KRA Refrigeration System Installation, Operation & Maintenance Manual
L-3571 ENGLISH
5
C
ONDENSING
U
NIT
All units have an R-404A compressor, air-and-water-cooled or water-cooled only titanium condenser, a receiver/dryer, and a
sight glass.
The KRA condensing unit comes in an enclosed, painted aluminum box. It is available with a 1/3 HP or 1/2 HP compressor.
There are no service valves. The unit includes low-loss quick-connect couplings, a moisture indicating sight glass, a receiver
and filter/dryer and a sufficient charge of R-404A for almost all applications. Condenser cooling is either air-and-water-cooled
(slanted-top models) or water-cooled only (flat-top model).
•
Air cooling is beneficial because the unit can operate even with the boat out of water.
However, the unit suffers a
drop in performance in ambient temperatures above 100°F/38°C where the unit is located, and the compressor
may be damaged.
•
Water-cooled units are not affected by ambient temperature and do not need ventilation. Water flow should be a
minimum of 1.5 GPM for the KRA 433X, and 2.0 GPM for the KRA 450X.
•
The combination air-and-water-cooled units provide the best of both applications.
E
VAPORATOR
The evaporator will be either stainless steel plate(s), or a wrapped box in which the evaporative tubing is located against the
outside surface of the box. See Figure 2: Various Flat-Plate Evaporators, page 16 and Figure 3: Wrapped-Box Evaporator,
page 16.
Stainless plates should be designed to cover about 30% to 40% of the box’s interior surface area for good freezer performance.
For instance, a box that is 36" long, 16" deep and 20" wide (92 cm x 41 cm x 51 cm), should use plates that are about 36" x 16"
(92 cm x 41 cm). This calculates to about 35% of the box’s interior surface area. Less plate area is needed for a box that is to be
only a refrigerator.
Box design is extremely important and requires considerations that are mostly outside the scope of this manual. For the power
boat industry, where constant power is available, evaporators must have a minimum of 2" (5 cm) of insulation (R7 per inch) for a
freezer. When power availability is not constant, 4" (10 cm) of R7 insulation is required. It is best to use as much insulation as is
practical. Call the Dometic Applications Department for advice on larger sized boxes, or if you are unsure.
S
UCTION
L
INE
The suction line returns refrigerant gas from the evaporator to the condenser and should be fully insulated.
L
IQUID
L
INE
The liquid line is the bare, 1/4" (7 mm) copper line. You must install the filter dryer assembly (packaged with the unit) at the KRA
unit on the liquid line female quick-connect fitting or the warranty will be voided.
R
EFRIGERANT
M
ETERING
D
EVICE
Refrigerant metering is handled by a thermostatic expansion valve (TXV) that is externally equalized. This is used to
compensate for varying installations. The TXV regulates the refrigerant flow into the evaporator depending on load. The valve is
either set at the factory or at installation, and does not require further adjusting. A valve which has been severely misadjusted
can damage the condensing unit, and will void warranty.
The metering valve must be insulated airtight!
The TXV has a sensing bulb that must be firmly attached and thoroughly
insulated to the suction line leaving the box. Bulb placement is important. Place at 3:00 to 9:00 o’clock on the side of the suction
line.
The TXV controls the flow rate of liquid refrigerant entering the evaporator in response to the superheat of the refrigerant gas
leaving the evaporator. It keeps the entire evaporator active, without permitting unevaporated refrigerant liquid to be returned
through the suction line to the compressor. The function on the TXV is to feed liquid into the evaporator coils at a rate that will
keep liquid evaporating in all the coils, and to control that feed so that only vapor will reach the outlet (return line to the
compressor).
T
HERMOSTAT
A thermostat controls the operation of the condensing unit, based on box temperature. An easy-to-use digital Dometic
thermostat control is required for the installation, and digital input must be enabled (see “Parameter Programming” on page 11).
No controls other than the Dometic digital control should be used; other controls are not supported.
WARNING
Do not drill through the inside of a wrapped box evaporator. If the refrigerant tubing is punctured by doing so, it will
not be covered under warranty.
