Cooling the Sponge

The property of humid air is a very interesting subject. Yet for many it’s a subject that still not fully understood. And its one of those, you just cant learn from reading about it alone but need to be explained to you in person and practiced to be able to be a competent HVAC Designer/Engineer.

Mr Google can fish out for you many articles and videos related to the subject matter. Therefore I will not attempt repeat the same jargon that’s out there. Instead I will try explain in layman’s term and try to simplify the process of how air behave as gently as I can. So buckle up and lets fly through ins and outs of air.

Air is exactly like your kitchen or bathroom sponge. It carry's water or moisture, if you prefer, inside its molecules. When air is heated ( or I should say gain heat) ?it expands and become a bigger sponge therefore its capacity to carry/hold moister naturally increases. And when its cooled (heat is lost) ?air shrinks therefore its capacity to carry moisture reduces. So air, basically, is a naturally expandable water tank.

Air, also, carry heat as well as moister, the higher the ?air temperature?the higher the heat content and the higher the moisture content also the higher the heat content. How is this possible?

The heat that causes air temp to change is called Sensible heat while the heat related to change in moisture content is called Latent heat. ?So sensible heat , gained or lost, affect the air temperature but latent heat do not affect air temperature. Because heat energy is stored in the moisture so it can only be transferred through evaporation (moisture gain) or condensation (moisture loss). However any medium used to cool the air will have to have enough cooling capacity to absorb both sensible and latent heat. That’s why when establishing any cooling load it must be expressed in Sensible and Total ( Sensible and latent) because both are needed to select the right equipment. Am always astonished seeing AC equipment specifying a singular figure of a cooling load. When air is heated only sensible heat need to be considered as moisture and it heat are absorbed naturally from surrounding surfaces and areas. ???????

The relationship between air temperature, moisture content and heat is presented in a graph format, internationally known, as Psychrometric Chart (pronounced psychRo not psycho metric). As indicated below.????

The graph has to main axes; horizontal is the temperature and the vertical is the moister content. ?The physical relationship between the two is presented by curved lines rising from left to right exponentially. Each line represent a constant relative humidity % (RH) and presented in a multiple of 10 RH lines. Since the air condition (temperature , humidity and heat content) ?can occur anywhere within the graph area the lines are there just to aid the user to plot a specific ?air condition so as to read other properties ( as indicated in colour over the graph above) as well as to plot a line between two points to determine sensible and latent loads. The use of this graph has more than what is in this report. ??

RH (relative humidity) is a ratio of moisture content (quantity measured in gm of water/kg of air) in the air relative to its maximum carrying capacity at a certain condition. Its like comparing the water level in a tank to its storage capacity or like fuel gauge in your car. The gauge do not show actual storage but an indication relative to full and empty limits. So RH is a percentage of full capacity.

The point where air capacity to carry moisture reach its limit, is called 100% Relative Humidity (RH) also called saturation line (full tank limit).

When the air temperature reaches the saturation line (100 RH) is called dew point (air has reached full capacity). And when air is cooled beyond its dew point it sheds its moister content ( like when you squeeze a sponge it start to lose water or when water level exceeding tank capacity water start to spell out ) and that what happens in cooling coils . If the coil surface temp is lower than air dew point moister start to condensate (physical appearance of the moister hidden in the air)?and therefore air moister content naturally reduces.

As mentioned, the psychrometric chart is a map of humid air behavior in a grid format with temperature and moister forming the main axes with RH lines forming the relationship between the two. Temperature is indication of the size the container and moister content is indication of how full the air is.

Other related physical property of humid air can also be established such as heat content (kJ/kg). , specific volume (M3/kg) ?which is the reverse of density and vapour pressure. ?

You would notice that as the air gets cooler the lines of constant RH gets much closer to each other. Which make sense since the capacity of air gets smaller at cooler temperature. That’s why it don’t make much difference if air near Zero deg is 10 or 100% RH. And if you bring in air at zero deg and heat it up to 20 deg C. The heated outside air is below 30% RH when it is at 20 deg C and that’s why it causes dry skin and dry throat. Because the air literally sucking up all the moisture out of your body like a big sponge. ??

Uses of Psychrometric Chart:

1-???Determination of room/space RH :

RH can be determined by the measuring the air Dry bulb and Wet bulb temperature by mechanical means using apparatus such as Hygrometer. Both readings are plotted on the chart to determine RH of the air. The wet bulb will always read lower than the dry bulb . This is due to the water evaporation from the wet bulb. The drier the air the higher the evaporation resulting in reading much lower wet bulb temperature than dry bulb. If the wet is equal to dry bulb temperature it means that there no evaporation bcs air is saturated i.e.?100% RH. Other air properties can also be determined from wet and dry bulb temperature readings.

2-???AC Equipment Load Estimation :

Cooling or heating as well as moister gain and loss processes can be presented on the psychrometric chart to allow designers to estimate the capacity of AC units in terms of heating, cooling , humidification and dehumidification.

The HVAC system designer can plot the required heat or cooling process on this graph in order to determine capacity of coils as well humidifier /dehumidifiers.

?Cooling coil duty is expressed in sensible and in total (sensible and latent) as mentioned before.

The following simple formulas are used to determine each duty:

Sensible cooling= Air heat capacity x air flow x 1/specific volume x ( air off – air on temp)

?????????????????????????????= Kj/kg. k x m3/s x kg/m3 x k = Kj/s

Total cooling = Air enthalpy (on-off coil) x air flow rate x 1/specific volume= ?

?????????????????????=Kj/kg x m3/s x Kg/m3 = Kj/s????????????????????

After the cooling duty is determined, AC Units (AHUs, FCUs, Blowers) selection need to consider the coils type and construction. Normally manufacturers provide a selection tables or a software to select the most suitable unit matching the required duty.???

However it is not possible to match the coil performance exactly with same the off coil condition as per the chart, even if the cooling duty is matching. This is because of something called by-pass factor.

Due to cooling coil fin spacing, not all the air passing the coil will be in contact with the coil surface. The ratio of air that passes but had no contact with coil compared to total air passing is called by-pass factor. Coils with higher BP factor are useful where moisture removal is not required. ( such as computer rooms cooling application ) this is also called sensible cooling.

Efficiency of a cooling coil can then be expressed with a contact factor or a bypass factor (both are interchangeable). If the designer require a certain off coil condition this can plotted on the chart to determine the required bypass factor that cooling coil has to have in its construction. Or if the designer require highest efficiency form the coil then best to utilise a cooling coil selection software and try different configuration to reach optimum efficiency.

Why is this important? Because the actual off coil condition will determine the chilled water flow rate and not based on the design point on the 100% RH line. If bypass factor not considered then potentially the coil will be commissioned with higher flow rate than what is required and result in cooler and drier air which could have adverse effect on human comfort and can cause electrostatic issues as dry air can become like an electric charge capacitor. ?

The amount of moister reduction in the air depends on the temperature of cooling medium running through the coil and the size of the coil in terms the depth (number of rows) and or cross-section area.?The cross-section area of the coil determine the velocity of air in contact with coil surface The lower the velocity the more time air and coil are in contact in heat exchange.

?While the deeper the coil (number of rows) the more contact area resulting in condensation /moisture content reduction.

Since air is in a constant flux its temperature and moisture content varies all the time. That’s why its important to select the various air conditions such as room condition, external air condition, mix air condition (mix of ?indoor and outdoor air) ??and required air off coil condition to plot the air conditioning process and determine loads. ??

?It is to be noted that room cooling load is not the same as AC unit load.?As AC unit have to account for Fresh air load and heat recovery. That’s why using the chart is the best way to size cooling coils and not room load.

??The graph above shows a typical air cooling process. The ideal off coil condition to overcome the sensible and latent room load is plotted ( 1) then the ?room condition and outside air are also plotted ( 2&4) ( as per required comfort condition and as per selected limit of outdoor condition) ?and straight line is drawn between the two design points. The mixed air condition which represent the on coil condition?of the cooling coil is determined based on the ratio of outside air to the total supply air. ?For example if fresh air is 10% of supply air the mix point will be on 10th of the distance between the two points( 5). By this you will have on coil and off coil conditions ( 5 &1) to estimated the required total and sensible cooling load of the cooling coil.

This is how low I can go to break it down for you. Hopefully you can ?find it useful introduction to the magic of air and its property.

Fuad Habayeb ??