The variety of systems using 5F,H compressors make it impractical to cover all aspects of hot gas bypass operation. The following guidelines will aid in determining the proper application.
The hot gas bypass valve is basically a pressure regulating valve installed to hold a constant compressor suction pressure. It should operate over as small a pressure range as possible. The normal set point of the valve should be coordinated with cylinder unloaders so that the bypass valve starts to open at a pressure where the last cylinder bank unloads, and is fully open at a slightly lower pressure. Types, ratings and published appli- cation guides for various available valves must be evaluated to determine the proper valve and installation practice for each application.
If a compressor system is to operate down to zero load, the valve capacity should equal compressor capacity when fully unloaded. For systems using multiple evaporators, it may be necessary to use multiple hot gas valves.
Hot gas should be taken from a point as close as possible to compressor discharge and fed through a hot gas solenoid valve and then through a hot gas valve. The hot gas solenoid valve can be controlled by a pressure switch or temperature switch. On compressors equipped with an electrically actuated cylinder unloader, the hot gas solenoid should be wired in parallel with the solenoid that unloads the final cylinder bank so that bypass- ing starts immediately when all cylinders are unloaded.
HOT GAS INJECTION INTO LIQUID LINE — When amount of bypass is small and the evaporator has a low pres- sure drop distribution system and existing system piping does not present problems, hot gas is frequently injected into the liquid line between the thermostatic expansion valve (TXV) and the evaporator. The ideal point for hot gas injection is into the side inlet of a side connection distributor, where inlet is downstream of distributor orifice. If too much hot gas is injected upstream of a distributor orifice, gas binding and erratic expansion valve operation will result. Injection into liquid line is recommended whenever practical, since agitation in the evaporator and normal operation of the TXV will tend to thoroughly desuperheat injected hot gas and prevent compres- sor overheating.
HOT GAS INJECTION INTO COMPRESSOR SUCTION — Hot gas injection into compressor suction is sometimes neces- sary but must be done with caution to ensure sufficient desuperheating of hot gas and to prevent liquid slugging in the compressor. Following guidelines should be observed:
1.Inject hot gas as close as possible to the evaporator outlet.
2.Install a TXV bulb at least 3 or 4 ft (further if possible) downstream from the hot gas injection point to ensure good gas mixing before the bulb.
3.Install a separate small TXV to inject liquid refrigerant into the suction line along with bypass gas. This valve should have capacity approximately 25% of hot gas valve capacity since hot gas must be superheated but not condensed.
4.Install a suction (knockout) drum in the suction line immediately before the compressor and downstream of the hot gas inlet and liquid injection inlet. Only larger industrial systems or systems with many remote evapora- tors can normally justify the extra expense of injecting hot gas into the compressor suction.
Motor Selection Data — Motor selection data based on brake horsepower occurring at design operating condition is usually satisfactory for applications in air conditioning suction temperature range.
Required compressor starting torque is dependent on dis- charge pressure as well as pressure differential occurring during
In selection of a motor, the required motor starting torque must exceed the compressor starting torque only when the compressor is operating at same speed as the motor. If com- pressor speed is less than motor speed, as on some belt drive units, the motor starting torque requirements are reduced in proportion to the speed ratio between the compressor and motor because of mechanical advantage available to the motor.
In special applications or systems where there is a large pulldown requirement, the bhp requirement during pulldown may significantly exceed bhp at design conditions. The motor must not be overloaded during pulldown operation. If the motor is sized for pulldown, it will be only partially loaded during design operation and will run inefficiently. Therefore, select a motor that will be optimized for system design require- ments and not for pulldown requirements. Two ways for handling this are:
1.Install a crankcase pressure regulator in the system to maintain a given saturated suction temperature, thereby controlling bhp requirement, or
2.Install a current sensing device so that the motor current draw does not exceed the maximum rated motor current.
Drive Packages — Table 20 indicates drive package components for 5F,H standard belt drive packages. Figure 17 and Tables 21 and 22 indicate data for the flywheel used in each of these packages.
Table 19 — Compressor Starting Torques
| % |
|
| SATURATED DISCHARGE TEMPERATURE (F) |
|
| |||||
|
| 80 F |
|
| 100 F |
|
| 120 F |
| ||
COMPRESSOR | UNLOADING |
|
|
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|
|
|
|
|
| |
SIZE | DURING | ||||||||||
| STARTING |
|
|
| |||||||
|
|
|
|
| Maximum Starting Torque |
|
|
| |||
5F20 | None | 19 | 30 | 32 | 27 | 42 | 45 | 34 | 53 | 57 | |
5F30 | None | 22 | 34 | 37 | 30 | 47 | 50 | 39 | 61 | 65 | |
5F40 | 75 | 18 | 28 | 30 | 25 | 39 | 42 | 32 | 50 | 53 | |
5F60 | 662/ | 22 | 34 | 37 | 30 | 47 | 50 | 39 | 61 | 65 | |
| 3 |
|
|
|
|
|
|
|
|
| |
5H40 | 75 | 42 | 65 | 70 | 57 | 89 | 95 | 74 | 115 | 123 | |
5H46 | 75 | 53 | 81 | 87 | 71 | 111 | 119 | 92 | 144 | 154 | |
5H60 | 662/ | 51 | 79 | 85 | 69 | 107 | 115 | 90 | 140 | 149 | |
5H66 | 3 | 64 | 99 | 106 | 86 | 134 | 144 | 113 | 175 | 186 | |
662/ | |||||||||||
| 3 |
|
|
|
|
|
|
|
|
| |
5H80 | 75 | 58 | 90 | 96 | 79 | 123 | 130 | 102 | 158 | 169 | |
5H86 | 75 | 73 | 113 | 120 | 99 | 154 | 162 | 127 | 197 | 212 | |
5H120 | 662/ | 91 | 141 | 151 | 123 | 191 | 204 | 160 | 249 | 266 | |
5H126 | 3 | 114 | 176 | 189 | 154 | 239 | 255 | 200 | 311 | 332 | |
662/ | |||||||||||
| 3 |
|
|
|
|
|
|
|
|
| |
27