- Why is a sub-atmospheric steam system called a variable vacuum (vari-vac) or sub-atmospheric system?
The pressure on the supply as well as the return side of the system can very.
- Variation of pressure in vari-vac steam heating system?
- On the supply side vary from 2.3 psig and 219.4* F during morning heat up period, or sever weather to 26” of vacuum and 125* F during mild weather condition.
- How does a sub-atmospheric system differ from a vapor steam heating system?
- Vapor system with an air eliminator can operate with an induced vacuum on the supply side of the system. The steam condenses and air can not re-enter the system.
The vacuum that is created is not controllable. - The vacuum pressure on the supply and return side of a vari-vac is controllable.
- Describe the operation of the sub atmospheric steam heating system?
- The steam flow into the supply main is continuous. A special pressure reducing valve is installed to control the rate that steam is to be supplied into the steam main.
- The steam supply lines feeding the heating units are equipped with outlet valves with adjustable orifices, or they are equipped with orifice plates. The orifice regulate the amount of steam flowing into each heating unit.
- Thermostatically controlled radiator valves are often installed in sub-atmospheric heating systems.
- The vari-vacuum pump discharges condensate to the boiler or boiler feed pump. When the vacuum pump is in operation and air is being exhausted from the heating system, a pressure differential is established.
This in turns cause steam to flow from the steam supply main allowing air and condensate to be returned to the vacuum pump. - When the vacuum pump shut off, the pressure tends to equalize between the supply and return piping. To insure proper circulation of steam throughout the heating system there must be a pressure differential between the supply and return piping. This differential must be maintained throughout the range of various pressures, both above and below atmospheric.
- What type of controller must be installed on a high head vacuum pump to make sure there is steam circulation?w
When the vacuum pump cycles off, the pressure in the supply and return piping equalizes. If there is a call for heat and a radiator valve opens there is no steam flow, because there is no pressure differential. In order to make sure there is always steam circulation a differential controller must be installed. The differential controller maintains a pressure differential between the supply and return sides of the system.
- List five types of equipment and controls in sub-atmospheric steam heating system?
- Differential controller.
- Control valve.
- Heat balancer.
- Control panel.
- External control devices (outdoor sensor and system thermostat).
- What is differential controller?
Device that maintains the pressure difference between the supply and return piping in a sub-atmospheric steam heating system. The controller is factory set to maintain this differential. It starts and stops the vacuum pump thus maintaining the system differential. For example, if the vacuum in the steam supply is at 10”, the pump will maintain 12” of vacuum in return piping.
- The preferred location for the differential controller?
Near the end of the steam main farthest from the control valve, it may, if necessary be located near the vacuum pump.
- What must two precautions be taken, when installing a differential controller?
- The steam connection to the controller should be taken off the steam main at least 20’ beyond the control.
- The connection between the return main and the controller should be taken off the return main at least 20’ ahead of the vacuum pump.
- Control valve location?
Zone valve must be installed in a horizontal main and never in a vertical pipe. There should be at least 10’ of straight pipe beyond the control valve.
- Pressure requirements for control valve?
A pressure of 2 to 10 psig should be maintained ahead of the control valve at all times while the system is in operation. A compound gage with a pipe siphon should be installed about 10’ from the control valve.
- How is the control valve positioned and operated?
By either a low voltage electric motor, a pneumatic relay, or a combination pneumatic-electric relay according to the temperature reading at the window selector and room thermostat, as corrected by the heat balancer.
- What is heat balancer for?
It is used to measure the heat supply rate.
- How does heat balancer work?
It is specially constructed convector with its heating element connected to the supply and return piping. Two resistance coils, one located below the heating element and the other located above, measure the temperature of the air entering and leaving the heat balancer. Any variation in steam supply to the system is thus measured and the extent of the control valve opening is automatically changed to balance the heat supply with the heat demand.
- Location of heat balancer?
It should be located between 1/4 and 1/2 the distance from the control valve to the end of the longest steam main. It should not be located close to the control valve and not closer than 5’ to an elbow or offset in the main. It should never be installed so close to steam piping that the inlet air temperature would be affected. The heat balancer should be installed in a room with a normal air temperature of not less than 60F or more than 80 F.
- How does control panel work?
The control panel takes continuous readings of the existing heating conditions in the system and of the amount of heat being supplied. If the steam supply is not in accord with heat demand the position of the control valve is changed accordingly. Any change in control valve setting is limited to approximately 3% per minute. This is done so there are no radical changes in the pressure and steam temperature in the system piping.
- What are external control devices?
The outdoor sensor and the system thermostat are both considered external control devices.
- How does External Control devices work?
They send data back to the control panel. This data along with the information provided by the heat balancer determines the degree of opening of the control valve and the amount of heat supplied to the system heating units.
- How shall outdoor sensor be installed?
It must be protected from the direct rays of the sun.
- What is the system thermostat?
It is a simple temperature sensing device. The thermostat serves as a high and low-limit control when other than normal temperatures exist in the building or zone. In addition to controlling the heat up period, the thermostat is also in command of steam flow during the night and during weekend shutdowns, when the selector switch of the control panel is positioned manually or changed automatically by the time switch assembly.
- What special consideration must be given to system piping?
- The piping is sized the same as a standard vacuum heating system.
- Steam supply and condensate return piping should be pitched at least 1” per 30’ of pipe length.
- The minimum pitch for runouts for risers and branch piping should be pitched not less than 1/2” per foot.
- There must be gravity flow from all steam traps to the return piping. The use of lift fittings in the condensate return piping is not permissible.
- Since this system is expected to operate at a vacuum of 26” of mercury (Hg), the piping and radiation units must be leaktight.
- Testing of piping variable vacuum steam heating system?
The design engineer specifies a hydrostatic or steam pressure test. This is followed by a vacuum test.
- How is the vacuum test for variable vacuum steam heating system make?
A vacuum pump lowers system pressure to 20” Hg. The pump is shut off and after two hours, a vacuum of at least 17” Hg must remain in the system. If there is hess the system is not tight and all leaks must be repaired.
- Design requirements for steam trap of variable vacuum steam heating system?
They must operate at any pressure from 15 psig to 26” Hg. Many steam traps will operate down to 15” Hg.At vacuum deeper than 15” the traps will snap shut, preventing circulation.
