FLOAT VALVE
3.3.2
3.3.2 FLOA FLOA T T V V AL AL VE SYST VE SYSTEMS EMS
Figure 22:
Figure 22: ElectroElectronic expansnic expansion valve on direcion valve on direct expansit expansion air cooleron air cooler. . (sour(source: ce: ETSU)ETSU)
correct,
correct, i.e. i.e. the the system system is is said said to to be be critically critically evaporator and evaporator and operates operates at at evaporator evaporator pressure.pressure.
charged.
charged. To To ensure ensure correct correct operation, operation, the the This This liquid liquid level level affects affects the the pressure pressure in in the the pilot pilot line,line, evaporator
evaporator must must be be fitted fitted with with a a level level gauge gauge which which and as and as the pressure the pressure varies the varies the expansion vexpansion valvealve iis s cchheecckkeed d rreegguullaarrllyy.. mmoodduullaattees s tthhe e ssuupppplly y oof f lliiqquuiid d ffrroom m tthhe e rreecceeiivveer r ttoo
the evaporator.
the evaporator.
LP float valves are used on systems which have LP float valves are used on systems which have more than one evaporator connected to one more than one evaporator connected to one compressor or to several
compressor or to several compressors in parallel.compressors in parallel.
A typical LP float system is shown in
A typical LP float system is shown in Figure 24. AnFigure 24. An
It is important that the expansion valve is fitted It is important that the expansion valve is fitted at aat a LP float valve is used to maintain a liquid level in the
LP float valve is used to maintain a liquid level in the
3.3.2.2 LOW PRESSURE (LP) 3.3.2.2 LOW PRESSURE (LP)
FLOAT VALVE FLOAT VALVE
Figure 23: High pressure float valve. (source: ETSU) Figure 23: High pressure float valve. (source: ETSU)
Figure 24: Low pressure float expansion system. (source: ETSU) Figure 24: Low pressure float expansion system. (source: ETSU)
level
level below below the lithe liquid quid surface surface in the in the receiver, receiver, in in are are used, used, such such as as electronic electronic expansion expansion valves.valves.
order to prevent refrigerant gas going through order to prevent refrigerant gas going through thethe valve and hence reducing efficiency. A level gauge valve and hence reducing efficiency. A level gauge must be fitted to the
must be fitted to the receiver so that the liquid levelreceiver so that the liquid level can be checked to ensure adequate performance is
can be checked to ensure adequate performance is In an air-cooled condenser the refrigerantIn an air-cooled condenser the refrigerant maintained.
maintained. condenses inside tubes over which air is forced by condenses inside tubes over which air is forced by fans. To improve the heat transfer, the
fans. To improve the heat transfer, the tube surfacetube surface is usually extended using corrugated metal fins.
is usually extended using corrugated metal fins.
A well designed plant should operate with a A well designed plant should operate with a condensing temperature no higher than l4ºC above condensing temperature no higher than l4ºC above There are three types of condenser in widespread
There are three types of condenser in widespread
the ambient temperature. With
the ambient temperature. With larger condenserslarger condensers use:
use: it is common practice to control the head pressureit is common practice to control the head pressure by switching off or slowing down fans, although this by switching off or slowing down fans, although this
air-cooled (using ambient air);air-cooled (using ambient air);
is inefficient.
is inefficient.
water-cooled (using mains, river or water-cooled (using mains, river or coolingcooling tower water);
tower water);
If air-cooled condensers are being used in a If air-cooled condensers are being used in a
evaporative cooled (using ambient air andevaporative cooled (using ambient air and
corrosive atmosphere (for example, near the sea or corrosive atmosphere (for example, near the sea or recirculated water).
recirculated water).
in polluted air) then a suitable tube/fin material in polluted air) then a suitable tube/fin material combination or a coating should be used.
combination or a coating should be used.
The two latter types take advantage of the lower The two latter types take advantage of the lower wet bulb ambient temperature and the greater heat wet bulb ambient temperature and the greater heat
Air-cooled condensers are susceptible to blockage Air-cooled condensers are susceptible to blockage transfer affect of water, and
transfer affect of water, and therefore operate withtherefore operate with
by air borne debris such as dust, feathers, packaging, by air borne debris such as dust, feathers, packaging, lower condensing temperatures. When comparing
lower condensing temperatures. When comparing
and so on. They must be
and so on. They must be regularly cleaned (but notregularly cleaned (but not different condenser types the
different condenser types the power requirementspower requirements
with refrigerant) to prevent a build up of with refrigerant) to prevent a build up of of associated fans, pumps and heaters should be
of associated fans, pumps and heaters should be
contamination, as this will reduce the air flow and contamination, as this will reduce the air flow and taken into account. In general, systems under 100
taken into account. In general, systems under 100
hence increase the condensing pressure.
hence increase the condensing pressure.
kW capacity use air-cooled condensers unless kW capacity use air-cooled condensers unless there is a space or noise restriction.
there is a space or noise restriction.
For a given capacity, a larger
For a given capacity, a larger condenser will result incondenser will result in a lower condensing temperature and hence better a lower condensing temperature and hence better efficiency. Problems can be caused on
efficiency. Problems can be caused on installationsinstallations
Water-cooled condenser
Water-cooled condensers are of s are of the shell and the shell and tubetube which use thermostatic expansion valves if the
which use thermostatic expansion valves if the
type. The cooling water flows in tubes inside the type. The cooling water flows in tubes inside the condensing (head) pressure varies widely. Such
condensing (head) pressure varies widely. Such
shell, and refrigerant inside the shell condenses on shell, and refrigerant inside the shell condenses on valves are unable to control refrigerant flow reliably
valves are unable to control refrigerant flow reliably
the outside of the cold tubes. Heat transfer is the outside of the cold tubes. Heat transfer is under such conditions, and reduced efficiency and
under such conditions, and reduced efficiency and
improved as the water velocity is increased. An improved as the water velocity is increased. An reliability will result. Some form of head pressure
reliability will result. Some form of head pressure
efficient system will work with a temperature rise of efficient system will work with a temperature rise of control may be used to raise the head pressure
control may be used to raise the head pressure
5ºC for the
5ºC for the water passing through the condenser,water passing through the condenser, artificially, although this is inefficient and is not
artificially, although this is inefficient and is not
and a difference of 5ºC between the condensing and a difference of 5ºC between the condensing necessary if more
necessary if more sophisticated expansion devicessophisticated expansion devices
3.
temperature
temperature and and that that of of the the water water leaving leaving the the likely to likely to cause cause a a problem, problem, cleanable cleanable condenserscondensers condenser.
condenser. On On very very small small commercial commercial installations installations should should be be used.used.
mains water is often used directly, although this is mains water is often used directly, although this is becoming less common on new installations due to becoming less common on new installations due to water metering.
water metering.
On larger installations the water will be
On larger installations the water will be cooled in acooled in a cooling tower, where the cooling effect is achieved
cooling tower, where the cooling effect is achieved In an evaporative condenser, refrigerant isIn an evaporative condenser, refrigerant is by evaporating some of the cooling water into the
by evaporating some of the cooling water into the condensed in tubes which are wetted and over condensed in tubes which are wetted and over air. Blockages in the air or
air. Blockages in the air or water side will significantly water side will significantly which air is forced. The water used to wet thewhich air is forced. The water used to wet the reduce the efficiency of the cooling tower. Such
reduce the efficiency of the cooling tower. Such outside of the tubes is recirculated, although aoutside of the tubes is recirculated, although a blockages are common and are
blockages are common and are normally caused by normally caused by
certain amount of make up water will be needed.
certain amount of make up water will be needed.
hard water deposits or algae
hard water deposits or algae growth. Water shouldgrowth. Water should
Evaporative condensers should operate with
Evaporative condensers should operate with similar similar be treated to prevent these and also to prevent
be treated to prevent these and also to prevent
temperatures to the water-cooled temperatures to the water-cooled conden-bacteria growth. The cooling tower should cool the
bacteria growth. The cooling tower should cool the
ser/cooling tower combination above. The water ser/cooling tower combination above. The water water to within 13 - l8ºC of the
water to within 13 - l8ºC of the wet bulb ambientwet bulb ambient used will require treatment, as used will require treatment, as described for water-described for water- temperature (which can be up to 10ºC lower than
temperature (which can be up to 10ºC lower than cooled condensers above.cooled condensers above.
the dry bulb temperature).
the dry bulb temperature).
The water side of the condenser is also liable to The water side of the condenser is also liable to blockage caused by hard water deposits. If this is blockage caused by hard water deposits. If this is
Air and other non-condensable gases in a Air and other non-condensable gases in a refrigeration system will increase the condensing refrigeration system will increase the condensing temperature and hence reduce efficiency. For temperature and hence reduce efficiency. For
example, in a medium temperature ammonia example, in a medium temperature ammonia system working with a condenser which contains system working with a condenser which contains 15% air, the running costs will increase by 12%.
15% air, the running costs will increase by 12%.
Air can remain in a system after installation or Air can remain in a system after installation or service, if the system has been inadequately service, if the system has been inadequately evacuated prior to charging with
evacuated prior to charging with refrigerant. Whilerefrigerant. While running, air can be drawn into a
running, air can be drawn into a system operating atsystem operating at a suction condition lower than atmospheric a suction condition lower than atmospheric pressure, if there is a leak on the low side of the pressure, if there is a leak on the low side of the system.
system.
It is possible to check for air and other It is possible to check for air and other non-condensable gases when the system is
condensable gases when the system is not workingnot working