Brochure

Parker Hannifin Corporation
Climate & Industrial Controls Group
Cleveland, OH
Catalog CIC-2003-1/US
Thermostatic and Constant Pressure (Automatic) Expansion Valves
162
General Information Thermostatic Expansion Valves
Anti-Hunt Ballast Bulb (“X” Charge)
The power element sensing bulb for an “X” charge contains
an internal ballast material and the entire assembly is
gas cross charged.
The combination of the cross charge and internal bal-
last results in a variable rate time constant dampening
that reduces or entirely eliminates undesirable system
hunt or instability caused by overfeeding or underfeed-
ing the evaporator.
The top two graphs at the right illustrate the thermal
ballast time delay characteristics. The top graph shows
the bulb response to temperature change. The thermal
bulb pressure will decrease rapidly when the tempera-
ture is decreased from point A to B causing the valve to
modulate toward a lower flow position. As the tempera-
ture is increased back to point A, considerably more
time is required to increase the thermal bulb pressure
The second graph is an illustration characterizing the
operating superheat variation of a typical refrigeration
system. When the system load decreases, the suction
line temperature and flow decrease and the operating
Gas charge condenses
at coldest point into
droplets of liquid.
Gas condenses
on pore surface of block.
superheat rises rapidly. As the suction line temperature
increases, the bulb pressure will slowly increase and
the operating superheat will decrease slowly to the
predetermined level. This results in a sawtooth wave
form which minimizes the system floodback. After
several cycles of continuous dampened amplitude, the
system will operate at the predetermined superheat
with minimum suction line fluctuations (anti-hunt).
The bottom graph illustrates the operation of a non-
ballast bulb charge. Since it will respond quickly in an
opening and closing manner, the valve may overfeed
and underfeed causing undesirable system fluctuation
referred to as hunt.
6
4
2
8
10
12
14
Ballast Bulb
(Superheat °F)
Time
Operating Superheat Variation of a Typical Refrigeration
System
A
B
Bulb Response to Temperature Change
Bulb Temperature
Closing Opening Time
Bulb Response to Temperature Change
BULB COLDER THAN
BALLAST BLOCK.
BALLAST BLOCK
COLDER THAN BULB.
2
4
6
8
10
12
14
Non-Ballast Bulb
(Superheat °F)
Operation of a Non-Ballast Bulb Charge