Psychrometric charts

[HashMakerOilProducts]

Ok this is going to be a long one. Here is some information about the basics of air. energy, and water vapor. I think it is important for people in our community to understand what they can and cannot do to control a grow room.

 

Introduction

 

If HVAC problems are to properly be diagnosed and designed, it is essential that the Psychometric chart and psychometrics in general be clearly understood. This will review the essential elements of psychometrics (the behavior of mixtures of air and water vapor under different conditions of heat) and the chart that represents it. With this knowledge you will be able to understand many of the HVAC challenges that are certain to arise, as well as anticipate problems before they occur and incorporate them in your design.

 

Like many self study courses, you will only get as much from it as you put into it. Plot each step on your own psychometric chart. BE certain to understand the basics before studying the examples. Try to solve examples for yourself. This is not the type of course you just read through and understand it. YOU HAVE TO DO IT. YES THAT MEANS YOU POTHEAD If you do your best you should be able to understand the basics whether you’re a math guru or not.

 

Disclaimer-Nothing in this document on OZ Stoners should be considered consulting engineering for your specific application - each situation requires individual analysis.

 

Definitions and Concepts

 

It is important to understand the primary concepts and definitions before we begin our study. For some of you with will be review, and for others this will be an introduction. There are many articles on the internet that cover the fundamentals in more detail but here are the minimum essentials.

 

-HVAC --Heating, Ventilation, and Air conditioning

 

-Psychometrics -- The behavior of mixtures of water vapor under varying conditions of heat

 

-Enthalpy ENTH-AL-PEE --Total heat in the air = sensible + latent heat

 

-Sensible Heat --Changes in temperature that do not alter moisture content of air

 

-Latent Heat --Related to level of moisture in the air

 

-BTU (British thermal unit) --The amount of heat that must be added to or subtracted from a pound of water at 60 deg F to affect a temperature change of 1 deg F

 

-BTUH or BH -- BTU's per hour

-MBH -- 1000 BTU's

 

-Ton --One ton equals the amount of heat needed to melt 1 ton of ice in one day or 12,000 BTUH or 12 MBH

 

-Dry bulb Temperature --The temperature reading given by a dry thermometer that gives a direct indication as to the sensible heat content of air

 

-Wet bulb Temperature --The temperature reading form a wetted bulb that gives a direct indication as to the total heat content of the air

 

-Dew Point Temperature --The temperature at which air will begin to release moisture

 

-Relative Humidity(RH)

--The actual amount of moisture in the air expressed as a percentage of the amount of moisture the air is capable of holding

 

-More Technically

-- the amount of water vapor in the air divided by the amount of water vapor the air can hold(at the same temperature and pressure)

 

-- The ratio of the air's vapor pressure to its saturation vapor pressure

 

Example: An air sample is at 50% RH is holding half the moisture it is capable of holding at the same temperature (at dew point)

 

RH is inversely related to temperature for the same moisture level (grains of moisture per pound of dry air) -- warm air can hold more moisture

 

RH is what we sense

 

High RH == mould and sticking

 

Low RH

 

-- Affects electronics, promotes static

-- Low RH air is seeking saturation, absorbing moisture wherever it can

 

-Specific Humidity or Humidity ratio

 

--The weight of the water vapor in each pound of dry air

--Typically grains of moisture/pound of dry air

--grain == 1/7000 pound

 

-Density -- unit weight of dry air at a given temperature and moisture content, #/ft^3

 

-specific volume -- Space occupied by dry air at a given temperature and moisture content (the reciprocal of density), ft^3/#

 

 

If you have made it this far without throwing up I'm surprised <BR>

 

The Psychometric Chart Template

 

Please print off a few of these. Make multiple copies, as you will need them in my course. As noted previously, if you are to benefit from the course, you must plot each step yourself. There are no shortcuts in learning this material. The smaller example charts are not intended for you to plot on but rather instruction -- use a clean large copy to plot each example.

 

 

The Psychometric Chart - an Overview

 

Before we can proceed further in our study, let's learn of refresh regarding the Psychometric chart. The following is a summary of the major elements of the chart. Do not continue to the next section until you can find the following on the chart.

 

-- Constant dry bulb Temperature: Vertical Lines

-- Constant Dew Point and humidity ratio: Horizontal Lines

--Constant Wet bulb temperature: upward left sloping lines

-- Relative humidity: Curving lines (100% line is the saturation curve or correlates with Dew Point)

-- Constant specific volume, ft^3/# of dry air, nearly-vertical; sloping lines

-- Enthalpy or total heat, BTU/Pound of air: Staggered scale left of saturation curve and left sloping lines

-- Humidity ratio: Right hand scale, grains of moisture/pound of dry air

-- Saturation Curve: 100% R curve (or the point at which an air mixture can hold no additional moisture at a given temperature); temperature on the curve of the dew point

 

The following is a chart with the above noted( note: this is a standard chart curve at sea level)

 

I can't find a decent chart I can edit on the internet so I will upload mine while I am at work tommorrow but here is a chart anyways.

McQuayChart.pdf

[HashMakerOilProducts]

Ok so I assume you have read the whole thing and looked at the chart. so here we go...

Let's say you have 85%RH at 74 deg. How cold must our dehumidifier get before we remove water from the air? find the point where temprature and relative humidity meet. follow along the X axis until we reach the dew point. and presto that point is the temp we need to cool to start to drop the humidity. Lets say we want 50% humidity so draw a line down from your start point to 74F and 50 RH%, follow it across to the dew point and we get 55F. thats how much we need to cool the air to remove that much water before we heat it up again.

 

Why is this important? becuase dehumidifiers will not go below 55 F therefore the colder your grow room the less RH you can remove.