Compressor and jet vacuum system:

Compressor:

• Polytropic process: The polytropic process equation can describe multiple expansion and compression processes which include heat transfer. PV(n)=const.

• Isentropic process: is an idealized thermodynamic process that is both adiabatic and reversible. In this case, the heat should be kept during the compression. It means that pipeline must be isolated. The temperature after the compression must be higher than before the compression. An adiabatic process is defined as a process in which no heat transfer takes place. This does not mean that the temperature is constant, but rather that no heat is transferred into or out from the system.

PV(cp/cv)=const

• Isothermal process: in which the temperature of the system remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and the change in the system will occur slowly enough to allow the system to continue to adjust to the temperature of the reservoir through heat exchange (see quasi-equilibrium). No isolation is required.

How to calculate temperature before and after the compressor:

Known: pressure before and after the compressor,

Unknow: temperature before and after the compressor

• assume it as an isentropic process and solve it

The power of compressor:

Method 1:

Known: pressure before and after the compressor, mass flow rate

Unknow: power

• Calculating the enthalpy before and after the compressor
• The difference of the enthalpy* mass flow rate

Method 2:

1. Positive Displacement Compressor: The chamber then becomes smaller, decreasing the volume and, at the same time, increasing the pressure of the gas (PV(Cp/Cv)=constant).

• Piston compressor: They have normally a suction valve and a pressure valve for the regulation of flow in to the chamber.
• Rotary screw compressor

3. Rotary Lobe Blowers

2. Dynamic Compressor: radial dynamic compressors are called Centrifugal or Turbo compressors.

In a dynamic compressor, the pressure increase is achieved by accelerating the gas using an impeller then slowing the fast moving air in a diffuser and volute to transfer the kinetic energy into pressure.

• The main difference between an air compressor and an air blower is the pressure ratio of each. An air compressor operates at a high pressure to volume ratio, while an air blower tends to operate at a low-pressure ratio.

Vacuum Jet System:

He must also supply the vendor with operating conditions which include:

The higher the molar mass, the more the jet pump can extract; (The higher molar mass can result in the lower pressure according to the ideal gas equation)

The higher the temperature, the lower the suction flow and vice versa.The higher temperature can result in the higher pressure according to the ideal gas equation)

Single-stage ejector without condensable gas components:

1. Flows of all components to be purged from the system (often air plus water vapor).
2. Temperature and pressure entering the jets and pres- sure leaving if not atmospheric.

3. Temperature and pressure of steam available to drive the jets.

4. Temperature and quantity of cooling water available for the intercondensers. Also cooling water allow- able pressure drop for the intercondensers.

5. Air Equivalent:

ERTA: The ratio of the weight of air at 70°F to the weight of air at a higher temperature that would be handled by the same ejector operating under the same conditions.

ETRS: Same as above for steam

6. Horsepower: required to compress non- condensing components from the jet inlet pressure and temperature to the outlet pressure.

Use an adiabatic efficiency of 7% for cases with jet intercondensers and 4% for non- condensing cases.

7. Steam consumption:

Method1: Estimate the steam consumption to be the theoretical amount which can deliver the previously calculated total horsepower using the jet system steam inlet and outlet conditions.

Method2:

In general, only air or water vapor is available as a suction medium for the tests. Such measurements are therefore carried out with equivalent suction flows, i. H. with suction flows that are converted to air or water vapor.

The higher E is, the less motive steam is required; but the higher K is, the more motive steam is required.

Following are some general rules of thumb for jets:

1. To determine number of stages required, assume 7 : 1 compression ratio maximum per stage.

2. The supply steam conditions should not be allowed to vary greatly. Pressure below design can lower capacity. Pressure above design usually doesn’t increase capacity and can even lower capacity.

3. Use Stellite (a range of cobalt-chromium alloys) or other hard surface material in the jet nozzle. For example 316s/s is insufficient.

Multi-stage ejectors:

Intermediate condensers are used for condensing motive steam from ejectors, additionally to the steam aspirated from the facility where vacuum is being created, thereby suction load is reduced in each stage.

• Steam and Gas in Vacuum:

The condensation during compression. You just need simple relationships.
The process goes from p₁ and t₁ to p₂ and t₂.
For T₁ you calculate the vapor pressure according to Antoine for the evaporating component. You do the same for T₂. For the sake of simplicity, let’s assume a system made up of two components: a vapor and a non-condensable gas. So you have the difference between the initial pressure p₁ and the saturated steam pressure p*(T₁). This is the partial pressure of your gas in state 1. Now you compress to a new pressure and cool. Your vapor partial pressure can only be as large as the saturation vapor pressure at T₂. The remainder is the partial pressure of the gas. If the partial pressure of the gas after compression is now three times as great as before, then the theoretical partial pressure of the vapor must also have become three times as great. The difference between the three times the initial partial pressure and the new saturation vapor pressure is the fraction that condenses. So if you calculate the theoretical condition, you can use it to calculate the amount of condensate.

If, for example, there is a mixture of 2 components — an inert gas and a condensable vapor — then the following applies: If mixtures of vapors and gases are condensed under vacuum, the gases and non-condensed vapor components must be sucked off by a vacuum pump in order to achieve the desired vacuum in the to hold the capacitor.

Total pressure (P)=partial pressure of inert gas(Pi)+partial pressure of steam (Pd)(the saturated vapor pressure of the vapor at the temperature of the gas-vapour mixture at the condenser outlet)

Now you can apply the general gas law by imagining a space with the volume of the mixture to be extracted. This volume V is filled on the one hand with the inert gas, which is under the partial pressure pI, and on the other hand with the vapor under its partial pressure Pd.

Example:

is e.g. B. from a condenser at a total pressure of p = 250 mbar a saturated mixture of air, water vapor and benzene vapor with a temperature of 30 °C can be sucked off and is the mass flow the air with 100 kg/h known, so finds the saturation amounts of water vapor and benzene vapor are as follows:

At 30 °C one reads from the steam tables:

• Water vapor and air in vacuum

From a below 42.4 mbar, i. H. condenser working with a condensation temperature of 30 °C must remove 2 kg of air every hour.

WANTED: How many kg/h of steam/air mixture must be extracted if the temperature at the ventilation nozzle is 25 °C?

At 25 °C, one finds pD = 31.7 mbar from the water vapor table (see water vapor temperature table). At a total pressure of p = 42.4 mbar, the partial pressure of the air is:
pL = 42.4–31.7 = 10.7 mbar

This results in a vapor/air mixture of 2 + 3.7=5.7 kg/h to be extracted.

Measuring Air Leakage:

Air is usually the basic load component to an ejector, and the quantities of water vapor and/or condensable vapor are usually directly proportional to the air load.

It is desirable to select a capacity which minimizes the total costs of removing the noncondensable gases which accu- mulate in a process vacuum system.

To determine the amount of air leak in an existing system, estimate the total volume of the system. Operate the ejector to secure a pressure somewhat less than a pressure. Then isolate the ejector from the system. Measure the time required for a rise in pressure in the vessel.

GUIDELINE VALUES FOR LEAKAGE IN VACUUM APPARATUS AND PLANTS

In the case of normal flange connections with larger nominal widths, a leakage air incidence of 200 to 400 g per hour and meter of gasket length is to be expected. Thanks to specially designed flange connections, e.g. g. with tongue and groove or finely machined sealing surfaces and when using special seals, the value can be reduced to 50 to 100 g/hm.

Permissible flow velocities in vacuum lines

How high the flow rate can be allowed in a vacuum line depends on how high the pressure loss of this line can be. Higher pressure loss means increased energy requirements for the vacuum pump. A pressure loss of up to 10% of the total pressure can generally be accepted.