HVAC Submittal revisions, Total Load 5

[PuertoRicoMEP]

Projetc Type:retro-fit
One of the unit(split system)submitted to my project has a nominal capacity of 4 ton, but the total required load is 52,000BTU/hr wich is equal to 4.33(rnd to 4.5)ton. I asked the contractor about this difference and reponded me saying that the OA will compensate the difference!!! How, I do not see any AO DW connections or OA coils. Does the 0.5ton difference will affect the performance of my unit or the air conditioned effect? By the way, the project is in South Florida !.

Thank You

 

[MintJulep]

The 4 ton nominal capacity is measured at some condition. If your design condition is lower that the unit capacity for your application may be higher.

 

[CESSNA1]

PUERTORICOMEP:

Ask the contractor for his calculations (he probably does not have any). He is not responsible for the design, you are. Outside air usually requires additional cooling, more if it is humid. It also depends on the application. If is a system with a high latent heat load such as a theater or restaurant there may be other concerns MINTJULEP is correct, look at the design conditions also. Contractors are usually more interested in speed, cost, and profit than good design.

Regards
Dave

 

[HVAC68]

Am not sure about the peak conditions or design conditions in South Florida. However, nominal capacity is generally calcualted at 95 deg F (35 deg C) outside ambient temperature, and normal room conditions of 75 deg F (24degC). Most manufacturers indicate this in their catalogues.

Who has calculated the 52,000 Btu/hr load ? Ask the contractor to submit the capacity of the unit at your design conditions. Don't go by the "nominal" capacity. In hot climates like Middle East, where the design conditions are 46 or 48 deg C, the difference between the nominal and design required capacities would be to the tune of 20% to 25% !!!.

Alternately, if you have decided on the 4ton nominal capacity unit, back work and find out what temperature it can deliver at your design conditins. See, if it's acceptable to you - maybe 77deg F instead of 75deg F may be acceptable ???

HVAC68

 

[PuertoRicoMEP]

Thank you guys for your help. Here are the design conditions provided by A/C suplier: (SI)

EDB = 80(F)
EWB = 67(F)
OUTDOOR DB= 95(F)
CAPACITY @ARI = 52,000 (BTUH)
NET TOTAL CAPACITY = 52,000 BTU/HR
SENSIBLE CAPACITY = 38,100 BTU/HR
LATENT CAPACITY = 13,900 BTU/HR
LDB = 57.6 F
LWB = 56.3 F
SEER@ARI = 13 BTUH/WATT
EER @ ARI = 11.15 BTUH/WATT
FLOW = 1600CFM
MIN FLOW =1400 CFM
MAX FLOW = 1800 CFM
ELEC HEAT CAPACITY @208v = 25,427.00 BTU/HR
ELEC HEAT CAPACITY @240V =33,993.00 BTU/HR

Again, these are the calculations provided in the submittals. As you can see my load is 52,000 btu/hr. Is 4 ton is OK? Units are Trane 2TTA2048A3000 w/ TWE048P13F.

Thank you
PuertoRicoMEP

 

[HVAC68]

Capacity at ARI = 52,000 Btu/hr and if I am correct, the ambient DBT as per ARI is 95 deg F which is the same you have indicated. The submittal also indicates 52,000 Btu/hr. Where did 4 ton come from ?

HVAC68

 

[PuertoRicoMEP]

HVAC68:
The area to be conditioned has one existing 7.5ton split system. The proposed design is to split the area in two zones. The existing load was calculated back in the 80's.
For budget purpose it was indicated to use two 4ton units, but the awared contractor will be responsable for design/built. The contractor suplier, indicated in the load calculations results that each system will required 52,000BTH but the proposed units will have a nominal capacity of 4ton each!!!
It this correct? maybe I do not understand in full this topic(no YET!), but until then, this do not make sense to me.

Again, thank you
PuertoRicoMEP

 

[quark]

If the total capacity is 52000btu/hr for two zones and if you plan for 2x4tons then there is no problem.

 

[PuertoRicoMEP]

52000BTH for each zone.

 

[MintJulep]

As you can see my load is 52,000 btu/hr

I can't see that at all. The "design conditions" that you posted above look suspiciously like the unit capacity, not that space cooling load.

What are the loads in the space? Has the useage of the space changed from the original calculation?

Off-hand, it seems that you are replacing a single nominal 7.5 ton unit with two nominal 4 ton unit for a total of 8 tons of capacity. If the loads in the space have not changed significant, then I don't see much risk (assuming the 7.5 ton unit was sufficient). However, if the use has changed, or if the remodel has changed the envelope then the loads may have changed.

 

[HVAC68]

Let me get it clear again.

You are replacing a 7.5ton unit - I suppose there's no change in the load which again means 2 x 3.75 tons should be good enough - i.e. 2 x 45,000 Btu/hr. So if you provide 2 x 4tons - i.e. 2 x 48,000 Btu/hr - it's good enough.

The catch here is - Has the load remained the same from the 80's ?

My suggestion would be - Recalculate load based on the today's scenario and see whether the 2 x 4tons is good enough or not.

HVAC68

 

[PuertoRicoMEP]

Yes, If the has 1-7.5Tn system and if it is replaced with 2-4ton units, it shloud not be a problem. Now to be in the safe size, we required a new load calculation. Originally 80's, today 2005, almost 25 yrs. This a nature center(Park and recration Dept.) When it was build, I am really sure that it did not have so much vegetation around the building(the building is cover by trees 360), no computers, displays, and the wood that cover the building probably has deteriorated.

As you can see in the report the load calculated per unit 52MBH unless I intepreted wrong. This is equal to 4.33tn per zone. And if the TOTAL load is 52MBH and they are using 2-4ton then they overloading the building, creating other problems.

I will wait for contractor's repond and will let you know. Thank for your help, it was really interesting to share knowledge.

PuertoRicoMEP