How
to Use a Two-Column Pressure-Temperature Chart
Because
the properties of the new zeotropic refrigerant blends are
different than traditional refrigerants it's useful to know
how to read a two-column PT chart
By Jim Lavelle
The pressure-temperature
(PT) chart is a valuable tool that service technicians
use to check proper system operation. PT charts are most
often used for three purposes: to set a coil pressure so
that the refrigerant produces the desired temperature,
to check the amount of superheat above the saturated vapor
condition at the outlet of the evaporator and to check
the amount of subcooling below the saturated liquid condition
at the end of the condenser.
The saturated
liquid condition is often referred to as the bubble point.
Imagine a pot of liquid sitting on a stove; as it begins
to boil it forms bubbles in the liquid. The saturated vapor
condition is referred to as the dew point. Imagine a room
full of vapor and dew drops forming on the furniture. PT
charts for the zeotropic blends list two columns next to
each temperature: one for the saturated liquid (bubble
point) and the other for the saturated vapor (dew point).
Some of the zeotropic
blends have very low glide (from I° F to 2.5° F).
For these blends, the vapor and liquid pressures are only
separated by 1 or 2 psi. Because the difference is quite
small between the two values some manufacturers' PT charts
will only list one column for these blends. Blends with
higher glide (greater than 5° F) will generally have
both columns listed.
Using
a two-column PT chart
When checking a superheat or subcool temperature the procedure is the same as
for a single-component refrigerant. Superheat is checked by measuring the temperature
of the vapor line, measuring the pressure, then subtracting the saturated temperature
from the measured temperature. In the case of a blend, you Simply read the saturated
temperature next to the pressure in the vapor (dew point) column of the chart.
When
checking the subcool condition the technician will measure
the temperature of the liquid line, the pressure at that
point and subtract the measured temperature from the saturated
temperature at the end of the condenser. With the blend
you read the saturated temperature next to the pressure
in the liquid (bubble point) column of the chart.
For a
single-component or azeotropic refrigerant, the operating
pressure for the low side of a system can be found by cross
referencing the desired coil temperature on the PT chart.
For high-glide blends, however, the desired coil temperature
is the average (or midpoint) temperature of the coil.
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The
problem with two-column PT charts is that the conditions
at the endpoints of the temperature glide are listed,
not the midpoint. In this case, you must add half of
the temperature glide to the desired midpoint temperature,
then read the saturated vapor column to determine the
operating pressure (see Figure 3).
If
the vapor column is read directly at the desired
temperature, then the end of the evaporator will
be the correct temperature, but the re st of the
coil will be too cold. If the liquid column is used
directly, then the beginning of the coil will be
the correct temperature, but the rest of the coil
will be too warm.
Two-column
PT charts are every bit as useful as the traditional
ones. The charging and service procedures are very
similar for both single-component refrigerants and
zeotropic blends, and the specialized liquid and
vapor data correct for the effects of the blends'
temperature glide.
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Just
remember to keep track of the phase of the blend at the
point you are interested: saturated vapor uses the vapor
(dew point) column and saturated liquid uses the liquid
(bubble point) column.
Jim
Lavelle is the technical sales manager at National Refrigerants,
Inc.
For more information, call 800-262-0012 or send e-mail to info@refrigerants.com.
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