Cold Link Africa VOL. 01 - No.01 | September / October 2015

TECHNICAL INCORPORATING COLD CHAIN The reduced liquid to suction line temperature difference reduces suction/ liquid heat interchanger capacity. This is especially so if it is intended that saturated refrigerant pressures are dropped even further below the already speculated 20oC, which is most often the case. Optional heat interchangers required for these low liquid to suction temperature differences may be so large as to make their use wholly impractical. Liquid Pressure Amplification (LPA) Primarily LPA is about achieving floating head pressures. On DX systems a liquid line pump is added to the liquid line in order to maintain expansion device design operating pressure difference while head pressures is permitted to float with colder ambients. The pump also provides added sub-cool, by pressure amplification, needed to match the increased saturated refrigerant ∆T/∆P ratio. Of course, with LPA, a small additional charge quantity may be necessary to ensure at least 1K sub-cooling at pump inlet. However, this charge addition is far less than the extra charge needed to achieve required sub-cool for the same liquid line pressure losses without the use of a pump. The inclusion of a liquid line pump can provide for re-condensing of the liquid line vapour portions mentioned earlier in this article. In addition, it obviously raises liquid pressure, which in most cases acts to reestablish design EEV and TEV pressure drop. The advantage here then is that in many instances the cheaper and more proved TEV could be used in preference to the EEV. A carefully sized capillary can be installed parallel to the TEV to supplement TEV orifice size when design valve pressure drops are still not being achieved. Even when floating head pressures there are still often head pressure constraints requiring head pressure control only at some lower limit. Considering the colder liquids increased saturated ∆T/∆P ratio, head pressure controls cycling in and out can cause a ‘frothing’ of the liquid line again reducing EEV and TEV capacity. The liquid pump will most often clear this liquid line frothing. Acknowledgement Thanks to Dave Nicholls of Afcon, Durban, for submission of the article. Hysave liquid pressure pumps have been around for some years; as a matter of fact, the late Lynn Thompson introduced them to South Africa about 28 years ago. “As can be seen from the article, unless the quality of liquid reaching the EEV or TEV is good, the money spent on EEVs is largely wasted,” said Nicholls. The full version of this abridged article can be viewed via the website: www.hysave.co.uk. CLA know what’s happening when it’s happening INCORPORATING COLD CHAIN e-newsletter go to www.coldlinkafrica.co.za to subscribe COLD LINK AFRICA • September | October 2015 www.coldlinkafrica.co.za 43

Cold Link Africa VOL. 01 - No.01 | September / October 2015 | Page 43