Low recirculation rate evaporators

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Courtesy of Evapco Inc.

Abstract
Fin coil evaporators with enhanced internal tube surfaces allow for optimum performance with a minimal overfeed rate. Reducing the overfeed rate requiresless pumping power and smaller line sizes to the evaporator. Additionally, less compressor power is required for the same refrigeration capacity because the wet suction line pressure drop is reduced. Lower compressor power results in substantial electrical power savings over the life of the facility. With internal tube enhancement, top-fed evaporators can perform as well as bottom-fed evaporators while using a reduced refrigerant charge during full and part load operation.

Design history
Forced-air, tube, and fin evaporators have been used to refrigerate air for more than 75 years. In the 1960s, several papers documented the performance advantages of pumping ammonia through tubes with an excess amount of liquid, and pumped overfeed evaporators have been the popular choice for industrial ammonia refrigeration systems ever since.

Due to the high latent heat of ammonia, the quantity of liquid that enters an evaporator tube is very small, often occupying less than 2% of the cross-sectional area of the tube. For efficient heat transfer to result in optimum performance, excessliquid ammonia is pumped into the evaporator tube. This excessliquid “sloshes” around the inside perimeter of the tube ensuring a liquid ammonia film coats the inside of the tube to absorb large amounts of heat. If not for this excess liquid, more areas of the inside of the tube would dry, greatly diminishing the performance of the evaporator.

Tube and fin evaporators are generally multiple tubes high, and of a set length, to establish the face area of the evaporator. Typically, 6 to 12 rows of tubes are in the direction of airflow. Determining which of these tubes are in series and which of these tubes are in parallel relative to the ammonia flow, defined as circuiting, and is the job of the evaporator designer. Two competing factors must be balanced in any circuiting choice. A longer circuit has more surface area so it will absorb more heat and vaporize more ammonia, creating a higher velocity within the tube. Higher-velocity vapor is more turbulent, which distributes the liquid ammonia via agitation against the full inner perimeter of the tube wall. A shorter circuit has less surface area and does not absorb as much heat or vaporize as much ammonia, resulting in less turbulence and ammonia liquid that settles along the bottom the tube. This leaves the upper portions of the inner tube perimeter dry, so no high heat transfer rate due to boiling can occur, and greatly reduces the evaporator performance.

However, the design factor to balance against velocity and turbulence is pressure drop. If the saturated suction temperature at the outlet of the circuit is -25 °F, a 1.0 psi internal circuit pressure drop will result in a saturated evaporating temperature of -23 °F at the entrance of the circuit. This is an 11% loss of log mean temperature difference with -15 °F air on temperature. Bear in mind that temperature difference drives heat transfer.

Two factors, velocity and mass flow, primarily drive pressure drop within the circuit. Pressure drop is roughly proportional to the velocity squared. Therefore, in light of the previously described temperature penalties, pressure drop within the circuit must be great enough to establish turbulent flow, but not in excess of that. Circuit designers target a lower velocity with less pressure drop by adjusting circuit length or limiting the amount of excess liquid, referred to as overfeed or recirculation rate. Recirculation rate is defined as the total mass flow into the circuit or evaporator relative to the evaporated mass flow. As an example, three times the evaporation rate entering the evaporator is expressed as 3:1 recirculation rate.

Contemporary evaporator designs typically call for 4:1 or 3:1 recirculation rates at the higher temperatures (40 °F to 0 °F) and favor 3:1 or 2.5:1 at the lower temperatures (-20 °F to -45 °F). The pressure/ temperature relationship of ammonia at the lower temperatures discourages higher pressure drops within the circuit due to the large effect on the evaporating temperature.

In addition to wetting the inside perimeter of the tube, the thermal conductivity of the tube and fin material must be considered. For many years ammonia evaporators were constructed of carbon steel and hot dip galvanized after assembly, or they were aluminum tubes expanded into aluminum fins. In both instances the fins and the tubes have the same thermal conductivity. Heat transfer occurs equally along the perimeter of the tube and from the surrounding fin. Carbon steel has a thermal conductivity of 21 Btu/hr ft °F and that of aluminum is 118 Btu/hr ft °F.

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