Some veteran industrial refrigeration practicioners will state flatly that direct expansion should never be used with ammonia. This absolute condemnation of direct expansion for ammonia air coils should be challenged, because some of the unsuccessful attempts of the past can be prevented by proper coil design.
Furthermore, a direct-expansion coil operates with a relatively low refrigerant quantity in the coil, and low-charge ammonia systems are the goal of safety conscious designers.
The term direct expansion is synonomous with the use of a superheat controlled expansion valve, as shown in Fig. 6.9. Typical expansion valves of this type require as much as 7°C (12.6°F) to open completely. For low temperature coils the temperature difference between entering air (which is the same temperature of the space) and the refrigerant as listed in Table 6.7 is usually a maximum of 5.5°C (10°F). Consequently, there is not a temperature available that is high enough to open the valve completely. This situation is different than for spaces above the freezing temperature, and certainly for air conditioning where there are large air-to-refrigerant temperature differences. A blanket statement, then, that direct expansion should not be used for low temperature coils is normally good advice.
But even for the potential application of direct expansion in above-freezing applications, the coil must be properly designed24. Because of the high latent heat of ammonia, the flow rate to achieve a given refrigerating capacity will be low such that with the typical 3/4- or 1-in tubes the refrigerant velocity will be too low. The result of low velocity is that wavy and stratified flow results in which the heat-transfer coefficient is lower than when high velocities cause annular flow. A further requirement is to construct the circuits long enough that a sufficiently high circuit loading results. Reference 24 recommends no larger than 5/8-in tubes and each circuit carrying a refrigeration load of at least 7 kW (2 tons of refrigeration).
Another development that should be watched because of its pertinence to ammonia direct expansion is the availability of oils mutually soluble in ammonia. The coil could then operate in a manner similar to halocarbon installations where a sufficiently high velocity of the vapor leaving the evaporator carries oil back to the compressor.