A large evaporator coil has been determined to have a pressure drop exceeding 5 psig. How can this be compensated for in maintaining 10° of evaporator superheat as the system requires and avoid starvation of the evaporator? adjusting the TXV spring compression and raising the super heat value installing an externally equalized TXV
• Thermostatic Expansion Valve (TXV) operation and purpose • Difference between internally equalized and externally equalized TXVs, especially on large coils with pressure drop • Effect of pressure drop across the evaporator on bulb sensing and superheat control
• If there is a significant pressure drop through the evaporator, what pressure does an internally equalized TXV actually "see" when it tries to control superheat? • How does changing the spring compression on a TXV affect the superheat setting, and does that fix a pressure-drop problem or only change the target superheat? • Why might a large evaporator with more than 5 psi pressure drop be specified to use a particular type of TXV equalization?
• Check how superheat is defined (where temperature and pressure are measured in relation to each other). • Verify what condition typically requires an externally equalized TXV (hint: location of equalizer line and coil outlet pressure). • Determine whether adjusting TXV spring tension can compensate for coil pressure drop or if it just shifts the superheat setting without correcting the sensing error.
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