Effectiveness (epsilon) is the ratio of actual heat transferred to the maximum thermodynamically possible:<\/p>\n
It is the number you see quoted as “70% efficient” for a recovery core, and it is directly related to the \u52b9\u7387\u6307\u6a19<\/a> reported for air-to-air units.<\/p>\n The Number of Transfer Units scales with core size and inverse with flow:<\/p>\n For a counter-flow plate core the effectiveness is approximately:<\/p>\n with Cr = Cmin \/ Cmax. Raise NTU (bigger or slower core) and effectiveness rises toward its limit. Cross-flow cores sit a little lower than counter-flow for the same NTU – a design trade-off covered under \u5727\u529b\u640d\u5931<\/a>.<\/p>\n Given flow rates, fluid properties and a target effectiveness, the epsilon-NTU relation sizes the required UA – the bridge between a performance spec and a physical \u30d7\u30ec\u30fc\u30c8\u71b1\u4ea4\u63db\u5668<\/a>.<\/p>\n Is higher NTU always better?<\/strong> It raises effectiveness but also core size, material and fan power; designs stop where payback peaks.<\/p>\n What is Cr?<\/strong> The capacity ratio; when both streams are equal air flows, Cr is near 1 and effectiveness is most sensitive to NTU.<\/p>\nepsilon = actual_heat_transfer \/ (Cmin x (Thot_in - Tcold_in))<\/code><\/p>\nNTU: the size-and-flow term<\/h2>\n
NTU = (U x A) \/ (m_dot x cp)<\/code> where UxA is the overall conductance and m_dot x cp is the smaller heat capacity rate (Cmin).<\/p>\nFrom NTU to outlet temperatures<\/h2>\n
epsilon = (1 - exp(-NTU x (1 - Cr))) \/ (1 - Cr x exp(-NTU x (1 - Cr)))<\/code><\/p>\nWhy engineers use it<\/h2>\n
Frequently asked questions<\/h2>\n