Carrier XRV Spécifications télécharger pdf (Page 9)

VFD COOLING CYCLE

The unit-mounted variable frequency drive (VFD) is cooled

in a manner similar to the motor and lubricating oil cooling

cycle (Fig. 3).

If equipped with a unit-mounted VFD, the refrigerant line

that feeds the motor cooling and oil cooler also feeds the heat

exchanger on the unit-mounted VFD. Refrigerant is metered

through an orifice. The refrigerant leaving the heat exchanger

returns to the cooler.

LUBRICATION CYCLE

Summary —

The oil pump, oil filter, and oil cooler make

up a package located partially in the transmission casing of the

compressor-motor assembly. The oil is pumped into a filter

assembly to remove foreign particles and is then forced into an

oil cooler heat exchanger where the oil is cooled to proper

operational temperatures. After the oil cooler, part of the flow

is directed to the gears and the high speed shaft bearings; the

remaining flow is directed to the motor shaft bearings. Oil

drains into the transmission oil sump to complete the cycle

(Fig. 4).

Details — Oil is charged into the lubrication system through

a hand valve. Two sight glasses in the oil reservoir permit oil

level observation. Normal oil level is between the middle of the

upper sight glass and the top of the lower sight glass when the

compressor is shut down. The oil level should be visible in at

least one of the 2 sight glasses during operation. Oil sump

temperature is displayed on the ICVC (International Chiller

Visual Controller) default screen. During compressor opera-

tion, the oil sump temperature ranges between 125 and 150 F

(52 and 66 C).

The oil pump suction is fed from the oil reservoir. An oil

pressure relief valve maintains 18 to 30 psid (124 to 207 kPad)

differential pressure in the system at the pump discharge. The

normal oil pressure on compressors equipped with rolling

element bearings is between 18 and 40 psid (124 and

276 kPad). This differential pressure can be read directly from

the ICVC default screen. The oil pump discharges oil to the oil

filter assembly. This filter can be closed to permit removal of

the filter without draining the entire oil system (see Mainte-

nance sections, pages 84 to 88 for details). The oil is then piped

to the oil cooler heat exchanger. The oil cooler uses refrigerant

from the condenser as the coolant. The refrigerant cools the oil

to a temperature between 120 and 140 F (49 and 60 C).

As the oil leaves the oil cooler, it passes the oil pressure

transducer and the thermal bulb for the refrigerant expansion

valve on the oil cooler. The oil is then divided. Part of the oil

flows to the thrust bearing, forward pinion bearing, and gear

spray. The rest of the oil lubricates the motor shaft bearings and

the rear pinion bearing. The oil temperature is measured in the

bearing housing as it leaves the thrust and forward journal

bearings. The outer bearing race temperature is measured on

compressors with rolling element bearings. The oil then drains

into the oil reservoir at the base of the compressor. The PIC III

(Product Integrated Control III) measures the temperature of

the oil in the sump and maintains the temperature during shut-

down (see Oil Sump Temperature and Pump Control section,

page 46). This temperature is read on the ICVC default screen.

During chiller start-up, the PIC III energizes the oil pump

and provides 45 seconds of pre-lubrication to the bearings after

pressure is verified before starting the compressor. During

shutdown, the oil pump will run for 60 seconds to

post-lubricate after the compressor shuts down. The oil pump

can also be energized for testing purposes during a Control

Test.

Ramp loading can slow the rate of guide vane opening to

minimize oil foaming at start-up. If the guide vanes open

quickly, the sudden drop in suction pressure can cause any

refrigerant in the oil to flash. The resulting oil foam cannot be

pumped efficiently; therefore, oil pressure falls off and lubrica-

tion is poor. If oil pressure falls below 15 psid (103 kPad)

differential, the PIC III will shut down the compressor.

If the controls are subject to a power failure that lasts more

than 3 hours, the oil pump will be energized periodically when

the power is restored. This helps to eliminate refrigerant that

has migrated to the oil sump during the power failure. The

controls energize the pump for 30 seconds every 30 minutes

until the chiller is started.

Oil Reclaim System — The oil reclaim system returns

oil lost from the compressor housing back to the oil reservoir

by recovering the oil from 2 areas on the chiller. The guide

vane housing is the primary area of recovery. Oil is also recov-

ered by skimming it from the operating refrigerant level in the

cooler vessel.

PRIMARY OIL RECOVERY MODE — Oil is normally re-

covered through the guide vane housing on the chiller. This is

possible because oil is normally entrained with refrigerant in

the chiller. As the compressor pulls the refrigerant up from the

cooler into the guide vane housing to be compressed, the oil

normally drops out at this point and falls to the bottom of the

guide vane housing where it accumulates. Using discharge gas

pressure to power an eductor, the oil is drawn from the housing

and is discharged into the oil reservoir.

SECONDARY OIL RECOVERY METHOD — The sec-

ondary method of oil recovery is significant under light load

conditions, when the refrigerant going up to the compressor

suction does not have enough velocity to bring oil along. Under

these conditions, oil collects in a greater concentration at the

top level of the refrigerant in the cooler. This oil and refrigerant

mixture is skimmed from the side of the cooler and is then

drawn up to the guide vane housing. There is a filter in this line.

Because the guide vane housing pressure is much lower than

the cooler pressure, the refrigerant boils off, leaving the oil

behind to be collected by the primary oil recovery method.

Bearings — The 19XRV compressor assemblies include

four radial bearings and four thrust bearings. The low speed

shaft assembly is supported by two journal bearings located

between the motor rotor and the bull gear. The bearing closer to

the rotor includes a babbitted thrust face which opposes the

normal axial forces which tend to pull the assembly towards

the transmission. The bearing closer to the bull gear includes a

smaller babbitted thrust face, designed to handle counterthrust

forces.

For most 19XRV compressors the high speed shaft assem-

bly is supported by two journal bearings located at the

transmission end and mid-span, behind the labyrinth seal. The

transmission side of the midspan bearing also contains a tilting

shoe type thrust bearing which opposes the main axial forces

tending to pull the impeller towards the suction end. The

impeller side face of the midspan bearing includes a babbitted

thrust face, designed to handle counterthrust forces.

For 19XRV Frame 3 compressors built since mid-2001, the

high speed shaft assembly has been redesigned to utilize rolling

element bearings (radial and thrust). Machines employing the

rolling element bearings can be expected to have higher oil

pressure and thrust bearing temperatures than those compres-

sors using the alternate bearing design.

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