User Guide

ManualsBrandsSporlan ManualsBF/EBF/SBF SeriesSBFSE-AA-Z 3/8" x 1/2" ODF Thermal Expansion Valve w/ 30" Capillary (1/4 to 1/3 Ton)

Figure 4 shows the same internally equalized valve on a

system having the same evaporator pressure at the sens-

ing bulb location. The evaporator coil, however, now has a

pressure drop of 6 psi. Since an internally equalized valve

senses evaporator pressure at the valve outlet, the total

pressure in the closing direction becomes 58 psig plus the 12

psi spring pressure, or 70 psig. A bulb pressure of 70 psig is

now required for proper valve regulation, which translates

to a 41°F bulb temperature. The superheat becomes 13°F, or

4°F higher than the superheat calculated in Figure 3. This

rise in superheat is due to the pressure drop in the evapora-

tor. Consequently, pressure drop between the valve outlet

and the sensing bulb location causes an internally equalized

TEV to operate at a higher than desired superheat.

Page 6 / BULLETIN 10-9

Figure 5 shows the same system as in Figure 4, but with

an externally equalized TEV installed. Since an externally

equalized TEV senses evaporator pressure at the evapora-

tor outlet, it is not influenced by pressure drop through the

evaporator. As a result, the TEV senses the correct pressure,

and controls at the desired superheat.

These diagrams can be used to show the influence evapo-

rator pressure drop has on internally equalized TEVs as

evaporating temperatures fall. Table 1 provides general

recommendations for maximum pressure drops that can be

safely tolerated by internally equalized valves. These recom-

mendations are suitable for most field installed systems.

Use externally equalized TEVs when pressure drops exceed

values shown in Table 1, or when pressure drops cannot be

determined. An externally equalized TEV should be

used whenever a refrigerant distributor is used with

the evaporator.

Refer to Bulletin 10-11, TEV Installation, Field Service and

Assembly, regarding recommendations for the location of

the sensing bulb and external equalizer connection to the

suction line.

Thermostatic Charges

As previously mentioned, the TEV’s sensing bulb transmits

pressure to the top of the diaphragm by a length of capillary

tubing. The thermostatic charge is the substance in the

TEV’s sensing bulb which responds to suction line tempera-

ture to create the bulb pressure, and it is designed to allow

the TEV to operate at a satisfactory level of superheat over

a specific range of evaporating temperatures. The subject of

thermostatic charges is best approached by describing the

categories into which charges are classified. These catego-

ries are the following:

1. Liquid Charge

2. Gas Charge

3. Liquid-Cross Charge

4. Gas-Cross Charge

5. Adsorption Charge

The conventional liquid charge consists of the same refriger-

ant in the thermostatic element that is used in the refrig-

eration system, while the liquid-cross charge consists of a

refrigerant mixture. The term cross charge arises from

the fact that the pressure-temperature characteristic of the

refrigerant mixture used within the sensing bulb will cross

the saturation curve of the system refrigerant at some point.

Both the liquid and liquid-cross charges contain sufficient

liquid such that the bulb, capillary tubing, and diaphragm

chamber will contain some liquid under all temperature condi-

tions. This characteristic prevents charge migration of the

thermostatic charge away from the sensing bulb if the sensing

bulb temperature becomes warmer than other parts of the

thermostatic element. Charge migration will result in loss of

valve control. An additional characteristic of these charges is

their lack of a maximum operating pressure (MOP) fea-

ture. A thermostatic charge with an MOP feature causes the

TEV to modulate in the closed direction above a predetermined

evaporator pressure, thereby restricting flow to the evaporator

and limiting the maximum evaporator pressure at which the

system can operate.

Similarly, the gas charge consists of the same refrigerant in

the thermostatic element that is used in the refrigeration

system, while the gas-cross charge consists of a refrigerant

mixture. Unlike the liquid type charges, both gas charges are

distinguished by having a vapor charge in the thermostatic

element which condenses to a minute quantity of liquid when

41°

Bulb Pressure

70 psig

Evaporator

Inlet Pressure

58 psig

Evaporator

Outlet Pressure

52 psig

Diaphragm

Spring Pressure

12 psi

Closing Pressure............................................................................= 58 + 12 = 70 psig

(Evaporator Inlet Pressure Plus Spring Pressure)

Bulb Pressure Necessary to Open Valve.........................................................70 psig

Bulb Temperature Equivalent to 70 psig................................................................41°F

Saturated Temperature Equivalent to Evaporator Outlet Pressure..........................28°F

SUPERHEAT....................................................................................................13°F

Bulb Temperature Minus Saturated Evaporator Temperature

Converted to Temperature = 41°F

37°

Bulb Pressure

64 psig

Evaporator

Inlet Pressure

58 psig

Evaporator

Outlet Pressure

52 psig

Diaphragm

Spring Pressure

12 psi

Closing Pressure.............................................................................= 52 + 12 = 64 psig

(Suction Pressure at Bulb Plus Spring Pressure)

Bulb Pressure Necessary to Open Valve..........................................................64 psig

Bulb Temperature Equivalent to 64 psig.................................................................37°F

Saturated Temperature Equivalent to Evaporator Outlet Pressure...........................28°F

SUPERHEAT............. ......................................................................................... 9°F

Bulb Temperature Minus Saturated Evaporator Temperature

Suction Pressure

at Bulb 52 psig

Converted to Temperature = 37°F

Figure 5

Figure 4