<strong>Refrigeration</strong> Division Grasso TABLE OF CONTENTS C.II <strong>Installation</strong> <strong>and</strong> <strong>Maintenance</strong> <strong>Manual</strong> <strong>RC</strong>(U)6 v001.99.01.en
<strong>Refrigeration</strong> Division 3.1 GENERAL Grasso The capacity control, i.e. the decrease in capacity at constant speed <strong>and</strong> under constant operating conditions, takes place stepwise by successively cutting out cylinders in pairs. This is effected by isolating the discharge space direct above the valves for each individual cylinder head from the common discharge chamber in the compressor housing <strong>and</strong> by putting it into communication with the common suction chamber, also located in the compressor housing. This socalled “internal recirculation control” is, apart from the flow resistance in valves <strong>and</strong> channels, free from losses because the cut out cylinders do not compress gas but just circulate it under suction pressure. 3.2 DESCRIPTION OF CAPACITY CONTROL As shown in figure 3.1, each controllable cylinder head cover is provided with a by-pass slide valve (item 9), which can move horizontally forward <strong>and</strong> backward in a guide bush (10) <strong>and</strong> is connected via a push rod (3) to a hot gas operated control piston (5) in a horizontal control cylinder (4). Slide valve, push rod <strong>and</strong> control piston form one integral steel part. Via a three-way solenoid valve (7,8), which is mounted outside on top of the cylinder head cover against the sealing cover (6) of the control cylinder, the space to the left of the control piston can be communicated either to suction pressure (via lines c <strong>and</strong> b, the former connected to the control cylinder <strong>and</strong> the latter to the by-pass chamber 2) or to discharge pressure (via lines a <strong>and</strong> c, the former connected to the permanent discharge chamber 12 in the cylinder head cover). Actually, only line a, coming from the top connection of the solenoid valve, is an external one; the two others b <strong>and</strong> c are internal bores provided in the sealing cover of the control cylinder <strong>and</strong> in the walls of the cylinder head cover. These internal bores are not indicated in the sections of figure 3.1. 3.3 OPERATION OF CAPACITY CONTROL If the solenoid valve is not energized, line a is connected via line c to the control cylinder, so that hot gas from the permanent discharge chamber enters this cylinder <strong>and</strong>, as the diameter of the control piston is substantially larger than that of 3. DESCRIPTION AND OPERATION OF CAPACITY CONTROL the slide valve, the combination piston-rod-valve will move to the rightmost position, causing the slide valve to shut off entirely the permanent discharge chamber from the space direct above the discharge valves (11) <strong>and</strong> the latter to be communicated via a ring of circular openings in the guide bush with the space between control piston <strong>and</strong> slide valve (by-pass chamber), which is in permanent communication with the common suction chamber (1) in the compressor housing. Both cylinders then operate unloaded as a result of the by-pass thus effected. This situation is shown in the upper part of figure 3.1. When the solenoid valve is being energized, line b is connected via line c to the control cylinder, so that there is no pressure difference across the control piston <strong>and</strong> the combination piston-rod-valve is kept in its leftmost position by the pressure prevailing in the permanent discharge chamber <strong>and</strong> exerted onto the righth<strong>and</strong> face of the slide valve. The space direct above the discharge valves then communicates, via the afore mentioned circular openings in the guide bush of the slide valve, with the permanent discharge chamber, so that both cylinders operate normally. This situation is shown in the lower part of figure 3.1. For the st<strong>and</strong>ard capacity control part-load steps of the various compressor types <strong>and</strong> the corresponding energizing sequence of the solenoid valves, refer to the figures 3.2, 3.4 <strong>and</strong> 3.5 for compressor types <strong>RC</strong>6 <strong>and</strong> to figures 3.6, 3.8 <strong>and</strong> 3.9 for compressor type <strong>RC</strong>6W. 3.4 PARTIALLY UNLOADED STARTING In order to be sure that all controllable cylinder pairs always operate fully unloaded when there is no pressure difference between discharge <strong>and</strong> suction, a spring is mounted between each sealing cover <strong>and</strong> control piston to ensure that in that case all control pistons <strong>and</strong> slide valves are kept in their rightmost position, regardless of whether the corresponding solenoid valves are energized or not. On the other h<strong>and</strong>, to create sufficient discharge pressure to operate all available capacity control slide valves when starting the compressor at zero pressure difference between discharge <strong>and</strong> suction, one cylinder head cover of each compressor type is non-controllable, i.e. it has no built-in slide valve <strong>and</strong> control piston, but is v001.99.01.en <strong>Installation</strong> <strong>and</strong> <strong>Maintenance</strong> <strong>Manual</strong> <strong>RC</strong>(U)6 page C1.1
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