will thus have air leak rates that are mainly a function of the volume under vacuum. Systems that operate at lower absolute pressures will tend to have O-Ring seals, generally higher quality fabrications, better maintenance attention and no to little planned load components. These systems can and do achieve near zero leak rates.

    In Summary; for typical chemical process vacuum systems the design air leak rate, (as a general rule based on experience with many successful applications), will be equal to the following amounts based on volume under vacuum in the 1 to 50 mm HgA range:

    Volume Under Vacuum (ft3) Air Leak Rate (lb/hr)
    100 5
    500 10
    700 15
    1000 20
    1500 25
    3000 50

    There is a chart of air leak rates vs. volume under vacuum in the publication "Standards for Steam Jet Vacuum Systems" published by the Heat Exchange Institute, Cleveland, OH. This chart is based on actual field measurements of operating vacuum systems with no other load components being generated (i.e. just under vacuum, no processing). These vacuum levels were referenced to the performance curve of the steam jet system producing the vacuum. The resulting air flow numbers were then charted against the volume under vacuum for that system. The chart thus reflects the results of a study of actual operating conditions although that study was made over 40 years ago. Specific tests of an existing process system can be made with a given vacuum pumping system whose performance curve is available,  This test and/or an analysis of company specific construction and maintenance standards may provide a more up to date estimation of air leakage rates for the specific case at hand.

  2. Other Non Condensable Components (from the process)

    A.    Sparge or Sweep Gas. Some systems are purposely injected with an inert gas (typically Nitrogen) to aid in mixing or evaporation. This gas will become a direct load on the vacuum producing system.

    B.    Saturated Gas. Some liquids to be processed will release gas that has been saturated in the liquid when the pressure over the liquid is lowered and/or the temperature is changed (up or down depending on the fluids involved). This released gas will become a direct load on the vacuum producing system and should be estimated from saturation conditions prior to processing.

    C.    Evolved Gas. Some processes will include a chemical reaction under vacuum. The amount of gas evolved can be estimated from the reaction equation and the volume of material reacted. This gas will become a direct load on the vacuum producing system.

    D.    Non Condensable Vapors. Some components that are vaporized under vacuum will not subsequently condense in the steam jet vacuum system condensers. These components should be completely specified so that a vapor-liquid equilibrium analysis can be made by Thermo Systems Inc. for each condensing point in the system.

  3. Condensable Load Components

    A.    Saturated in the Total Non Condensable Flow. The design amount of non condensables (air and other gases predicted to come to the vacuum producing system) that is in contact with a liquid or vapor on the way will become saturated with the vapor phase of that liquid at the pressure and temperature of the mixture. 100% saturation will occur if there is sufficient a.) liquid to vaporize and b.) exposure time and contact.

    A full vapor-liquid equilibrium analysis and mass balance should be made at the appropriate points in the process system by the process system designer to determine all the vapor components coming out of the vacuum vessel. There may of course be an overhead condenser positioned to remove or knock back the vapor components if they are condensable under the operating vacuum level given the available cooling water temperature. The analysis is then made at the gas/vapor outlet of that condenser assuming a significant amount of sub-cooling is available.

    A simple system (with one non condensable and one vapor component to saturate) can be determined using the following equation based on Dalton's Law of Partial Pressures:

    Rvap = (Pvapor x M.W.vapor)/(Pgas x M.W.gas)

    Where (with consistent units for pressure):
    Rvap = Ratio of vapor saturated in the gas in lbs/lb
    Pvapor = Partial pressure of the vapor, equal to the vapor pressure at the temperature of the mixture at the gas/vapor outlet connection
    Pgas = Partial pressure of the gas, equal to the total pressure of the mixture at the gas/vapor outlet minus the partial pressure of the vapor component
    Rvap is then multiplied by the total gas flow rate in lbs/hr to determine the vapor mass flow rate.

    Water vapor is the most common vapor component to be saturated in the gas. Given the use of steam tables an example follows:

    Example: Given 20 lb/hr air at 1.0" HgA + 72°F (at 72°F the water vapor pressure = 0.3887 Psia = 0.7914 in HgA)

    Rvap = (0.7914 x 18)/((1.0-0.7914) x 29) = 2.35 lb/lb
    20 x 2.35 = 47 lb/hr water vapor load

    This condition presents a good example of how much sub cooling below the saturation (vapor) temperature for the total pressure (79°F is 0.4912 Psia in this case) is appropriate for stability in the prediction of how much vapor is saturated in the non condensable. It is also an example of the type of standard chosen for turbine condenser outlet conditions in the power industry (ref. "Standards for Steam Surface Condensers" by the Heat Exchange Institute, Cleveland, OH). A ratio much higher than this means a condition that is becoming very sensitive to the mixture or connection outlet temperature (the curve becomes asymptotic) and is hard to control as well as predict. As the outlet temperature approaches the saturation temperature the gas will become infinitely saturated with vapor. It is no longer possible to predict the amount of vapor saturated in the gas other than by the total amount of vapor produced form the available heat transfer and the amount of each liquid present.

    B. Evaporated Condensable Components. Some process systems produce condensable flows well beyond the amount that the process gases can hold in saturation. These condensables are evaporated as a purposeful and fundamental design goal of the process system and can be well defined from the process intention. Their successful prediction however will depend on the actual heat transfer analysis made and the actual equipment and temperatures provided for the process vessel. Some of these condensable components may be condensable in a precondenser or knock-back condenser at the operating absolute pressure (less a design pressure drop to the outlet of the condenser if not a knock-back type). A low enough temperature at the condenser outlet must of course be available with a reasonable amount of sub-cooling for the outlet flows to be predictable. If they are not condensable at the process operating pressure then they may be condensable at some downstream point in the steam jet system where the ejectors are creating a compression to a higher condensing pressure in each intercondenser (this extra condensing point is sometimes an advantage of a steam jet system in preventing undesirable vapors from being vented to the atmosphere).


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