Carrier 09RH specifications Selected Example

Discharge Valve Springs — When 5H compressors are used for booster applications where discharge pressure is below 10 psig, the standard discharge valve springs furnished with the machine should be replaced with an equal number of lighter weight springs, Part Number 5H41-1801.

No change in discharge valve springs is recommended for

5F compressors.

Water-Cooled Heads — Standard 5F,H compressors are not equipped with water-cooled heads but they are avail- able on special order. Water cooling of heads is generally not necessary in R-12 or R-502 booster applications. For applica- tions with R-22 involving high compression ratios, 5 or above, 5F,H booster compressors should be equipped with water- cooled heads.

Motor Selection Data — In staged refrigeration sys- tems, the high stage compressor starts first and runs until low stage pressure has been reduced to a predetermined level before the low stage machine starts. With direct staged arrange- ments, the high stage machine draws gas from the evaporator through low stage machine bypass during this initial period. Size of the selected motor must be related to the maximum condition at which booster compressor can operate.

Compressor may run under heavy loads during periods of high suction pressure, especially on starting when system is warm. To handle these situations the motor must be sized larger than the actual balanced operation brake horsepower indicates, or special attention must be paid to operation of the system when starting initially. Tables 25-27 give balanced brake horse- power values at 1750 rpm.

If the system is to operate only at a fixed low temperature, it is possible to avoid oversizing of motors providing careful op- eration is followed when the system is first put in operation.

On applications requiring reduction from ambient condi- tions to some extremely low temperature, the compression system will be operated at high suction pressures for consider- able periods of time. General practice is to drive the high stage compressor with a motor that will operate compressor at the highest expected evaporator temperature. This is generally the “air conditioning” rating of unit. For intermediate or low stage compressors, it is generally sufficient to size motor to take care of double the balance load indicated horsepower plus friction horsepower.

Also consider compressor starting torque requirements when selecting motor for a booster compressor. Starting torque of a motor only large enough to provide required normal operating bhp for booster applications may not be large enough to start the compressor. Recommended minimum motor sizes shown in Table 28 have been selected to assure adequate starting torque. Actual motor size selected is usually larger, depending on the maximum bhp conditions under which the compressor will run during pulldown or other abnormal operat- ing periods.

It is good practice to select motors with allowance for 10% voltage reduction unless there is a certainty that this cannot occur.

Compressor Starting Torque — Required compres- sor starting torque is dependent on the discharge pressure as well as the pressure differential occuring during start-up. Maximum expected torque required during the starting period for 5F,H compressors, used as boosters, is shown in Table 28 at 2 saturated discharge temperatures.

Selection Procedure — Selection of a 5F,H booster compressor requires that the load, saturated suction tempera- ture, saturated discharge temperature, type of system and refrigerant are known.

After the saturated intermediate temperature is determined from Fig. 21, the booster rating (Tables 25-27) can be entered and the compressor selected. Low stage load is then multiplied by the “R” factor from Table 23 to obtain high stage compres- sor load. With this information, the Compressor Ratings tables on pages 7-15, and page 17 can be entered and the high-stage compressor selected.

SELECTED EXAMPLE:

Given:

Refrigeration Load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.7 tons Saturated Suction Temperature . . . . . . . . . . . . . . . . . . . . . –60 F Saturated Condensing Temperature . . . . . . . . . . . . . . . . . . .80 F Open-Type Intercooler

Refrigerant 22

Find: Compressor size and motor size.

Solution:

1.Figure 21 indicates an optimum saturated intermediate temperature of –2 F. Allow a 1 degree or 2 degree drop from the booster compressor to intercooler and from the intercooler to the high stage compressor.

Booster Saturated Suction Temperature = –60 F Booster Saturated Discharge Temperature = 0° F

2.At –60 F suction and 0° F discharge, the 5H60 booster compressor has a capacity of 6.8 tons with 12.1 bhp input at 1750 rpm.

The safety factor at 1750 rpm:

( 6.85.7 ) x 100 – 100 = 19.3 or 20%

This is satisfactory from Fig. 19 and a 5H60 compressor is selected.

3.Indicated hp (ihp) = bhp – Friction hp (fhp)

Where bhp is given in Table 26 and fhp is given in Table 28.

Indicated hp (ihp) = 12.1 – 3.07 = 9.03 Recommended minimum hp

=(2 x ihp) + fhp

=(2 x 9.03) + 3.07 = 21.13

Tentatively select a 25-hp motor. Assume that low stage will never start against a saturated discharge higher than 30 F. At 30 F discharge, Table 28 indicates a starting torque of 54 lb-ft. Therefore, a normal starting torque 25-hp motor is selected.

4.With –60 F suction and 0° F discharge, Table 23 indicates an “R” value of 1.303. Therefore, the high stage load is:

1.303 x 6.8 = 8.86 tons (actual load)

5.Allowing a 1 degree drop from the intercooler, the high stage saturation suction temperature is –3 F.

Allowing a 2 degree drop between the compressor and condenser, the high stage saturated discharge temperature = 80 + 2 = 82 F.

6.Referring to the 5F,H Compressor Ratings table, 5F60 at 1450 rpm (using multiplier in compressor capacity notes) has a capacity of 9.21 tons at –3 F suction and 82 F dis- charge (through interpolation). The 5F60 is selected and requires 13.0 bhp at 1450 rpm.

7.Assume that maximum load during pulldown occurs at 50 F suction and 90 F discharge. For this condition, the rating tables (using the multiplier in Step 6) indicate 15.8 bhp, thus a 20-hp motor is selected.

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