Sample Specification H Series
Refrigerated Compressed Air Dryers General Vendor shall supply one packaged, fully assembled refrigerated type dryer. Dryer is to be complete with: heat exchangers, separator, automatic condensate drains and interconnecting piping, and a refrigeration system and controls completely wired, piped and mounted on a structural steel frame ready for start-up after utility connections are made. (Air-cooled models with remote condensers will require refrigeration and electrical connections at site.)
Description An air-to-air heat exchanger shall be provided so that half of the heat load in the incoming air is removed by the outgoing air, minimizing refrigeration capacity and energy requirements. The heat exchanger shall be a shell and tube type with extended surfaces on the outside of the tubes and smooth surfaces on the inside. Inlet air is to be routed through the tubes where a combination of moisture condensation and twisted tape turbulence enhancers increases the heat transfer coefficient. The increased heat transfer coefficient inside the tubes balances the extended surface on the outside of the tubes, increasing heat exchanger efficiency and minimizing pressure drop. Turbulence enhancers shall resist fouling and are to be removable so that tube cleaning, if necessary, can be accomplished. A prefilter shall not be required upstream of the dryer. Dry, filtered outlet air is routed on the shell side to take advantage of the extended surface without threat of fouling. The air-to-refrigerant heat exchanger shall be a shell and tube type with removable heads. Refrigerant is to be routed in multiple passes on the tube side. Cross sectional area of the tubes is to be varied (smaller where the refrigerant is mostly a liquid, greater where it is a gas) to increase the velocity of the refrigerant through the heat exchanger and ensure quick response to changing loads. Twisted tape turbulence enhancers shall be used in gas phase passes to promote refrigerant droplet evaporation. Removable heads are to be baffled, flat head style to minimize oil or liquid refrigerant accumulation. The separator/filter shall be of the filter cartridge type in a top access pressure vessel. Vessel internals are to be painted. Filter cartridges shall contain two stages. The first stage is to be a cleanable, stainless steel separator core for removal of bulk liquids. The second stage is to be a replaceable coalescing type filter sleeve capable of collecting all droplets three microns in size and larger and maintaining this high efficiency at reduced flows. Efficient liquid removal prevents an increase in the outlet dew point by preventing liquid carryover and reevaporation. The second stage shall also act as an in-depth filter media capturing all solid particles three microns in size and larger. All pressure vessels shall be built and stamped in accordance with the ASME boiler and pressure vessel code. Interconnecting piping is to be welded by ASME qualified welders and is to be hydrostatically tested as a complete assembly with all other pressure vessels attached, per that code. Heat exchangers, separator and interconnecting piping shall be fully insulated and the insulation protected with a durable shroud. The dryer is to be supplied with three, rebuildable automatic condensate drains. One drain is supplied for each heat exchanger and one for the separator. Drains are to discharge automatically and are to be routed to a common piping connection at the frame edge. Each drain is to be supplied with an isolation valve. A non-cycling, direct expansion refrigeration system shall be supplied and include Copeland semi-hermetic, direct coupled, suction gas cooled refrigeration compressor(s) with oil pressure pump, oil strainer and oil sight glass (one compressor, 7.5 through 40 HP; two compressors, 50 HP and larger). The control system shall allow automatic and continuous operation at all load conditions from zero (0%) load to full (100%) load while tolerating large load swings, in the specified ambient range, without adjustment. Outlet dew point shall be maintained at ± 3°F (± 1.7°C) within the specified pressure dew point at all loads. A three valve control system shall be supplied and include: a hot gas by-pass valve to control pressure in the evaporator; a thermal expansion valve (TXV) to control superheat; and a liquid injection (desuperheating) valve to control the return gas temperature. Control valves shall be preceded by a strainer and are serviceable without removing valve bodies from the line. By-pass gas shall be routed into the suction piping downstream of the evaporator. This enables a constant evaporator temperature to be maintained by preventing interference with TXV heat load control. It also prevents refrigerant floodback at low loads. Units with 20 HP and larger condensing units shall be supplied with compressor unloaders that work in conjunction with the hot gas by-pass valve to control evaporator pressure.